Discuss 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses in the Electrical Forum area at ElectriciansForums.net

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markc17

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Hi all,
I commissioned the renewal of some mains intake equipment at a building that involved a full shut down. The building runs 24/7 and so a shut down is a VERY rare event. Upon completion of the works the contractor informed me that the 95mm 4 core cable feeding the building was being protected by 200 amp fuses. I looked at table 4D4A and this suggests that the maximum currenct carrying capacity for a cable ot this type,running in ducting, is only 169 Amps.
I asked for old records and test results (which are sketchy at best) and it does seem to point at the fact that this was how the install was done from the outset. Periodic results from ten years ago conveniently have the cable size missed out on the results sheet. Unless i am missing something, this would mean i have to recommend they change their fuses to 160 amp ( which would be a virtual impossibility) or upsize the mains incoming cable which is going to be very costly. Before i recommend these works and upset the boat.. am i missing something?
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Forget table 4D4A, and look at the manufactures data for this cable!! It's probably still going to be close to the mark!! Any idea of the maximum demand of the overall load of this buildings equipment??
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Any idea where i would find the information on the cable? as i say the installation is fairly old and information is pretty sketchy. I put a metering pack on the mains of the building for a week and during the day there were peak readings up to 200 amps on the phases, with one even reading as much as 225 during a busy lunch time. If no information is available, surely table 4D4A is what i will have to go by... am amzaed at how much the cable derates being in a ducting. I have to provide the owners with a recommendation....
 

i=p/u

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Arms
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

interesting................ id say stick to book, but you could try doing calc out on full.. but what do i know just interested
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I'd say it's fine, it could potentially carry 260 amps, we would need specific information to be sure.

Exact cable type, a precise description of installation method etc.
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Who here would go by the book, rather than the cables manufacturer themselves?? Remember that all derating factors and installation methods applied to cables, are based on stated full load current of that cable!!

Quite appalled at the lack of understanding here, and the total reliance of taking figures out of tables that are also not being fully understood!!
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Elecsa table actually confirms the rating of 169 Amps as it is a 4 core cable. The switch for the building is fed via main busbars.. 200 amp fuses... then a 30 m run underground up into an intake cupboard to main isolator for building. I can probably see a metre of the cable at each end if i am lucky so am honestly asking how i find out manufacturers information on the cable.. if i cant see anything pointing to manufacturer.. surely then tables must be used.... i would understand if the drop was negligeable.. but 169 amps.. to 200 amps is not...
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Check out a few cable manufactures data, they will generally be pretty much in the same ball park!! Check the insulation too, PVC or XLPE. XLPE insulation tends to be more transparent and thinner than PVC insulation, which will also be much brighter in colour than XLPE!! From only my bad memory, 95mm 4 core SWA, should be capable of supplying in excess of 200A rather than the 169A being banded about just because it's in a 30m duct... but i'll stand to be corrected if wrong!! lol!!

We are talking here of only around 30A, and if this is an original cable, that has been in service for some time and has provided the means of supply to the same loading of this building without any ill effects, then my guess is that it will continue to be adequate. Having said that, if the owner is thinking of extending the loading in the foreseeable future, then a cable upgrade would be a prudent move to make now, while the installation as a whole is being renovated. How come this supply cable has only come to light, at or towards the end of the job, surely this would have been one of the first things to assess before the job started!!
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Not here to stand to correct you my friend, im only going off whats in the book. It states 169A. Pretty sure its not XLPE. As i say, am suprised myself at how much the cabel derates just from being in a duct.
I agree with you that it should have been assessed at the beginning of the job, assumptions were made as the mains are never ever shut down due to servers and other complications with the building as it is never really not in use. To be honest... whether this assessment had been made at the beginning or the end the job would still have had to go ahead as the incoming mains had no means of isolation and was so crowded that a pannel was essential..
The building is adding more and more all the rime in the way of computer rooms etc.. and as i say.. i had a peak reading of 225 amps on a friday.. it lasted for a good minute or three above 200 amps...
 
B

Bobster

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Is it showing signs of overheating?
 
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Megavolt

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Stick with cable manufactors every time
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Not here to stand to correct you my friend, im only going off whats in the book. It states 169A. Pretty sure its not XLPE. As i say, am suprised myself at how much the cabel derates just from being in a duct.
I agree with you that it should have been assessed at the beginning of the job, assumptions were made as the mains are never ever shut down due to servers and other complications with the building as it is never really not in use. To be honest... whether this assessment had been made at the beginning or the end the job would still have had to go ahead as the incoming mains had no means of isolation and was so crowded that a pannel was essential..
The building is adding more and more all the rime in the way of computer rooms etc.. and as i say.. i had a peak reading of 225 amps on a friday.. it lasted for a good minute or three above 200 amps...
Mark, you have benn given good advice, your sure ist not XLPE, id be pretty sure it is since the standard has changed requiring it!
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Not here to stand to correct you my friend, im only going off whats in the book. It states 169A. Pretty sure its not XLPE. As i say, am suprised myself at how much the cabel derates just from being in a duct.
I agree with you that it should have been assessed at the beginning of the job, assumptions were made as the mains are never ever shut down due to servers and other complications with the building as it is never really not in use. To be honest... whether this assessment had been made at the beginning or the end the job would still have had to go ahead as the incoming mains had no means of isolation and was so crowded that a pannel was essential..
The building is adding more and more all the rime in the way of computer rooms etc.. and as i say.. i had a peak reading of 225 amps on a friday.. it lasted for a good minute or three above 200 amps...

