Electrics for a shed

[jaff]

I know this has been covered countless times before on this forum and yes I have done a search but my question, I think, is a little more complex.

My CU is full, there is no space to fit any more MCB's
therefore, I wish to supply the shed using a fused spur from the kitchen ring (or possibly the heating circuit which is a gas combi boiler so should be very low load)

I will take the supply into the shed using 2.5mm^2 SWA.

At this point, I was planning on fitting a small CU with a 6A MCB for the lighting circuit and a 10A RCBO for the sockets. My reason for using a 10A RCBO is that it should trip before the MCB/RCD on the main CU in the flat protecting my freezer. The last thing I wanty is the electrics in my shed tripping at 9am and me coming in at 6pm to find 150quids worth of frozen food has defrosted.

Is this an acceptable way of connecting the power to the shed?



If not, is it possible to move the RCD in the CU so that instead of having 2 non protected and 3 protected, I have 3 non protected and 2 protected? that would be the ideal option as I could then transfer the heatingcircuit to a fused spur and use the MCB in the CU for the shed?

 

[electronicsuk]

At this point, I was planning on fitting a small CU with a 6A MCB for the lighting circuit and a 10A RCBO for the sockets. My reason for using a 10A RCBO is that it should trip before the MCB/RCD on the main CU in the flat protecting my freezer. The last thing I wanty is the electrics in my shed tripping at 9am and me coming in at 6pm to find 150quids worth of frozen food has defrosted.

If you're definitely going to supply the shed from a 13A FCU then there is little point installing a DB in the shed. It would be far more sensible to use a 3A switched FCU in the shed for the lights, and let the protection for the sockets be provided by the 13A fuse back in the house.

If the circuit you intend to spur from already has 30mA RCD protection, there is no point wasting money on an RCBO for the shed. There will be no discrimination between the two devices, so in the event of an earth fault, either RCD/RCBO or even both may well trip.

Is this an acceptable way of connecting the power to the shed?

It's not the best way of doing it, but it's certainly permissible if you follow BS7671 and notify the work to your LABC.

If not, is it possible to move the RCD in the CU so that instead of having 2 non protected and 3 protected, I have 3 non protected and 2 protected? that would be the ideal option as I could then transfer the heatingcircuit to a fused spur and use the MCB in the CU for the shed?

Perhaps, if it's a flexible split load. An alternative and perhaps easier option would be to split the incoming tails using Henley blocks, into a small DB with a suitable MCB to protect the shed cable. This would also mean that all RCD protection could then be located in the shed, without fear of the rest of the house losing power due to an earth fault.

Again, it's all notifiable.

 

[jaff]

that last option sounds good! although sounds like it could be expensive.

Yeah I realise it is all notifiable. The shed isnt even built yet so this is all just forward planning.

Once the hall is decorated there is no chance of running anything to or from the CU as everything has to be fun under the floor.

I should have an electrician coming on monday so I could enquire about the CU then.

Thanks for the help.

 

[ban-all-sheds]

My CU is full, there is no space to fit any more MCB's

You could get a larger CU.

therefore, I wish to supply the shed using a fused spur from the kitchen ring (or possibly the heating circuit which is a gas combi boiler so should be very low load)

You have a dedicated circuit just for a gas boiler?

I will take the supply into the shed using 2.5mm^2 SWA.

How far away is the shed?
Have you looked into the following:
- how the cable will be buried
- how you will gland it at each end
- whether you have the issue of TN-C-S export with extraneous-conductive-parts

At this point, I was planning on fitting a small CU with a 6A MCB for the lighting circuit and a 10A RCBO for the sockets. My reason for using a 10A RCBO is that it should trip before the MCB/RCD on the main CU in the flat protecting my freezer.

There is absolutely no guarantee of that, and unless you use a DP RCBO a neutral fault will definitely trip the house RCD as well even if the shed RCBO does fire first.

You're also not guaranteed discrimination on overcurrent operation of a 10A and 32A device. More guaranteed, but not absolute. Knowing your PFC at the shed would be useful to you when looking at discrimination tables.

Is this an acceptable way of connecting the power to the shed?

It doesn't seem unsafe, but it's not without inconvenience risks.

If not, is it possible to move the RCD in the CU so that instead of having 2 non protected and 3 protected, I have 3 non protected and 2 protected? that would be the ideal option as I could then transfer the heatingcircuit to a fused spur and use the MCB in the CU for the shed?

I can't work out how that would help, but to move RCDs around would probably need new bus-bars, and you can't remove RCD protection from circuits with buried cables.

Why not have the shed completely separate by splitting the tails and taking the supply to it from a switchfuse?

Some general points:

All of this is notifiable - what is your LABCs policy regarding DIY electrical work, and how do you plan to comply with P1?

How will you test your work?

 

[jaff]

> A bigger CU will cost me extra £££

> Yes I have a dedicated boiler circuit, no idea why.

> shed is around 1-2m from the flat.

> can you elaborate on the risks?

As I am not an electrician I probably wont do any of this work myself.

I just want to find the best way to do it rather than give an electrician free reign to do what he likes which will undoubtandly add un nesecary cost to the project.

 

[OwainDIYer]

It might be worth considering replacing the CU - you have 2 non-protected circuits, so presumably a "16th Edition" split CU and fairly recent wiring. As well as being considerably neater which miught be an issue if your CU is visible, you would also gain full RCD protection to all circuits and some spare ways for future expansion.

Putting the fridge/freezer and boiler on their own RCBOs prevents them being affected by faults elsewhere.

If you use power tools in the shedn then an emergency lighting unit can stop you plunging into darkness with power tools spinning,

 

[ban-all-sheds]

> A bigger CU will cost me extra £££

Cheap designs and installations are worth every penny.

> can you elaborate on the risks?

MCB or RCD trips in the house CU because of faults in the shed.

As I am not an electrician I probably wont do any of this work myself.

I will take the supply into the shed using 2.5mm^2 SWA.

At this point, I was planning on fitting a small CU with a 6A MCB for the lighting circuit and a 10A RCBO for the sockets. .... I could then transfer the heatingcircuit to a fused spur

I just want to find the best way to do it rather than give an electrician free reign to do what he likes which will undoubtandly add un nesecary cost to the project.

No it won't.

 

[ericmark]

The 10ma RCD is expensive and the cheap option seems a 10ma socket see here on ebay I did find which must be about cheapest option at £13 which means you will need of course to plug in for your supply. Using a box like this is an easy way to change from flex to SWA both can be glanded into the box and the FCU serves as a good junction box even though you don't need the fuse. Using a duel box in the shed

will allow both a socket and switched FCU to be mounted side by side and so a 3A fuse in the fuse connection unit will reduce power for lights and switch them.

Note some pictures are also links.