Cable size

[yesitsashock]

you can wing the 3% for lighting if you fit all LED lights. this could allow a 25mm SWA for > 40A.

LEDs all the way now lol, wouldnt fit anything else. Prob what hes gonna end up with.

 

[MJPD29]

If your going with a 20a supply don't bother with a ring. No point fitting a 32a mcb downstream from a 20.

 

[pc1966]

Is there any other factors i need to take into consideration for that run of cable?

If the supply is TN-C-S and you don't TT the remote building then you need 10mm copper equivalent for the CPC.

For 2-core SWA that only happens by time you get to 70mm, expensive!

If smaller SWA (but above 10mm) it can be cheaper to run a 10mm coper CPC in parallel with the SWA.

 

[timhoward]

If the supply is TN-C-S and you don't TT the remote building then you need 10mm copper equivalent for the CPC.

I must admit this sentence has perplexed me, and I'm probably missing something.
If we were talking about the main earthing conductor for the installation at the house I'd get it.
But we are talking about a circuits CPC here aren't we, so doesn't it just have to meet 543.1.3, and be equal to line conductor if less than 16mm sq?

 

[telectrix]

If the supply is TN-C-S and you don't TT the remote building then you need 10mm copper equivalent for the CPC.

For 2-core SWA that only happens by time you get to 70mm, expensive!

If smaller SWA (but above 10mm) it can be cheaper to run a 10mm coper CPC in parallel with the SWA.

but only if there are extraneous parts in the garage. ?

 

[telectrix]

I must admit this sentence has perplexed me, and I'm probably missing something.
If we were talking about the main earthing conductor for the installation at the house I'd get it.
But we are talking about a circuits CPC here aren't we, so doesn't it just have to meet 543.1.3, and be equal to line conductor if less than 16mm sq?

think he means as a bonding conductor. see post #15.

 

[timhoward]

think he means as a bonding conductor. see post #15.

Aha thank you.

 

[yesitsashock]

If the supply is TN-C-S and you don't TT the remote building then you need 10mm copper equivalent for the CPC.

For 2-core SWA that only happens by time you get to 70mm, expensive!

If smaller SWA (but above 10mm) it can be cheaper to run a 10mm coper CPC in parallel with the SWA.

Its a TT system at the house and will be made TT at the shed, Im not using the armour as a cpc for the shed, only for the sub-main cable which will be protected by a 100ma s-type RCD. I will install a spike at the shed.

 

[pc1966]

but only if there are extraneous parts in the garage. ?

The OP described it as "16mx8m metal shed" so I assumed the building itself would be bonded by the supply CPC and is an extraneous conductive part in itself.

 

[pc1966]

Its a TT system at the house and will be made TT at the shed, Im not using the armour as a cpc for the shed, only for the sub-main cable which will be protected by a 100ma s-type RCD. I will install a spike at the shed.

Is there any chance the house will become PME (TN-C-S) at any time? If not, then I would use the armour to link both as well as an earth rod. Generally the lower the Ra the better and multiple earths allow less chance of a single point of failure.

Will the sub-main be off the supply (i.e. split tails and the 100mA delay-RCD as an isolator) or is it coming off the house CU that already has a 100mA delay RCD up-front? Fine in both cases, but ideally all final circuits at the out building on 30mA RCD/RCBO so you don't lose the lot on a fault.

If it is planned as a workshop you might also want to consider some LED lighting with built-in emergency battery, that way you don't end up plunged in to darkness with tools spinning, etc, if the light supply goes off for any reason (including supply interruption).