It's your call, at the end of the day!! Personally i wouldn't derate a 95mm SWA cable down from around 250'ish Amps or so to 169A because it runs in a duct for 20m or 30m!! Why don't you check the two ends of this existing cable for signs of stress, i doubt very much if you'll see any!!
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Mark, you have benn given good advice, your sure ist not XLPE, id be pretty sure it is since the standard has changed requiring it!
Heya Chris, I am not doubting any advise thats for sure.. i appreciate people taking the time to help me out, including yourself. I was given layout drawings this morning which confrims the cable is PVC. Like everyone is suggesting on here.. i am actually in agreement.. there is no signs of stress or overheating.. but the contractor that has carried out the mains change has highlighted the fact that the fuses need to be downsized, and will only sign off from his side of the new isolator and the fixed wire test 5 years ago it seems made a recommendation to do the same thing. the cable derates nearly 100 amps putting it in the ground according to the book.. i am just trying to come up with the best way to advise the client what to do is all..
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

It's your call, at the end of the day!! Personally i wouldn't derate a 95mm SWA cable down from around 250'ish Amps or so to 169A because it runs in a duct for 20m or 30m!! Why don't you check the two ends of this existing cable for signs of stress, i doubt very much if you'll see any!!
Appreciate the advise. I think the derating is over the top too. Theres no stress or overheating. Just figures causing me headaches from a book. If a contractor brings the matter to the clients attention, as another, it appears has done in the past, then it is my job to advise and recoomend a course of action. Am compiling a report containing meter readings for the week.. now just writing covering letter..
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Is it showing signs of overheating?
No Rob.. noth ing to suggest stress..or overheating.. just a few peak times of the day when the amp readings are high (over 200)..and often over the 160 amps to satisfy the table.. problem is that a contractor has recommended a downsizing of fuses recently, as has one in the past. To upgrade the cable would be a huge job so dont want to advise anything rash but the building is obviously pulling more and more as the years go on.
 

darkwood

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Im in the thinking the original fuses may have been 160's in a 200amp isolator and probably through ill thinking they were changed to 200 to match the isolator without thought as to why they were 160amp in the first place. Id be inclined to sit down and discuss this with the client and express your concerns the fact your only having occasional peaks >200 amp for a few minutes at most is why you havent seen the 200amp fuses go and why there will be no cable damage as such....... check your time current curves on 200amp fuses?

If the plans are for expansion that will add load to the existing set-up then the existing cable won't be adequate even if it was sized correctly, also to check if additional loading is planned is KVA demands and what allowance you have and what the DNO can spare if any, pointless weighing up for increased supply if there isn't a buffer zone available from the DNO to give you extra without charging you for a new substation.

Your difficulty here is probably knowing who made the cable and without this info you have to adhere to the tabulated values and recommend a derating of the fuse or increase of csa of cable... either way i would note it down because your the last inspection and if anything occurs in the near future like an overload of cable or fuses going im sure the downtime will be costly and having written recommendation that it needs addressing will stop the company chasing you for some kind of compensation.

All companies cry poor when costs are unexpected but if they plan on fitting computer suites then maybe that money needs directing to sorting out the power supply issue.... it can all be done with minimal downtime by digging new trench and running new cable in while existing is still functional the added costs of doing this will normally outweigh the cost of shutdown.
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Im in the thinking the original fuses may have been 160's in a 200amp isolator and probably through ill thinking they were changed to 200 to match the isolator without thought as to why they were 160amp in the first place. Id be inclined to sit down and discuss this with the client and express your concerns the fact your only having occasional peaks >200 amp for a few minutes at most is why you havent seen the 200amp fuses go and why there will be no cable damage as such....... check your time current curves on 200amp fuses?



If the plans are for expansion that will add load to the existing set-up then the existing cable won't be adequate even if it was sized correctly, also to check if additional loading is planned is KVA demands and what allowance you have and what the DNO can spare if any, pointless weighing up for increased supply if there isn't a buffer zone available from the DNO to give you extra without charging you for a new substation.

Your difficulty here is probably knowing who made the cable and without this info you have to adhere to the tabulated values and recommend a derating of the fuse or increase of csa of cable... either way i would note it down because your the last inspection and if anything occurs in the near future like an overload of cable or fuses going im sure the downtime will be costly and having written recommendation that it needs addressing will stop the company chasing you for some kind of compensation.

All companies cry poor when costs are unexpected but if they plan on fitting computer suites then maybe that money needs directing to sorting out the power supply issue.... it can all be done with minimal downtime by digging new trench and running new cable in while existing is still functional the added costs of doing this will normally outweigh the cost of shutdown.
Id wait untill he aquires the relevent information before he does anything, potentially this cable could take upto 260 amp.
 
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Gage

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I agree with darkwood you should be concerned that the 200A fuses haven't blown at 220+ amps sustained for minutes.

Also there's videos on YouTube of 95mm taking 5000 amps ;) only for about 20 minutes but still
 
If you cant get any manufacturers details then you'll have to go from the regs book. If anything happens least your bum will be covered.

You can only advise, just as the last 2 contractors did. Advise them on the options.

Just because it's been ok so far doesnt mean it's going to be ok in the future with expansion.

Remember it's your name on it all the end...:thumbup:
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

What's the overall length of this 95mm 4 core cable?? You've only indicated you have 30m run in ducting, is that roughly the total length or is it much longer than that 30m??

If it's roughly around the 30m mark, it isn't going to be an overly expensive upgrade, especially if you use a parallel supply, using 2 X 25 or 35mm 4 core SWA cables, the later giving you around 325A, which will be more than enough to de-rate to your hearts content for the ducting!! lol!! Also be a lot easier to man handle during the installation....
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Mark, go down and get the BS number and manufacturer of the cable, forget what's being said, the oversheath may well be PVC, I doubt the insulation will be!

What's the length of cable entering and exiting the duct with regard to terminations? How is the duct installed, surrounding material etc, what is the ambient being taken as?.

I've got 263 in a duct at 90 C, even if I run it at 70 C it still greater than 200.

You need to assess the install before anything.
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Why's that squire?...

Remember this is an old installation...



Also worth remembering if designing the circuit for 90 degree cable then connected equipment needs to be rated at that too...
Old installation, New cable is it not? As I have said, SWA standards have changed.

I'm aware of the 90 C, I've already raised that issue with Mark.

No point going any further until Mark addresses the points I've raised.
 
