Voltage drop on swa

[Duncan Bennett]

Hi

I've been asked to provide a supply to a machine shop on a farm. The nearest supply is 170m away.

I'm working out the vd/a/m and I'm looking at having to install a 25mm 2 core SWA to compenate. But that is while working out the current carrying capacity of the cable at 152amps (Ref. E) or 25Kw.

However, the load isn't going to be anywhere near that: the main piece of equiptment is a mig welder at 10amps and some small hand tools. They don't even require lights.

Should I calculate the vd/a/m at the cable potential load, or can I use the expected load (closer to around 30amps) - I'm running three double sockets individually run on 2.5mm from separate 16amp breakers

Thanks for your time.

Duncan

 

[HappyHippyDad]

Hi Duncan,I always work out VD based on the max demand that the cable will be taking (not the CCC of the cable). Plus I think about what may potentially change/be added in the future, so add on a few amps when calculating VD. Please wait until you get some more input though as I am purely domestic and I wouldn't want to give you the wrong info. Good luck.

 

[Duncan Bennett]

Thanks HHDad :stooge_curly: I'm pretty much domestic myself and volts drop is never much of an issue - but 170 metres is a hell of a run!

 

[telectrix]

calculate the expected load. add 30%, then work out the VD from that.

 

[Duncan Bennett]

Cheers Telectrix. That'll bring the price down a bit :stooge_curly:

 

[UNG]

Hi

I've been asked to provide a supply to a machine shop on a farm. The nearest supply is 170m away.

I'm working out the vd/a/m and I'm looking at having to install a 25mm 2 core SWA to compenate. But that is while working out the current carrying capacity of the cable at 152amps (Ref. E) or 25Kw.

However, the load isn't going to be anywhere near that: the main piece of equiptment is a mig welder at 10amps and some small hand tools. They don't even require lights.

Should I calculate the vd/a/m at the cable potential load, or can I use the expected load (closer to around 30amps) - I'm running three double sockets individually run on 2.5mm from separate 16amp breakers

Thanks for your time.

Duncan

Cheers Telectrix. That'll bring the price down a bit :stooge_curly:

Don't see how it will assuming a load of 30 amp your still on 25mm² as your volt drop kills it

 

[Duncan Bennett]

Don't see how it will assuming a load of 30 amp your still on 25mm² as your volt drop kills it

Yes UNG. I spoke to soon, the difference is pretty marginal; assuming a supply of 230v and a load of 40a I still need to use 25mm to get below the acceptable drop of 11.5v.:sad3:

 

[telectrix]

your total load may be around 30A, but allowing for diversity ( not everything will be on at once ), your design load may well be down to around 15A. so 16mm will fly it.up to 20A. that's what i'd do.

 

[silverfox1]

Yes UNG. I spoke to soon, the difference is pretty marginal; assuming a supply of 230v and a load of 40a I still need to use 25mm to get below the acceptable drop of 11.5v.:sad3:

You may have an issue with disconnection times at 40A even using 25mm.

 

[Darkwood]

Once you calculated your load inc' diversity see what front end O/L protection will need to be to cover it then calculate volts drop to the rating of the device this will mean you have correct VD even if upstream O/L device if nearly fully loaded.

Is this 1ph or 3ph and will the machine shop require any bonding arrangements as this will be needed to be taken into account when cable sizing also the nature of the DNO supply can make a difference too.

 

[Duncan Bennett]

It's a 2 core single phase supply. I planned to protect the swa with a 40amp circuit breaker at the dno supply. The job only involves running three twin metalclad sockets in the work shop, which i will supply through three x 16amp cb's and a 60amp 30ma RCD main switch.

 

[Darkwood]

If your front end is 40amp then this should be your calculation figure unless you are supplying a fixed load. As you have a number of sockets the load can be variable and however unlikely it seems to get fully loaded you still have to consider the worst case scenario in your workings.

Don't forget you will have a Zs value to achieve and at such a run if you calculate using a estimated demand you will almost likely fail to meet the Zs requirements and at the design stage you should not be relying on the RCD to cover for this, you still haven't responded regarding any bonding requirements to structural or services and if the supply is TT, TNCS or TNS?

I would also assume lighting will be added at some point your wiring a sub-main to supply a work-shop it has a high chance lighting will be added regardless of what is said at this stage, so allow for this in your calcs...Part of your design is future proofing to a certain extent and with the length of that run you may want to be more cautious in your sizing and just allow that little extra.

 

[wozz]

Work out your Ib as thats what you should be using, no point in guessing till you have done that.

