Electrical supplies over long distance

[067256]

Hello there.
I have been asked to look at a new supply, 240 volt to a stable block. Problem being is distance likely to be between 150-175 metres. Volt drop gonna be a problem. DNO quoted £30k to run new cable from sub. The stable block includes a kitchen, toilets, office so they were looking at a 80/100 amp supply! This is a bit out of my comfort zone so some advice would be helpful. The customer, to cut down costs will dig trench and lay cable themselves.
Many thanks.

 

[Brightspark2]

You just need to do your calcs and select the correct cable for the volt drop. You need to work out the design current it wont be 80/100amp for that you would be looking at 70 or 95mm and a few thousand pounds. Get the design current then you can work out the volt drop. The cable wont be cheap but that's how much it's going to cost them. It will be cheaper than 30k though!

Use SWA buried with warning tape above it. Deep enough as to prevent danger. Is it a farm?

Sent from my Xperia S using next doors WIFI

 

[NEZ2490]

Be careful to check the system as well as its out in the sticks it may well be a TT in which case you'll need RCD protection.

 

[HandySparks]

Is the existing supply capable of taking the additional load?

 

[Engineer54]

Hello there.
I have been asked to look at a new supply, 240 volt to a stable block. Problem being is distance likely to be between 150-175 metres. Volt drop gonna be a problem. DNO quoted £30k to run new cable from sub. The stable block includes a kitchen, toilets, office so they were looking at a 80/100 amp supply! This is a bit out of my comfort zone so some advice would be helpful. The customer, to cut down costs will dig trench and lay cable themselves.
Many thanks.

After you have scared yourself silly, trying to find a single multi cored cable to suit your requirements, Think about the use of a smaller sized parallel cabled sub-main. You may well be pleasantly surprised!!! lol!! ...Just make sure, that whatever way you decide to supply this stable block ensure that the cable trench floor is free of sharp stones and debris etc, and lined with a suitable depth of soft sand on which to lay your cable(s). And it should be the cables covering medium also. Don't forget to conduct a sheath test on the cable(s) prior to the final backfill layer....

 

[Knobhead]

If the customers laying the cable(s) I think you can forget correct back fill and a sheath test.

 

[JD6400]

After you have scared yourself silly, trying to find a single multi cored cable to suit your requirements, Think about the use of a smaller sized parallel cabled sub-main. You may well be pleasantly surprised!!! lol!! ...Just make sure, that whatever way you decide to supply this stable block ensure that the cable trench floor is free of sharp stones and debris etc, and lined with a suitable depth of soft sand on which to lay your cable(s). And it should be the cables covering medium also. Don't forget to conduct a sheath test on the cable(s) prior to the final backfill layer....

As Engineer54 Say's , parallel is the way forward ! We have just laid 400m of 70mm 4c to a small barn conversion come holiday home on Wednesday , instead of a substantially larger 2 core .

Also he mentions the importance of the bedding and back fill material , I can unequivocally support that statement due to a length of 340m 95mm 4c we are about to replace next week under warranty !

Some may remember me mentioning a issue with this sub a few months ago and the need for some fault location equipment , well let's just say multiple fault's would not even cover the first couple of meters let alone the rest of the run !!!!!!!

Just to note though ! We did not lay it in the first place and did not even supply it , but as most things in life " it's complicated ! "

 

[Knobhead]

As Engineer54 Say's , parallel is the way forward ! We have just laid 400m of 70mm 4c to a small barn conversion come holiday home on Wednesday , instead of a substantially larger 2 core .

Also he mentions the importance of the bedding and back fill material , I can unequivocally support that statement due to a length of 340m 95mm 4c we are about to replace next week under warranty !

Some may remember me mentioning a issue with this sub a few months ago and the need for some fault location equipment , well let's just say multiple fault's would not even cover the first couple of meters let alone the rest of the run !!!!!!!

Just to note though ! We did not lay it in the first place and did not even supply it , but as most things in life " it's complicated ! "

Remember the thread. You’re having to replace under warranty when the customer installed the cable? That’s a bit harsh!

 

[JD6400]

Not the half of it Tony !

I do not want to go into it on a open forum so may pm you with a explanation latter if i have time to type it ( exceptionally slow typist !) , but we have essentially taken the hit as a matter of honour !

 

[Knobhead]

Please do.

 

[JD6400]

Will do but it ain't going to be fast !:computer:

 

[Knobhead]

067256 take note of what DP has said.

 

[Rockingit]

Parallel subs: If, for arguments sake and just for explanation, the calcs say you've got a 100m run of 3/100mm to lay, so instead you lay 2 x 3/50... is this the same as laying a 1 x 6/50 (were it to exist) and double up or even a 1 x 12/25 and quading the cores together?? Or do the 'bunching' rules start to affect you? I have no idea and have never thought about it before.

 

[johnboy6083]

ther cables do not split the current in the same way. So 2x 25mm will carry approx 200A, however 1x 50mm will not. Obviously volt drop is not taken into acount here.

 

[Rockingit]

ther cables do not split the current in the same way. So 2x 25mm will carry approx 200A, however 1x 50mm will not. Obviously volt drop is not taken into acount here.

Just looked it up in the tables - roughly 30% reduction between 2 x 25 and 1 x 50. At the risk of getting a ribbing if I've forgotten something basic - why?

 

[johnboy6083]

i think its to do with the skin effect. Ie in cables, the electricity only travels around the outside of the cable. This is why on HV lines, they can get away with having the centre of the cable as steel. (im talking about ACSR, Aluminium CAble, steel reinforced)
If anybiody has a better otr more informed answer, im all for it lol

 

[kingeri]

This is an interesting topic. Parallel conductors are something that folk often overlook, yet may prove to be a much more economical solution. Well, just look at the ring final - this is essentially a play on conductors in parallel, and we all know why they were introduced. I want to hear more from E54 on this.......

 

[HandySparks]

Just looked it up in the tables - roughly 30% reduction between 2 x 25 and 1 x 50. At the risk of getting a ribbing if I've forgotten something basic - why?

Isn't it because, for a given total csa, if you split the current between more than one conductor, the surface area is greater. This increases the ability of the cable to dissipate heat into the surrounding medium. Therefore it can pass more current for a given conductor and insulation temperature, which is the limiting factor for CCC.

 

[vjsmarwick]

I wouldn't bothering would just get the power supplier to give you another supply in the new building cheaper I would guess and safer

 

[Rockingit]

I wouldn't bothering would just get the power supplier to give you another supply in the new building cheaper I would guess and safer

I would bother to learn some maths, mate:

25mm = r 2.8209, C 17.725
50mm = r 3.989, C 25.066

Therefore 2 x 25mm = C of 35.45 which is roughly our 30% difference on capacity. Job Sorted, and money in the sparkies pocket rather than the DNO's.

Cheers chaps, every day's a school day.