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LlandrilloSpark

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A good example of this is the CCC of 10mm two core armour , I priced a job on Friday eve , bs7671 states approx 60 Amps , TLC website who use manufacturers Data states over 80A
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

especially if you use a parallel supply, using 2 X 25 or 35mm 4 core SWA cables, the later giving you around 325A, which will be more than enough to de-rate to your hearts content for the ducting!!QUOTE]

I may have read this wrong but the rating you have quoted is Reference Method E and what he has is Reference Method D, 115A for 35mm so 230A for parallel cables. If they are installed in the same ducting then you have to de-rate the cable by .75 (cables touching) so you would get 86A or 172A for 2 cables.
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

especially if you use a parallel supply, using 2 X 25 or 35mm 4 core SWA cables, the later giving you around 325A, which will be more than enough to de-rate to your hearts content for the ducting etc!!QUOTE]

I may have read this wrong but the rating you have quoted is Reference Method E and what he has is Reference Method D, 115A for 35mm so 230A for parallel cables. If they are installed in the same ducting then you have to de-rate the cable by .75 (cables touching) so you would get 86A or 172A for 2 cables.

I wasn't comparing ref methods, as i stated, more than enough leeway for any de-rating for duct installation...

They are the same supply cable, be they X 2, X 4 or whatever, you don't de-rate a parallel circuit, de-rating only applies if the cables being de-rated are two different supplies!! Would you de-rate a ring circuit if both legs were run in a conduit/trunking?? lol!!

Depending on any other criteria, i'd be more than happy rating a parallel 35mm supply at or around 280A +. I rarely take much notice of cable ratings in the BGB, i tend to mainly use manufacturers data. But that would be up to the OP and his own experience as to what sort of de-rating factors to apply on any given cable installation etc... Whatever way you go, 2 X 35mm SWA cables are going to fulfill the OPs supply cables requirement....
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

They are the same supply cable, be they X 2, X 4 or whatever, you don't de-rate a parallel circuit, de-rating only applies if the cables being de-rated are two different supplies!! Would you de-rate a ring circuit if both legs were run in a conduit/trunking?? lol!!QUOTE]

Isn't the de-rating of grouped cables down to heat dissipation? If there are two cables together then they don't dessipate heat as well if they are apart, so that is why they are de-rated. It doesn't matter if they are for the same circuit or different circuits. Its grouping of multi-core cables.
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Mark, go down and get the BS number and manufacturer of the cable, forget what's being said, the oversheath may well be PVC, I doubt the insulation will be!

What's the length of cable entering and exiting the duct with regard to terminations? How is the duct installed, surrounding material etc, what is the ambient being taken as?.


I've got 263 in a duct at 90 C, even if I run it at 70 C it still greater than 200.

You need to assess the install before anything.
Ok.. spoke with someobody in the know that can assure me the cable has been in 25 years plus... i did find a name on the cable.. says AWCO Electric cable 66/1000V BASEc...(crawling around at the back in the dark) No BS number..Cant find anything on AWCO anywhere.. unless im being stupid.. one meter of cable straight out of ducting set in concrete into 200 Amp switchfuses either end..one (old) MEM 200A HRC..off the busbars at the intake... the other 200A TPS-N Hager Fuse combination switch (New)... in building intake cupboard.
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

If an upgrade is to be looked at.. 2 x parallel 35mm cables will certainly be easier to pull than the 150 or 185 needed to upgrade properly according to 4D4A...lol..not that it will be easy.. the road will have to be dug up.. although we may get away with no hydraulic bending equipment and some elbow grease i guess...!..lol.. i think im going to have to quote similar manufacturers figures.. and the regs... and tell them they are full up and to make future plans based on my recommendadtions.. The place is running like a dinosaur anyway.. full of t8 wingback lights.. convection heating everywhere.. might be able to make a few savings the other way... and keep my arse covered..:)
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

They are the same supply cable, be they X 2, X 4 or whatever, you don't de-rate a parallel circuit, de-rating only applies if the cables being de-rated are two different supplies!! Would you de-rate a ring circuit if both legs were run in a conduit/trunking?? lol!!QUOTE]

Isn't the de-rating of grouped cables down to heat dissipation? If there are two cables together then they don't dessipate heat as well if they are apart, so that is why they are de-rated. It doesn't matter if they are for the same circuit or different circuits. Its grouping of multi-core cables.
This side of it i will need to look into IF they decide to do something.. which i very much doubt..)
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Depending on any other criteria, i'd be more than happy rating a parallel 35mm supply at or around 280A +. I rarely take much notice of cable ratings in the BGB, i tend to mainly use manufacturers data.....
Ever heard of AWCO? or am barking up the wrong tree
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

They are the same supply cable, be they X 2, X 4 or whatever, you don't de-rate a parallel circuit, de-rating only applies if the cables being de-rated are two different supplies!! Would you de-rate a ring circuit if both legs were run in a conduit/trunking?? lol!!QUOTE]

Isn't the de-rating of grouped cables down to heat dissipation? If there are two cables together then they don't dessipate heat as well if they are apart, so that is why they are de-rated. It doesn't matter if they are for the same circuit or different circuits. Its grouping of multi-core cables.
No it doesn't work like that!! The 2 or more cables of a parallel circuit are equally sharring a single point load, so it's a single circuit!! In other words it is considered as a SINGLE cable, not multible cables supplying different loads etc. As i said above, would you actually de-rate a ring circuit contained in a suitably sized conduit or other containment??
 
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Gage

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In debating whether people know the difference between series and parallel :confused:
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

If an upgrade is to be looked at.. 2 x parallel 35mm cables will certainly be easier to pull than the 150 or 185 needed to upgrade properly according to 4D4A...lol..not that it will be easy.. the road will have to be dug up.. although we may get away with no hydraulic bending equipment and some elbow grease i guess...!..lol.. i think im going to have to quote similar manufacturers figures.. and the regs... and tell them they are full up and to make future plans based on my recommendadtions.. The place is running like a dinosaur anyway.. full of t8 wingback lights.. convection heating everywhere.. might be able to make a few savings the other way... and keep my arse covered..:)
That's just how ridiculous some of these cable and de-rating tables can end up, ...with totally silly cable sizes!! Who in their right mind would install a 150 or 185mm cable to satisfy a 200A to 225A supply requirement??