You also seem to be allowing your full volt drop allowance on your submain, anything left for the final circuit?

Id also be concerned lighting will go in a somepoint worth using that volt drop instead?

 

[Duncan Bennett]

Cheers for that, Darkwood. I hear what you're saying about adding lights; they haven't asked me to price for any, but, yes, once I'm out they could plug in a few 3kw heaters, a tea urn and an electric cooker! I have allowed for a 30% addition to my calcs as advised by Telectrix.

There are no services (gas /water/oil) in the workshop. It has steel supports which I will bond through the board. The dno supply is TNCS.

The R1+R2 value of 16mm 2 core SWA is 0.00495 ohms/M which gives 0.8415 @ 170M, and assuming that the max Ze for a TNC-S system should be 0.35 ohms, then the total Zs = 1.195 ohms.

I'm running 3 sockets. 2 of these are side by side with the board and the other is about 3m away, so R1 + R" on the actual circuits should be minimal. I'm running these outlets through separate 16a (B) breakers which allow a max impedance of 2.87 ohms so I should be within parameters i think. Also, bonding the structural steelwork should help.

 

[Darkwood]

Read up on your earthing especially extending your equipotential zone or creating a local TT set-up its crucial you understand the requirements and know what size earthing is required and whether a separate core would be big enough to act as the additional requirements for bonding.

Mess this up may mean your install would be dangerous in a DNO fault for instance, if your not sure where Im directing you here you may need to work along side someone with experience in this area and have a hands on learning curve.

 

[Duncan Bennett]

Is there something wrong with my calcs?

 

[Darkwood]

Is there something wrong with my calcs?

Not checked them myself what Im getting at is the requirements of extending the equipotential zone with regards to additional earthing that may be required depending on the cable size chosen the csa of the armouring and whether a separate core would suffice, as I don't have the incoming N size I could tell you what is needed and as I don't know your final choice of CSA of the SWA nor can I say whether it will require additional earthing to allow for the bonding of the steel... that is if you choose to extend the equip' zone or whether settle to TT at the shed end.

 

[Besoeker]

Hi

I've been asked to provide a supply to a machine shop on a farm. The nearest supply is 170m away.

I'm working out the vd/a/m and I'm looking at having to install a 25mm 2 core SWA to compenate. But that is while working out the current carrying capacity of the cable at 152amps (Ref. E) or 25Kw.

However, the load isn't going to be anywhere near that: the main piece of equiptment is a mig welder at 10amps and some small hand tools. They don't even require lights.

Should I calculate the vd/a/m at the cable potential load, or can I use the expected load (closer to around 30amps) - I'm running three double sockets individually run on 2.5mm from separate 16amp breakers

Thanks for your time.

Duncan

Circular logic there.
The conductor current rating and voltage drop are two different constraints.

 

[Duncan Bennett]

But, surely, if I can meet the required disconnection times then the install method is valid?

 

[Duncan Bennett]

@besoeker. The conductor current rating is not an issue at all; 1.5mm would handle the amperage, the sizing has only to do with the VD on the length of the run :-(

 

[shanky887614]

@besoeker. The conductor current rating is not an issue at all; 1.5mm would handle the amperage, the sizing has only to do with the VD on the length of the run :-(

and the bonding unless he TT's the workshop

 

[Besoeker]

@besoeker. The conductor current rating is not an issue at all; 1.5mm would handle the amperage, the sizing has only to do with the VD on the length of the run :-(

Believe it or not, I do know that......

 

[wozz]

Your calculated Zs does it have any tempreture correction?

 

[shanky887614]

Your calculated Zs does it have any tempreture correction?

i believe its 10oC you should be using for the calc in the ground (you will have to check but I think it was 10)

of the constant temperature of the ground all year round

for when you Bury the swa or you will need to add a bit more for the temperature changes throughout the year

 

[wozz]

No the temperature correction for loaded conductors when calculating Zs. Unless it was corrected before he put it into the equation.

 

[Duncan Bennett]

I didn't think this could get so complicated, lol.

I'm using table 4E4A on page 348 of 7671. 16mm 2 core swa. Reference method E (the cable is being run on the surface) - It shows CCC of 115amps.

Then calculating the VD from table 4E4B 16mm 2 core single phase at 2.9mV.

I'm running from a TNC-S supply and have calculated a loop impedance of 1.1915 ohms.

I'm running 3 x twin sockets close to the board through individual 16amp breakers for which Table 41.3 allows a max. 2.87 ohms to meet the disconnection times.

