Shed Supply SWA

[Jase_82]

I am running a supply to a new shed that is already pre-wired with it's own CU which has a 40A main switch 30mA RCD protecting a B16 Ring and a B6 lighting circuit. It's 40m away and in the house CU there is space for an MCB, but nothing has RCD protection here.

1. Am I right in thinking that I can use a 32A MCB in the house and run SWA (earthed) direct to the shed CU without needing RCD protection for the SWA cable?

2.Haven't purchased the cable yet, but Im proposing its 6mm 3-core SWA to cover the volt-drop. Could I yet away with 4mm 3-core SWA? The shed will be used for light domestic DIY, the only fixed loads being a couple of lights, a TV and a pond pump.

Any advice would be appreciated, thanks. Jason

 

[ackbarthestar]

There is nothing in the regs that state that the sub-main cable requires RCD protection, but then you won't be burrying T&E ?
As long as the the conductors are mechanically protected by the use of armour and depth, the SWA is bonded at some point (preferably at the MET) then you would only need to RCD the final circuits in the shed.

 

[timo1]

Hello mate - 1. Yes the cable is mechanically protected so no RCD required, obviously make sure it is well buried etc... 2. For the extra price I would stick with 6mm given the longish run and it will future proof it in case there are any larger loads in the garage at a later date.

 

[Jase_82]

So using the 6mm as an earth going back to the CU in the house will be ok? Does this mean the ZS at the shed end of the SWA needs to be below1.44 if theres no RCD in the house?

 

[ackbarthestar]

So using the 6mm as an earth going back to the CU in the house will be ok? Does this mean the ZS at the shed end of the SWA needs to be below1.44 if theres no RCD in the house?

Its standard / safe practice to ensure that the correct sized MCB for the sub-main cable will disconnect within 5 seconds. It is unlikely that a resulting S/C will last any longer than >0.1s . So you are looking at 32A-B with a maximum of 1.44 * 0.8 = 1.14 Ohms.

 

[Guest123]

1. Am I right in thinking that I can use a 32A MCB in the house and run SWA (earthed) direct to the shed CU without needing RCD protection for the SWA cable?

Providing the house isn't part of a TT earthing system, yes.