Thought you said this cable was in a duct, why would you need to dig the road up for god's sake??
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

You need to take a look at ERA report 69.30

this has cable sizes for sustained and cyclic loads. It's one of the methods that DNO's use to size their cables.
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

You need to take a look at ERA report 69.30

this has cable sizes for sustained and cyclic loads. It's one of the methods that DNO's use to size their cables.
Have you got a link to this report??
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Ok.. spoke with someobody in the know that can assure me the cable has been in 25 years plus... i did find a name on the cable.. says AWCO Electric cable 66/1000V BASEc...(crawling around at the back in the dark) No BS number..Cant find anything on AWCO anywhere.. unless im being stupid.. one meter of cable straight out of ducting set in concrete into 200 Amp switchfuses either end..one (old) MEM 200A HRC..off the busbars at the intake... the other 200A TPS-N Hager Fuse combination switch (New)... in building intake cupboard.
Sorry Mark, misunderstood, i thought you had replaced the mains like for like lol for some reason.
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

No it doesn't work like that!! The 2 or more cables of a parallel circuit are equally sharring a single point load, so it's a single circuit!! In other words it is considered as a SINGLE cable, not multible cables supplying different loads etc. As i said above, would you actually de-rate a ring circuit contained in a suitably sized conduit or other containment??
Ok, (example 1) so we have a circuit taking 200A run in conduit. The intention is to use 2 cables, each taking 100A. Each producing heat.
Lets just say we also have 2 additional circuits (example 2) each using 100A, supplied with two cables in a ducting. (different ducting to example 1.)
You are saying that the re-rating factor applies to example 2 but not to example 1? How does that work?
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

That's just how ridiculous some of these cable and de-rating tables can end up, ...with totally silly cable sizes!! Who in their right mind would install a 150 or 185mm cable to satisfy a 200A to 225A supply requirement??

Thought you said this cable was in a duct, why would you need to dig the road up for god's sake??
It does enter the building in a duct...Im trying to explain the installation without EVERY detail.. but that duct goes out under the floor... a meter down the side of a buildingoutside into a duct in the road.. then around multiple bends with no access panels of any type..There are no draws in.. from my experience.. pulling any sort of heavy duty armoured around corners is tough.. i dont have any groundworks drawings or a CAT scanner to hand...lol... so i think my observations are a fair assessment.. but still..
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Sorry Mark, misunderstood, i thought you had replaced the mains like for like lol for some reason.
No worries Chris.. to be honest this has been a good learning curve for me.. thanks for your help.
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

You need to take a look at ERA report 69.30

this has cable sizes for sustained and cyclic loads. It's one of the methods that DNO's use to size their cables.
will do.. thank you
 
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markc17

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  • #45
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Anyone know where i can pull some manufactureres data for a cable like this.. one rpe XLPE.. every site i look at seems to be the latter.. or if anyone has heard of AWCO?
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Have you got a link to this report??
Unfortunately no. If anyone does have access to this, then I would be happy to have a copy.

ive tried looking for this myself, and can't get it anywhere.
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Ok, (example 1) so we have a circuit taking 200A run in conduit. The intention is to use 2 cables, each taking 100A. Each producing heat.
Lets just say we also have 2 additional circuits (example 2) each using 100A, supplied with two cables in a ducting. (different ducting to example 1.)
You are saying that the re-rating factor applies to example 2 but not to example 1? How does that work?
In simplistic terms Yes, the parallel circuit ensures equal current carrying between the two conductors, (and are considered as a Single cable), whereas the two separate circuits that are supplying different loads will not. The cable grouping de-rating table(s) in the BGB are based on the cables carrying the full load current being present, on each of the grouped cables/conductors, when in the real world that is almost never the case, and why you always end up with over sized cables!!...
 
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markc17

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Aluminium wire and cable co, good chance this is a Soidal Aluminium cable?
Im sure one of the previous tests (of which i only have the results for one...or this fixed wire test done in August would have said so.. definately says copper on the sheet...) Gulp.. ha.. i certainly hope so or the cable derates down even more! Must admit ive never worked with aluminium cable.. am hoping AWCO made copper SWA too...
 
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markc17

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  • #51
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

just found this on the IET forum.. a guy that used to be a cheif electrical engineer for BACO posted it...
''This BACO company had a cable manufacturing company in Swansea producing their "Solidal" cable.''


Qustion to post - ''I guess that was the old Aluminium Wire and Cable Co (AWCO) Jaymack ?''

Reply ''Correct, I took a couple of BACO maintenance electricians there for 2 day's hands on training.''

Hmmm.. anyone enlighten me on this soidal cable? if that is what it is?

- - - Updated - - -

Aluminium wire and cable co, good chance this is a Soidal Aluminium cable?
Very kind.. have you worked with it before?
 
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Stroppy

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

They will keep me amused tomorrow when I'm snowed off work. :)
 

darkwood

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Alot of confusion in this thread regarding grouping and same source/supply parallel cables...

Im on the understanding myself the grouping factor is to be applied to parallel runs same circuit too, the reason is the derating factor is designed to allow for the reduced surface area of which the thermal energy can dissapate away from the copper, with this in mind the fact it the same circuit has no bearing as surface area will still be reduced and cable sized will be selected with relation to ccc of each cable added together. When running parallel cables in trenches or cable-tray its common to seperate them apart so grouping isnt applicable....

The problem and why i believe most are misled is the reg's titles grouping factor for cables of different circuits but if you refer to appendix 10 and read the first paragraph of no'2 (overload protection....) then at the end of this paragraph it does clearly state 'with the appropriate grouping and other factors applied'.

In essence of the reason grouping factor is applied in the first place then both cables of a parallel supply are still subject to dissapating their thermal energy much the same as cables of different circuits i cant see the any reason why having parallel 'same circuit' cables would be exempt from the grouping factor.

