Cable size

[yesitsashock]

Hi everyone,

Ive been asked by the customer to supply a 16mx8m metal shed, it will be used as for his hobby of working at old cars it will have 5 circuits,
supply to another out building 50m away with a couple of sockets and 5 lights, prob 20amp be enough
a ring main
lighting circuit 6-8 led lights
roller door
car lift with 3hp motor.
The problem is the shed is 115m away from the house, hes asked me to price for a 63amp and 40amp supply. From my cable calcs a 2-core 16mm swa will have 12.88 volt drop for the 40 amp supply and 20v for the 63amp supply(too much). The calcs for the 25mm swa is 12.68v at 63 amps.
Ive worked out using k1/k2 X S that the armour is ok for the cpc. I will be fitting a 100ma S-type at the house end and driving a spike in at the shed end.
Is there any other factors i need to take into consideration for that run of cable?
Also only looking to use two cores to keep the cost down. Do for a pound what any fool can do for a fiver
Thanks in advance.

 

[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).