What size cable would you use????

[Gringoking88]

Hi all,

Bit of a dilemma, hoping someone can help.

Im running a supply to a garden out building, from a 40 amp RCD protected type b mcb at the main board. The first 8 meters or so are in twin and earth and will be in plastic conduit, the second lot of cable is swa, clipped direct to an outside wall and about 20 m. The question is will 6mm cores through out be ok?

Depending how you work it, 6 or 10 are both acceptable?

Thanks

M

 

[catdog1121]

Why not just dispense with the twin and earth in plastic conduit and run it all in SWA, no need for additional protection then? Or is it a TT system and you are relying on the RCD for fault protection?

40Amp I would go with 10 if nothing else it means you dont have to worry so much about your Zs.

You do know it needs to be registered with building control so you need an electrician to install it right?

 

[Dustydazzler]

As above , I would be inclined to do away with the plastic conduit and do the entire run in swa thus removing the joint in the middle.

if you truly need a full 40amps in the outbuildings then 10mm would be more suitable, if you can get away with less load I would install 6mm on a 32a

 

[timhoward]

Avoid joints if possible. And if not possible keep them accessible.

 

[DefyG]

Depending how you work it, 6 or 10 are both acceptable?

If 6 is why consider 10 with the extra costs?

 

[Strima]

There is no recognised method for running T&E in conduit so by the book we cannot confirm the current carrying capacity of the cable.

 

[brianmoooore]

What's behind this aversion to running SWA inside a house?? Safer, avoids joints, make compliance with regs. easier, higher current capacity. Why wouldn't you?

 

[Rockingit]

What's behind this aversion to running SWA inside a house?? Safer, avoids joints, make compliance with regs. easier, higher current capacity. Why wouldn't you?

Wrangling is the normal reason in my experience plus also the difficulties in glanding back to the CU if it's not done at original first fix.

 

[old man sparky]

If the board has high integ available I would go for a separate rcbo. If you need to load up to a 40 amp mcb go for 10mm, Especially as ambient temps will differ considerably through the seasons.

 

[FatAlan]

As above but surely you should be basing this on the load you want to cater for in the out building and then undertake the appropriate cable calcs taking into account distance and volt drop? Is it just that you have a spare 40amp MCB?

 

[brianmoooore]

It would be better fed from a switch fuse mounted alongside the CU, and RCD protection fitted in the shed. This would give discrimination for the shed MCBs and local RCD protection.
Only acceptable if the SWA goes all the way.

 

[old man sparky]

As others have said, you need to calc your max demand. 40 amps in a shed is a bit ott. Unless your using high current equipment of course. As far as the installation is concerned there are a few options before you now, depends which is the most practical in your given situation.

 

[pc1966]

As the rest have said, determine first what you need powered, and then what that will demand in terms of current and reliability.

For example:

• If it is a workshop or similar you should be considering emergency lights in case it all goes dark with power tools still running (for any reason, local fault or regional power failure).
• If there will be lots of computers, etc, then you need to consider a fused-switch & SWA feed so no RCD there, and then have a few RCBOs to divide up the sockets without too much accumulation of leakage.
• If you plan on having a freezer then have it on a separate dedicated circuit from a garage CU and potentially some alarm if the supply goes off.

But if all you really need is a handful double of sockets and some LED lights, then a 20A-32A RCBO on 4mm or 6mm radial (length and thermal path dependant) and a FCU with 3A fuse for the lights is simple and effective.