Ive been digging wider trenches for years and spacing the cables subject to calculation relative to diameter for yrs to avoid derating for grouping :uhoh2:
 

Gavin A

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Elecsa table actually confirms the rating of 169 Amps as it is a 4 core cable. The switch for the building is fed via main busbars.. 200 amp fuses... then a 30 m run underground up into an intake cupboard to main isolator for building. I can probably see a metre of the cable at each end if i am lucky so am honestly asking how i find out manufacturers information on the cable.. if i cant see anything pointing to manufacturer.. surely then tables must be used.... i would understand if the drop was negligeable.. but 169 amps.. to 200 amps is not...
that elecsa table is based on the following

Table 4D1A – Single core 70[SUP]0[/SUP]c thermoplastic insulated cables (e.g. PVC Singles), non-armoured with or without sheath. All values assume an ambient temperature of 30[SUP]0[/SUP]c and a conductor operating temperature of 70[SUP]0[/SUP]c.
Ambient ground temperature peaks at more like 15 deg C in summer, maybe a little more under a building if the basement is heated and not insulated from the ground, but it'll never get anthing like 30 deg.
 
200 amp fuses

best to follow manufacturers data rather than book.

for example 2.5 twin can carry around 27 amps ish

but 1.5 tri rated can carry 21 amps but they would question it if you put it on a 16a breaker
 
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Engineer54

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  • #57
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Alot of confusion in this thread regarding grouping and same source/supply parallel cables...

Im on the understanding myself the grouping factor is to be applied to parallel runs same circuit too, the reason is the derating factor is designed to allow for the reduced surface area of which the thermal energy can dissapate away from the copper, with this in mind the fact it the same circuit has no bearing as surface area will still be reduced and cable sized will be selected with relation to ccc of each cable added together. When running parallel cables in trenches or cable-tray its common to seperate them apart so grouping isnt applicable....

The problem and why i believe most are misled is the reg's titles grouping factor for cables of different circuits but if you refer to appendix 10 and read the first paragraph of no'2 (overload protection....) then at the end of this paragraph it does clearly state 'with the appropriate grouping and other factors applied'.

In essence of the reason grouping factor is applied in the first place then both cables of a parallel supply are still subject to dissapating their thermal energy much the same as cables of different circuits i cant see the any reason why having parallel 'same circuit' cables would be exempt from the grouping factor.

Ive been digging wider trenches for years and spacing the cables subject to calculation relative to diameter for yrs to avoid derating for grouping :uhoh2:

No confusion here!! lol!!

So you too would apply a grouping factor to a ring circuit if both legs were run in a conduit or other containment. That'll be a first for me!!! lol!!
 
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Gavin A

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

So you too would apply a grouping factor to a ring circuit if both legs were run in a conduit or other containment. That'll be a first for me!!! lol!!
Can you explain why not?

Is the derating factor not down to the additional heat stemming from the additional cable, and the lower surface area available for heat to be lost through?

And why would this not apply just because both cables are part of the same circuit?

if anything I'd think it should apply even more, as both cables will always be under full load at the same time, whereas in cables on different circuits will be under different loading levels to each other.

The only logical reason I could see for this not being the case would be if this was automatically taken into account already in the figures for those cables.
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Jesus, ...I give up!!
 
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Engineer54

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  • #60
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

It does enter the building in a duct...Im trying to explain the installation without EVERY detail.. but that duct goes out under the floor... a meter down the side of a buildingoutside into a duct in the road.. then around multiple bends with no access panels of any type..There are no draws in.. from my experience.. pulling any sort of heavy duty armoured around corners is tough.. i dont have any groundworks drawings or a CAT scanner to hand...lol... so i think my observations are a fair assessment.. but still..
Let's get things absolutely clear on the length of this cable, it's approximately 30m Yes?? What size is the duct this cable is running through and what's it made of ?? (Metal, PVC, Ceramic)

It's extremely unlikely that any of the bends in this duct are going to be tight bends, otherwise they would never have been able to pull the cable into the duct in the first place. All professionally installed ducting will have slow/long bends, for this very reason. So there shouldn't be any need to dig up a road or whatever to replace this cable. The only time you would need to dig up this road would be, if clowns installed this duct and fitted the duct over the cable during installation, a bit at a time! ...lol!! But then why bother installing a duct system, you might just as well direct bury the cable for what good or use the duct would be!! lol!!

Depending on the amount of bends in this shortish duct, you may well need to hire a pulling machine (and maybe cable rollers) to remove old cable and replace with new!!

The copper should be worth a few bob from the old cable, at the scrapie's!! lol!!
 
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markc17

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  • #61
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Ok.. well.. judging by the way every other aspects of electrical install has been carried out at this place i would hate to guess... duct is perhaps 12 inches across.. ceramic.. in the building.. but when it comes out of the building and down the wall.. it disappears into the road with tarmac covering any sign of ducting... Anyway.. im going to submit a report advising what the regs say.. and advising what a cable manufacturer would say.. and tell them that whatever happens.. they are at capacity.. THEN wait and see if they tell me tto investigate how to pull a new feed in..:)
Best go check to make sure conductors are copper first lol...although im sure they are..
Thanks for your time.. very much appreciated.
P.S. you are right.. would be worth a few... if i can get it away..;)
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

How about taking a few pictures of this 95mm cable installation when your back down there. It'll give a much clearer veiw of what were talking about!!

By the way, are there any other cables within this duct with the 95mm cable??
 

darkwood

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

No confusion here!! lol!!

So you too would apply a grouping factor to a ring circuit if both legs were run in a conduit or other containment. That'll be a first for me!!! lol!!
As i recall been taught that the unique nature of a ring main and its normal operating conditions give an exemption from grouping factor to itself where as supply cables with no spurs which are designed to handle up to full ccc of both cables have to include grouping factor... if you flick through the regs regarding Parallel cables you often see that ring mains are exempt from the regulation or are noted that the specific reg shouldn't be applied to ring mains ....

523.8 .... discussing achieving load sharing later states this does not preclude the use of ring mains with or without spurs....

This is repeated with many regulations regarding parallel cables and gives the overall impression that the nature of our flawed ring-main system is in itself a seperate area requiring its own regulation hence appendix 15 or reg 433.1.5 which all through there implementation give the ring main its exemption for the need to apply a grouping factor...
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

As i recall been taught that the unique nature of a ring main and its normal operating conditions give an exemption from grouping factor to itself where as supply cables with no spurs which are designed to handle up to full ccc of both cables have to include grouping factor... if you flick through the regs regarding Parallel cables you often see that ring mains are exempt from the regulation or are noted that the specific reg shouldn't be applied to ring mains ....

