Smallholding supply

[MrF]

Hi guys i hope you can help. i own a smallholding and have had EDF install a 20kVA SP&N supply to a kiosk in my field and they have fitted a 100A fuse before my meter and i have a 100A switch fuse after the meter. Now my question is this, what size armoured cable (burried underground) would i need to run 100m to an outbuilding to give me the same 100A supply at the far end. I assume its going to be quite big but the cable coming down the pole seems quite small. And do i need a two or three core cable if i have an earth rod at the far end in my outbuilding.

thanks
karl

 

[plugwash]

the cable down the pole can be quite small because it presumablly only runs a short distance to a transformer so volt drop is not such an issue.

I don't have exact figures to hand for such large cable sizes but from some rough calculations I think 70mm.

edit: oops forgot to finish post

 

[RF Lighting]

You're looking at a 70.0mm² by my calculations to allow you a full 100A supply

The suppliers cable will look small due to the small distace of the run, and it is probably concentric cable too.

Why are you planning on installing an earth rod at the outbuilding?

What type of supply have the DNO given you?

 

[JohnD]

maybe you could move the outbuilding?

 

[ricicle]

What are you putting in this kiosk that will need 100A?

 

[davy_owen_88]

And why didn't you just get the supply installed where you needed it in the first place?

 

[MrF]

Thanks for the info so far. To answer the questions. I have a TT supply. The smallholding is out in the stix and that is what EDF said it will be. So i assume run 2 core swa from the pole kiosk to my barn and have a earth rod there just to keep things simple. They have dropped the supply down the nearest pole and they would not go any further than 40m without a new transformer (big money) and the pole is 100m from where i wanted my supply. It is also cheaper fo me to run 100m of cable than to get EDF to run 40m so we built the kiosk at the bottom of the pole and paid for a single pole drop. As for the power requirements i just thought if i have 100A at one end why not have it at the other. Future proofing and all that.

Karl.

 

[davy_owen_88]

As for the power requirements i just thought if i have 100A at one end why not have it at the other. Future proofing and all that.

Because 100m of 70mm² 2-core SWA isn't going to be cheap and if you are never going to need 100a you are just p1ssing away money.

Just to give you an idea - 100m of 16mm² 2-core SWA is going to cost about £300. And 100m of 70mm² 2-core SWA is going to weight about 245kg or about 1/4 of a tonne.

 

[JohnD]

use the money you save to run a cable duct with a bit of string in it so you can pull a bigger one if you ever need it.

 

[Steve]

They have dropped the supply down the nearest pole and they would not go any further than 40m without a new transformer (big money)

This means the place they dropped it is right at the limit for volt drop.

It is also cheaper fo me to run 100m of cable than to get EDF to run 40m

Cheaper based on what? 100 metres of 2.5mm twin and earth?

 

[Aardvark Avo]

your expense for 70mm is
1)your de rating the cable because it's buried-so it doesn't overheat
2)voltage drop from distance

so you could always think about 63amp supply which drops cable size-but really you need to work out demand and diversity.

Also be aware that the regs state that earth not to be exported-this means tt system one earth stake per building

 

[RF Lighting]

There is nothing wrong with exporting an earth as long as the building it is exported to remains within the equipotential zone.


TT systems are a very poor earthing system. For what reason would you make a perfectly good TN-C-S system into a TT when a proper earth system already exists?