I've admitted - many times - on this board, that I am primarily a domestic installer. But i do come here to get educated when I'm not 100% sure of the odd job which falls my way. But, I am having a problem understanding why I can't export the equipotential earthing out to this cable. I do so when I run to garden huts etc. and while this particular building is 170m away, I can't see there being a problem so long as I can match disconnection times. As for a fault with the DNO supply, wouldn't this effect the farmhouse, milking shed and out-buildings which are all drawn from the same supply?

Apologies in advance if I seem dumb, but I can't get my head around it.

 

[Darkwood]

Firstly watch your terminology here we never discussed 'exporting' we were discussing extending the equipotential zone (2 very different things).

Secondly I never said you can't extend it either what I did say is if you choose this method to bond the structural steel work in the shed then it may influence the cable you choose and may mean you up the number of cores... this all depends on the size of the DNO supply, if you choose to isolate earthing at the load end and TT the shed then this is an alternative method that may reduce costs.

As info is limited I cannot say, we are not discussing the other shed etc and I can't confirm there compliance, its often the case that this set-up is wired up incorrectly due to lack of understanding but in the rare event of a N/E DNO loss the full current of the DNO TX may source alternative route to return to the TX - this could see circulating currents around your earthing system that are related to other businesses fed from the same TX.. this is why the bonding requirements of a TNCS are greater for larger supplies where TNS has a lower plateau for max size.

I understand your stepping into new areas out of your comfort zone but this is why such titles as 'domestic installer' that can be attained with short courses are strictly limited to staying within the domestic realm .... things get a whole lot more complex when DNO supplies increase in size and can influence your whole design just to ensure compliance.

You will gather by the nature of this Thread that not many picked up on this as been something you need to consider when designing your install, a slighty worrying thing TBH but yes what seems like a simple job with just a larger distance than your used to can easily get very complex and often these installs are done incorrectly by migrating domestic sparks widening there field, I'm not saying their is a need to redesign the install to allow for extending the equip' zone all I saying is you have to ensure the armour can accommodate the additional requirements to bond the steel which will be in addition to the required earthing size needed for the sub-main.

Depending on size of the DNO supply this could mean...

-No extra measures need to be taken and the Armour can accommodate both circuit and bonding needs.
-An extra core may be needed to allow for the bonding requirements.
-Due to the nature of a larger DNO supply the cable choice may still be too small to comply with additional core so a separate earth may need to be run.

In the worst case scenario where you have a large DNO supply to the complex your bonding requirements may be 50mm^2 which is additional to the requirements of the submain earthing. If you decide your 16mm satisfies VD etc then an extra 16mm core comes no where near the 50mm needed so an independent 50mm is run - Note! this is a worst case scenario!

Alternatively you can earth the Armouring as normal and isolate it at the shed end and TT the shed, not forgetting to take measures into the design that you would normally associate with a TT set-up.

Edit... just to add here the local DNO may require greater measures than that of the regulations due to their system layout and on occasion they may request even greater earthing arrangements for equip' bonding etc... but I've only come across this once where my installed complied to regs but they requested a 35mm additional earth to allow for their requirements.

 

[Duncan Bennett]

Darkwood, thanks for taking the time to deliver such a detailed answer. The very complexity of the arrangement, and your concerns over the size of the DNO supply (which I must admit I haven't even considered) informs me that I might well be out of my depth on this one. I think I'd probably be wise to hold my hands up and advise the customer to seek an industrial installer for this one. Again, though, thanks for taking the time to type out such a comprehensive answer - the very reason why I visit these boards in the first place. Cheers, Dunc.

 

[Darkwood]

Don't let my input shy you away, we all learn somewhere... you could just isolate supply earth at load end and TT the shed and forget any concerns about additional earthing, if your still not familiar with this concept then how about getting in a experienced guy and working along side him to pick up the jist of it all, he earns from it, you gain knowledge and widen your field of experience.

Only you can know if you are truly out off your depth but their was a time I couldn't wire a plug ... if I didn't take the path I did and stick to it I might have turned out a postman or worse a Plumber!!! :willy_nilly:

Remember there may not be any need for additional earthing here but I lack info so can't guide any further.

 

[telectrix]

I might have turned out a postman or worse a Plumber!!! :willy_nilly:

you would have failed the entrance exam for a plumber. IQ>25 = fail.

 

[Engineer54]

The armouring of a SWA cable is unsuitable for any form of bonding when the supply is TNC-S/PME.

Given that this cable run to a small workshop is 170m, then to be honest the only realistic route for earthing requirements is to TT this distant workshop building!!