523.8 .... discussing achieving load sharing later states this does not preclude the use of ring mains with or without spurs....

This is repeated with many regulations regarding parallel cables and gives the overall impression that the nature of our flawed ring-main system is in itself a seperate area requiring its own regulation hence appendix 15 or reg 433.1.5 which all through there implementation give the ring main its exemption for the need to apply a grouping factor...
There is nothing flawed in the nature of a ring circuit, so let's not go there!! lol!! If there was going to be any detrimental effect on ring circuit, you can be dammed sure a factor would be placed against it!! Grouping factors have always been about grouping of circuits rather than conductors/cables, otherwise you can start adding Trefoil-ed conductors into the equation too!!...

Never, and i do mean never, have i applied a grouping factor to a Paralleled single circuit. And many of those parallel circuits were feeders from TX's to main switchboards consisting of multiple conductors per phase, in fact i'm looking at a project drawing consisting of a similar situation now, consisting of 9 X 95mm insulated & sheathed singles per phase!! run on tray in trefoil (or should i say quad-foil lol!!) Can you imagine what any grouping factor for this single circuit would do to the size of those conductors!! Jesus, they would end up as 185mm's ... Other de-rating factors to apply Yes, grouping de-rating of conductors of a single circuit, ...No!!
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I have to disagree with you there Eng54. Single Circuit or Multiple Circuits, its all the same. Its all about dispersal of heat from the cables. If the cables are spaced correctly then you do not need to apply the grouping factors. As darkwood said "Ive been digging wider trenches for years and spacing the cables subject to calculation relative to diameter for yrs to avoid derating for grouping"
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I have to disagree with you there Eng54. Single Circuit or Multiple Circuits, its all the same. Its all about dispersal of heat from the cables. If the cables are spaced correctly then you do not need to apply the grouping factors. As darkwood said "Ive been digging wider trenches for years and spacing the cables subject to calculation relative to diameter for yrs to avoid derating for grouping"
On a trench with a single parallel circuit?? I don't think so!! On any trench with multiple cables, parallel circuit or otherwise, you would space out your cables, purely for natual movement of the ground.
 
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Engineer54

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  • #68
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

All a parallel circuit is, is replacing a single large CSA sized cable with smaller CSA cables, that can give many advantages and equally shares the current flowing. It remains at all times a SINGLE circuit, it cannot influence any other circuit!! (unless run with other cable/circuits of course)
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Get back to work then!
I'm thinking more of going to bed, rather than trundling off back to site!! lol!!
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

All a parallel circuit is, is replacing a single large CSA sized cable with smaller CSA cables, that can give many advantages and equally shares the current flowing. It remains at all times a SINGLE circuit, it cannot influence any other circuit!! (unless run with other cable/circuits of course)
Come on Eng54, you have to be taking the ---- now....
Ok, just look in the regs at correction factors for cables... they are all about heat dissipation of the cables.. Ambient temp, thermal insulation, buried in the ground.....
Look at the tables for the current capacity of cable eg. Table 4E4A the more surface area of the cable is in contact with the air the higher the current is.. Also heat dissipation...
Therefore two cables touching will not dissipate the same heat as two cables apart. Therefore you have to de-rate the cable. Again, single circuit or multi-circuits its all the same. The physics of it cant distinguish between 2 cables supplying 100A each for one circuit and 2 cables supplying 100A for 2 circuits. They are both producing the same heat....
 
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Engineer54

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Come on Eng54, you have to be taking the ---- now....
Ok, just look in the regs at correction factors for cables... they are all about heat dissipation of the cables.. Ambient temp, thermal insulation, buried in the ground.....
Look at the tables for the current capacity of cable eg. Table 4E4A the more surface area of the cable is in contact with the air the higher the current is.. Also heat dissipation...
Therefore two cables touching will not dissipate the same heat as two cables apart. Therefore you have to de-rate the cable. Again, single circuit or multi-circuits its all the same. The physics of it cant distinguish between 2 cables supplying 100A each for one circuit and 2 cables supplying 100A for 2 circuits. They are both producing the same heat....
I don't take the ****!!

Oh i forgot, none of this applies to a ring circuit does it!! It somehow doesn't need to have the same heat dissipation requirements of a similar circuit!!

Whatever you guy's want to do when running parallel circuits is up to you lot!! You'll be, all but defeating to whole point of a parallel circuit!!
I know that i'll wont be adding a grouping factor to any parallel single circuits i'll be seeing on this project!! And i'll be happily sleeping like a baby to the end of my days, in that knowledge!! lol!!!
 

Rockingit

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I think what people may be overlooking here E54 (and I'm in full agreement with you), is that when you do the maths you'll find that N + N has a greater surface area than (2N) [if you see what I mean]. This is why two parallel conductors together can still dissipate more heat.
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Ok mate, it looks like you are not going to change your mind. I have tried to tell you the physics behind grouping factors. I just hope that the people who search this site looking for info on parallel circuits just dont look at your post count and then mine and say "Eng54 has to be right" and they look at the whole picture.
Anyhow Im going home in 5 mins, so the kids can bully me. Ill see in the morning if anyone agrees with me. Have a good one mate.
 

Rockingit

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

The physics of it cant distinguish between 2 cables supplying 100A each for one circuit and 2 cables supplying 100A for 2 circuits. They are both producing the same heat....
a
Sorry mate, I'm with E54 on this. They're not producing the same heat! Two parallel conductors of identical nature carrying I current at U voltage along R resistance will both be identical, whereas two different circuits will be operating at different lengths and voltage drops, hence they will never create the same amount of heat - hence one can induce heat in the other, whereas our parallel conductors are incapable of transferring heat energy one to the other as the figure is the same for both of them to start with.

