Power to 4 sheds

Hi all, I am new to the forum, just looking to get some advice on a job I am doing. I am of a electronics background so forgive me if I get confused regarding anything above 24v! I have a decent knowledge of electrics although I am not up to date with the regs in anyway hence my visit to this knowledge bank!

I am looking to put power and comms to four sheds. I have had a local sparky to assess the situation although he was only around for 10 minutes so didn't give me the full story, as I just need to know what cables to lay as I have the next week off. I am doing the donkey work he is making the final connections testing and signing it off.

The proposal is to add a RCD switched fuse after the switched fuse I already have (meter is >3m away from CU) and for me to install 3core 6mm SWA to an RCD disboard is the main shed 25m away. From this disboard he recommends that I just install 2.5mm SWA to each of my sheds which 3 are 15m away and the 4th 30m away.

2 of the heds 15m away will only have small loads; light and socket for the odd power tool, the other one at 15m and the one at 30m I may be making it into a potential office and/or playroom in the future. All will have a small disboard for lights and sockets all rcd protected.

Does this seem reasonable to you?

I am a bit confused regarding the suggestion that 2.5mm seem correct for the radials? As I would of assumed 4mm was the norm for radials.
I am not sure I gave him the full story so I am just making sure everything knits together before purchasing the cable and burying it 500mm down with additional conduit for the comms.

Any thought are very much appreciated!

Avi46 said:

The proposal is to add a RCD switched fuse after the switched fuse I already have (meter is >3m away from CU) and for me to install 3core 6mm SWA to an RCD disboard is the main shed 25m away. From this disboard he recommends that I just install 2.5mm SWA to each of my sheds which 3 are 15m away and the 4th 30m away.

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I am at a loss to understand why you are asking this question here - the electrician has given you the instructions - he/she has assessed the situation on the ground - given you a quote - so carry on.
On the other hand if you do not believe the electrician can be trusted then get another in who can be trusted or at least obtain three quotes.

Avi46 said:

I am a bit confused regarding the suggestion that 2.5mm seem correct for the radials? As I would of assumed 4mm was the norm for radials.
I am not sure I gave him the full story so I am just making sure everything knits together before purchasing the cable and burying it 500mm down with additional conduit for the comms.

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Perhaps you need to get back in touch with him, explain the full story and ask him to repeat his instructions to you - better still get him to write them down as part of the contract you he should have signed. After all he will be responsible for the completion of all the testing and necessary paperwork and contacting the Local Authority Building Control if required.

I have no issues with the electrician, he has done many jobs for me before, but he is on holiday for the next week, so I cant ask him anything else.

I may not, as you mention, have given him the full story, so was hoping someone may be able to assist with some technical knowhow, so I can go ahead with confidence that the information I may have omitted from my electrician will not affect the size of the cable I am about to buy and lay, knowing on his return when the full story is given I will not have to dig them up and replace with something more suitable for my actual requirements.

Avi46 said:

I have no issues with the electrician, he has done many jobs for me before, but he is on holiday for the next week, so I cant ask him anything else.

I may not, as you mention, have given him the full story, so was hoping someone may be able to assist with some technical knowhow, so I can go ahead with confidence that the information I may have omitted from my electrician will not affect the size of the cable I am about to buy and lay, knowing on his return when the full story is given I will not have to dig them up and replace with something more suitable for my actual requirements.

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Doesn't work like that I'm afraid - the electrician will have worked out the various factors to determine the new circuits characteristics so only he can clarify the instructions.
If as you say you haven't given him the full story then it is possible that the cable calculations he has given you are wrong - but only he will be able to determine that.
So you will have to wait until he comes back from his holiday I'm afraid.

No such thing as cables that are two big (with in reason) go for 10mm feeder and 4/6mm to each building if you want wont cost much more

Thanks for the advice. I understand there is no hard and fast answer, and their isn't anything wrong with the theory behind his original thoughts. he does know there are comms ducting going in and he is well aware of what I would probably do if one shed was an office, so if there is no major regs issues issue with his proposal, I will carry on but may go with a 10mm feeder just in case!

cheers!

10.0mm² main feed, and then 6.0mm² to each of the sheds would be my plan. Maybe a little OTT, but it's not going to hurt, and gives you a bit of scope for the future too.

It won't cost much more to up the cable size, and will save a whole world of hassle in the future had the original cable not been big enough.


In your original plan, you seem to have gone a bit RCD crazy.

SWA cable does not need RCD protection.

I would put a HBC cartridge fuse at the incommer to supply the new mains board in the first out building.

From here an MCB supply to each of the smaller buildings, which would be fitted with an RCD socket, and a switched FCU to supply the light assuming none of the cabling will be flush mounted.

They are the lines I am thinking, although I will have to look at the associated costs and go from there.

Yeah RCD wise that was probably a misunderstanding in my behalf of what he mentioned to me. I will speak to him again and double checks proposed loads etc. to ensure everything is understood by both parties. But at least I can see the wood from the trees a little more now.

many Thanks to all

I find it odd he suggests putting an RCD switched fuse after another switch fuse: what about discrimination? Why not just an RCD?

Also, if (potentially) your greatest load is going to be the furthest away (55m), I don't think 2.5 is enough. Indeed, 6 might not be enough. Has he done the calcs?

All the sheds have got RCD protection: on top of the RCD switched fuse? Discrimination again.

Think your spark's reasoning does not add up.

Wouldn't use conduit for comms. I'd use 1½" waste pipe minimum.

There are some details missing, too. What is the size of the CPD protecting the 6 milli from the supply?

What is the 'main shed' and why is it so different/special to all the others?

To retain discrimination, and as you are a little unsure how your installation will change in the future, I would daisy chain 10mm² 3-core from the fuse switch in the house to each shed in turn and use a junction box like

View media item 65810 Wiska Combi 1010.

Then you can have whatever protection you desire at each shed for whatever load you're going to put in them. There's no point creating an unnecessary discrimination problem for yourself in the future by star feeding from what is the 'main' shed right now.

securespark said:

I find it odd he suggests putting an RCD switched fuse after another switch fuse: what about discrimination? Why not just an RCD?

Also, if (potentially) your greatest load is going to be the furthest away (55m), I don't think 2.5 is enough. Indeed, 6 might not be enough. Has he done the calcs?

Think your spark's reasoning does not add up.

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Do you really think there is a fully qualified (on holiday) electrician involved in this debacle?

securespark said:

Think your spark's reasoning does not add up.

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This was a bit tongue-in-cheek!

riveralt said:

Do you really think there is a fully qualified (on holiday) electrician involved in this debacle?

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I suppose it's at least partially because some OPs have familiarised themselves with the sort of reponses which may otherwise be expected in this forum that we hear so much about these on-holiday electricians.

Kind Regards, John

What type of earthing arrangement is there at the incoming to the house ?

If you want to work on the project before the electrician is back in touch then you could dig the trenches and lay in the ducts before he gets back.

Then he can decide what size cable to buy and they can be pulled through

Obviously the ends need to be be where the equipment will be located and no sharp bends anywhere.

Two ducts, one for power, one for comms etc.

Almost certainly the electrician can buy suitable cable cheaper than you can, you could get some cheaper stuff but the quality may not be to minimum standards.