300M Sub Main

[Lillpete]

A contract farmer I do work for has asked me to price up running a sub main from a redundant main coming into an unused shed to a new building 300 metres away!!

I am away at the moment so don't have my tables to calculate voltage drop, it doesn't have to be a full 100a/80a supply so we can reduce it down a little to help with cable sizing but I'm guessing its still going to be hard to get within voltage drop without some serious and hard to work with cable sizes.

I need to get back to him before I get back so any advice would be greatly appreciated

 

[ProDave]

Step 1 is to work out the actual demand. Until you do that, nobody can work out the cable size.

At least it will only be 2 core SWA.

I would also want to measure the loop impedance of the existing supply. It might already be at the end of a very long cable.

 

[Lillpete]

All they actually require is a 32a socket circuit and lighting so 38a (8740A) but obviously this wouldn't allow for any fututre increases. 

Unfortunately I don't even know the earthing system let alone the the Ze/Ra  

 

[ProDave]

As it's a farm it must be TT earth.

Use a local TT earth at the new building.

So you can fuse / size your submain at 40A then.

You NEED to see the job before you price it.  you NEED to measure the distance, taking into account the actual routing of the cable,  no good assuming it's 300M because that's what he said, and ordering 300M of cable, only to find it's 310M

It could well be cheaper to get a price for the DNO to move the existing supply.

 

[apprentice87]

Forget it.... Cable size will be about 95mm....

john..

 

[Steve3948]

Why not pay someone to visit site and specify for you. If you get the job you factor their fee in and if you dont get the job, well thats business.

 

[SPECIAL LOCATION]

Why not pay someone to visit site and specify for you. If you get the job you factor their fee in and if you dont get the job, well thats business.

Because you can ask someone on a forum to do your work for you for FREE!!!!

p.s.

I've been a bit busy today fetching daughter back from Southampton Uni...

So if anyone can sort out a few bits of my admin & paperwork before Monday...

That would be great!!

:Blushing

:C

 

[apprentice87]

Like Prodave said, i would think it was much cheaper [and much better] to get the DNO to put in a new supply...

john

 

[Blue Duck]

As it's a farm it must be TT earth.

Could also be on TNS/PME?

 

[NozSpark]

Could also be on TNS/PME?

Farms are not allowed to be on a TNCS / PME supply, nothing wrong with TNS or TT though from what I can remember

 

[Andy™]

Could also be on TNS/PME?

some installations TNCS/PME cannot be used. the supply into the building can be TNCS, but it would need changed to TT

 

[NozSpark]

some installations TNCS/PME cannot be used. the supply into the building can be TNCS, but it would need changed to TT

Yes, it can be presented by the DNO as TNCS/PME but it shouldn't be used

 

[Blue Duck]

I don't work on farms ever I live and work in London where I'm yet to see a TT

I figured that not all farms are out in the sticks and therefore connected to a more local supply which in their wisdom the DNO may have thought to PME it.

 

[batty]

In the old days they wouldn't give you an earth on a farm due to the fact if neutral was lost a lot of cattle could be wiped out remember milking parlors are all metal and have a metal grid under them. As paddler says nowadays they don't seem to worry but at the end of the day its down to the contractor if he connects to it if an earth is given.

For a 32 amp supply at that distance you are looking at least 70mm will be an expensive job.

 

[Steve3948]

In the old days getting a new supply and meter may have been a cheaper option, however in today's world don't bank on it.

On one of our jobs the DNO have been asked to re-site the pole mounted transfomer back 2 poles, then run new supply cable to joint to existing, in all about 50M.

Budget figure provided of around 22k with all trenching by others.

Using an earth provided has been discussed on the IET forum, where providing all metalwork within the building is connected to the provided earth and where the structure if steel entered the ground then this would be true earth so should not be an issue with RCD protection throughout also.

 

[Lillpete]

Thanks for your help I'm home now and have calculated the cable required is 70mm which comes out at around £5k on its own for 300 metres so I have suggested they look into a new supply, we'll see how it all stacks up

 

[Andy™]

i get 95mm due to volt drop at 3% due to lighting

and dont forget that the volt drop after the submain also needs to be taken into account, you cant use it all up on the submain unless your final circuits end 500mm from the DB

 

[Sidewinder]

It still may be cheaper, for a whole of life costing to run a sub-main, remember, that a new supply has a "standing charge".

Depending upon the proposed life of the premises, the standing charge for this life and the cost of the install of the new supply, may, outweigh the cost of the sub-main.

 

[batty]

This is a farmer so probably will shelve the idea when he knows how much it will cost.

 

[Andy™]

they will just stick in some 6mm or whatever they acquire FOC from somewhere