If I put a jug of boiling water next to another jug of boiling water I don't create 200 degrees in between them, I've still got 100 degrees. But if I place one jug at 50 and one at 90 side by side then we get (in a perfect sealed system) 70 degs resulting between them.
 

darkwood

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I think what people may be overlooking here E54 (and I'm in full agreement with you), is that when you do the maths you'll find that N + N has a greater surface area than (2N) [if you see what I mean]. This is why two parallel conductors together can still dissipate more heat.

Im always open to debate and just as you posted this i was thinking the same ... the surface area of 2 cables is larger than that of 1 conductor of twice the size thus could omit the grouping factor this would be in agreement of E54's posts but up until this point was made it was hard to understand the reasoning but yep E54 can sit down with his cuppa and slightly smug smile il possibly re-think this area of the regs and may explain why it dosn't actually mention grouping factor directly in association of a multi cable parallel supply ..... but hey all ive wasted is a few half charlies spacing the swa's before overfill so if im gonna be wrong id rather it not effect the integrity of my previous work lol....

Browsing the tinterweb though i get conflicting results so assume some countries base ccc calculations different to ours and thus do apply this grouping factor for parallel cables of 1 circuit as their initial ccc of cables may be higher but subject to more derating.
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Im always open to debate and just as you posted this i was thinking the same ... the surface area of 2 cables is larger than that of 1 conductor of twice the size thus could omit the grouping factor this would be in agreement of E54's posts but up until this point was made it was hard to understand the reasoning but yep E54 can sit down with his cuppa and slightly smug smile il possibly re-think this area of the regs and may explain why it dosn't actually mention grouping factor directly in association of a multi cable parallel supply ..... but hey all ive wasted is a few half charlies spacing the swa's before overfill so if im gonna be wrong id rather it not effect the integrity of my previous work lol....

Browsing the tinterweb though i get conflicting results so assume some countries base ccc calculations different to ours and thus do apply this grouping factor for parallel cables of 1 circuit as their initial ccc of cables may be higher but subject to more derating.
Work it out, 1 cable 300mm and 2 150mm, work out the mW/m, the larger cable will emit more heat.

When enclosed or grouped, the radiated heat loss is dramatically reduced, this is why we derate.

Practical tests were used with upto 37 grouped cables, if all were parallel, how would you expect the cables in the middle to perform?

There is also the theoretical models, all of which the table within BS 7671 is based upon.

I sure the NEC use the same method too, they just calculate In differently.
 
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Engineer54

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  • #77
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Im always open to debate and just as you posted this i was thinking the same ... the surface area of 2 cables is larger than that of 1 conductor of twice the size thus could omit the grouping factor this would be in agreement of E54's posts but up until this point was made it was hard to understand the reasoning but yep E54 can sit down with his cuppa and slightly smug smile il possibly re-think this area of the regs and may explain why it dosn't actually mention grouping factor directly in association of a multi cable parallel supply ..... but hey all ive wasted is a few half charlies spacing the swa's before overfill so if im gonna be wrong id rather it not effect the integrity of my previous work lol....

Browsing the tinterweb though i get conflicting results so assume some countries base ccc calculations different to ours and thus do apply this grouping factor for parallel cables of 1 circuit as their initial ccc of cables may be higher but subject to more derating.
I'm not being in anyway smug here, It's what and how i was taught, and what i've been applying for too many years to be told any differently now!! I've also never once seen a calculation placed in front of me, by any contractors Engineer, for approving parallel cable sizes, that have included a grouping factor. I work with quite a few different national standards, and too my knowledge, none of them apply or require a grouping factor for paralleled single circuits either!!

You haven't wasted anything, if your talking about spacing cables in a trench, I do the same. But my reasoning is for any future ground movement. I never run cables in trenches straight either, they are always laid with slight snaking along it's length for the same reason and to allow for any seasonal expansion/contraction that may occur etc....
 

Rockingit

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Work it out, 1 cable 300mm and 2 150mm, work out the mW/m, the larger cable will emit more heat.

When enclosed or grouped, the radiated heat loss is dramatically reduced, this is why we derate.

Practical tests were used with upto 37 grouped cables, if all were parallel, how would you expect the cables in the middle to perform?

There is also the theoretical models, all of which the table within BS 7671 is based upon.

I sure the NEC use the same method too, they just calculate In differently.
The larger won't emit MORE heat, it will emit the same amount of heat energy dissipated over a smaller surface area than the two parallel conductors which have a greater surface area - this is basic radiator principle. If N amount of electrical energy passing down R resistance of copper will produce J amount of heat, J becomes a fixed form. So all we are left with is simply J/circumference, of which 2 x ccc's are roughly 1/3 larger than a single one double the size. Do the maths if you don't believe me!
 

darkwood

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I'm not being in anyway smug here, It's what and how i was taught, and what i've been applying for too many years to be told any differently now!! I've also never once seen a calculation placed in front of me, by any contractors Engineer, for approving parallel cable sizes, that have included a grouping factor. I work with quite a few different national standards, and too my knowledge, none of them apply or require a grouping factor for paralleled single circuits either!!

You haven't wasted anything, if your talking about spacing cables in a trench, I do the same. But my reasoning is for any future ground movement. I never run cables in trenches straight either, they are always laid with slight snaking along it's length for the same reason and to allow for any seasonal expansion/contraction that may occur etc....

The smug was just humour and im not worthy ;) ....:mickey:
 
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Knobhead

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  • #80
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Like E54 I've run countless parallel feeds, never applied a de-rating to them. As has been said it would make the whole exercise pointless.
 
I'm with e54, why would you derate a parallel cable, it seems pointless, they are both of the same origins and circuit destination so why would you not treat them as 1 cable.?? Or am I missing something?
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

OK guys, it looks like I'm the only one here that applies a grouping factor to cables that are run in parallel (that are touching, Table ), when they feed a single source. So please can someone explain why they don't. As I posted in #40

(example 1:same source supply) We have 2 cables (touching, in ducting) each drawing 100A (the source requires 200A) The cables are of the same size and are both 30 meters long. You are saying you would not de-rate these cables as they are supplying a single sourse

(example 2: 2 separate sources) We have 2 cables (touching, in ducting) each drawing 100A (each source requires 100A) The cables are of the same size and are both 30 meters long. You are saying you would de-rate these cables.

You de-rate the cables due to heat disipation. Example 1 cables are producing the same heat as example 2 cables. In layman's terms, so a thicko like me can understand, why de-rate example 2 but not example 1? What is the logic in that... They are the same..
 
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Engineer54

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  • #83
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I think Rockingit has given you a more than good enough explanation!!

Again i stress, that were talking here about a single parallel circuit, not a situation where there are two or several parallel single circuits grouped/run in a containment. In this case you ''Would'' apply a grouping factor for the multiple ''circuits'', as you would normally, for running multiple ''circuits'' in a given containment.... But notice that i stated circuits and not conductors/cables!! lol!!

On another note, ''All'' these de-rating tables (correction factors) are based on the worst case scenario, which can be quite misleading to say the least, for those that lack experience. Blindly using these tables verbatim, can and will lead to grossly over sized cables if they aren't anywhere near worst case in the first place... The times i've seen a cable on paper, that has gone through a number of correction factors, without any thought being applied, ending up too large to be physically possible to connect!! lol!!
 
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

The larger won't emit MORE heat, it will emit the same amount of heat energy dissipated over a smaller surface area than the two parallel conductors which have a greater surface area - this is basic radiator principle. If N amount of electrical energy passing down R resistance of copper will produce J amount of heat, J becomes a fixed form. So all we are left with is simply J/circumference, of which 2 x ccc's are roughly 1/3 larger than a single one double the size. Do the maths if you don't believe me!
I have done the Maths lol, if think you need to!!


Example Table 4E1A

Ib = 700 amp, 2 Phase, Cables touching Ref Method C

2 x 150mm CSA

1 x 300 mm CSA

Your argument is missing a few key point's

2 x 150 = ccc = 952 amps
1 x 300 = ccc = 743 amps

Power dissiapted = I2R
 
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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

OK guys, it looks like I'm the only one here that applies a grouping factor to cables that are run in parallel (that are touching, Table ), when they feed a single source. So please can someone explain why they don't. As I posted in #40

(example 1:same source supply) We have 2 cables (touching, in ducting) each drawing 100A (the source requires 200A) The cables are of the same size and are both 30 meters long. You are saying you would not de-rate these cables as they are supplying a single sourse

(example 2: 2 separate sources) We have 2 cables (touching, in ducting) each drawing 100A (each source requires 100A) The cables are of the same size and are both 30 meters long. You are saying you would de-rate these cables.

You de-rate the cables due to heat disipation. Example 1 cables are producing the same heat as example 2 cables. In layman's terms, so a thicko like me can understand, why de-rate example 2 but not example 1? What is the logic in that... They are the same..
No your not on your own, i derate to!!
 

Spoon

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Eng54. I understand that we are talking about a single parallel circuit. If you are also refering to Rockingit post #75 then he deviates from my examples of the cables being of the same length and carrying the same current. It does not answer my question in post 83.

Chris. Regarding your post #85: My argument is about applying groupling factors to parallel cables that are touching. Not what your maths are about.. Thanks for your post #86
 
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Engineer54

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  • #87
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I have done the Maths lol, if think you need to!!


Example Table 4E1A

Ib = 700 amp, 2 Phase, Cables touching Ref Method C

2 x 150mm CSA

1 x 300 mm CSA

Your argument is missing a few key point's

2 x 150 = ccc = 952 amps
1 x 300 = ccc = 743 amps

Power dissiapted = I2R

Exactly my point about using worst case correction factors!! Your actually reducing any heat, by using two smaller CSA cables that ''CAN'' carry considerably more current, but actually isn't!! Who in their right mind would install a circuit that is going to be on it's limits?? lol!!

Do you apply a grouping correction factor to a circuit run in trefoil too?? Adds up to exactly the same thing after all!!
 

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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Deviating from me learning here :)
In my examples (post 83) all the cables are carrying the same current, are of the same length, same type of armoured cable.... therefore producing the same heat.
Please explain why you only apply de-rating factors for example 2.

If you use the argument ' ''All'' these de-rating tables (correction factors) are based on the worst case scenario' and 'Who in their right mind would install a circuit that is going to be on it's limits' then you wouldn't apply the correction factors for example 1 and 2. (This is not meant to come across insulting in any way or form)
 
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Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Exactly my point about using worst case correction factors!! Your actually reducing any heat, by using two smaller CSA cables that ''CAN'' carry considerably more current, but actually isn't!! Who in their right mind would install a circuit that is going to be on it's limits?? lol!!

Do you apply a grouping correction factor to a circuit run in trefoil too?? Adds up to exactly the same thing after all!!
Yes in open air the conductor operating temp will be lower in the parallel, but when we enclose and group the cables this greatly effects the parallels cable to effectively radiate that heat, thus causing the opersti.g temp to dramatically rise, so we have to derate.
 
S

Silly Sausage

  • Thread Starter Thread Starter
  • #90
Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I have done the Maths lol, if think you need to!!


Example Table 4E1A

Ib = 700 amp, 2 Phase, Cables touching Ref Method C

2 x 150mm CSA

1 x 300 mm CSA

Your argument is missing a few key point's

2 x 150 = ccc = 952 amps
1 x 300 = ccc = 743 amps

Power dissiapted = I2R
Great discussion!

Even if you applied worst case group factor from Table 4C1 of 80%, the 2 cables above would be 762A.


Exactly my point about using worst case correction factors!! Your actually reducing any heat, by using two smaller CSA cables that ''CAN'' carry considerably more current, but actually isn't!! Who in their right mind would install a circuit that is going to be on it's limits?? lol!!

Do you apply a grouping correction factor to a circuit run in trefoil too?? Adds up to exactly the same thing after all!!
So, your effectively derating the cable yourself.

Re the Trefoil, single cores, Note 5 of 4C1, if a group consists of n single core cables it may be considered as n/2 circuits of 2 loaded conductors or n/3 circuits of 3 loaded conductors. ie 1 circuit.




I think I'm starting to get it?!?!?!?
 
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