Wiring for outbuildings or sheds

[rob0602uk]

Hi, I've been getting some conflicting guidance/quotes and want to check what people's thoughts are on this setup. I will be getting an electrician to do this work and sign off, but i'm hoping to reduce costs by running the cable myself (as it is a lot of digging!). I'd also like to have a solid plan so I can finish building the outbuildings.

The electrics run from consumer unit, approx 7m to the outside of the building, then follows the path outlined above. Each consumer unit in the outbuildings (ob1 and ob2) are roughly 1.5m off the ground so that distance needs to be added on. If you can't see diagram, there is approx. 15m of cable run between the main CU and OB1, then 29m to OB2. Cable run will be buried for the most part, apart from where it comes up to connect to the CUs.

There is (10mm?) SWA cable in place at the moment and a consumer unit with RCBO in OB2. I'm looking to add in OB1 which is a shed and will have a freezer and some basic lighting in, but might also be used for electrical appliances like the mower or a power tool from time to time - probably a chop saw. Was going to have a double plug outlet on each wall in here and 2 old school strip lights.

OB2 will be a garden room (was previously a shed) so it currently has lights and a freezer in there. In future it is likely to have a double outlet on each wall, plus at maximum a laptop, 49inch tv, 2kw electric radiator and led spotlights x 6. At the minute this has a sub consumer unit in it already which

The CU in the house is very new and was put in when we had building work, the circuit for the shed is separate on that CU, in that no other sockets etc are running from it.

I'd like to check if the 10mm swa cable is enough and what ratings I should be looking at for the sub consumer units in each outbuilding. The most consistent guidance is that I should have a sub consumer unit in each outbuilding with an RCBO in each. I do want to make sure it's not on the limit in case someone plugs something extra in at some point etc.

 

[DefyG]

The 10sqmm cable on a 45A OCPD will be more than adequate for the load you have mentioned, with RCBO's in each of the OB 1&2's CU.

 

[littlespark]

Pity theres no drawing here to help... but how i understand is at the moment theres a 10mm out to the furthest away shed, and you want to put another shed on the line somewhere between house and existing shed?

So you need to pull the cable back to the new plot, and run a new cable to the further away?
The newer cable may not need to be as big as 10mm

 

[rob0602uk]

Pity theres no drawing here to help... but how i understand is at the moment theres a 10mm out to the furthest away shed, and you want to put another shed on the line somewhere between house and existing shed?

So you need to pull the cable back to the new plot, and run a new cable to the further away?
The newer cable may not need to be as big as 10mm

Thanks both, sorry I can't figure out how to upload my sketch, will try again later.

littlespark that's pretty much right, the 10mm has enough cable that it will be enough to add in without re-running etc. When we had the building work done, the electrician ran a long cable from the CU to the shed but I've since changed the shed into a garden room. The electrician was going to come back and do the other outbuilding and sign off for me but it's taken me ages and he has since moved to Australia lol. The council said they have an electrician that can come and sign it off to finalise the building work but I'd like to make sure it's going to pass everything before I do that so want to check everything here, get an electrician to install the final consumer unit in OB1 and then get signed off by the council's electrician (I think this has to be done by them retrospectively as unlikely someone will sign off work done by someone else).

so sounds like the cable is good, my unit in OB2 is 40A at the moment, do I need to get that uprated to 45? and am I right in thinking it's ok to have the feed running off my main CU (this is on one side of the board running off an MCB and RCBO in the same way the other house circuits are). This will then go to OB2 and OB1's feed will then be an mcb circuit in OB2?

I think that I read before about there being issues with tripping if you have multiple RCBOs and difficult to track it back if there is a fault, I'm not that concerned about this as I think that would still be possible. Also, some articles say about having separate earthing for outbuildings. I don't have that now and no one has mentioned this so far that's seen the setup, but will I need this.

 

[westward10]

 

[rob0602uk]

Got it, thank you. This is the CU in OB2 at the moment and the diagram

 

[Simon47]

It looks to me as though it would be better to pull the cable back from the house to OB1 and run house-OB1 as a new cable - that's a much shorter length of cable. But obviously that depends on practical considerations.
Is there any slack in the cable ? If so, then it might be possible to just tap into it mid run.
It wouldn't be my first choice, but there's scope for tapping off the cable mid run using a potted joint. That could be buried and not need any slack - saving you cost of cable and effort digging.

 

[DefyG]

I think that I read before about there being issues with tripping if you have multiple RCBOs

Only if you have them (RCD's/RCBO's) in series and regulation on selectivity isn't followed.

 

[DefyG]

do I need to get that uprated to 45?

No not with the load you have indicated, 40A is still ok.

 

[DefyG]

Also, some articles say about having separate earthing for outbuildings. I don't have that now and no one has mentioned this so far that's seen the setup, but will I need this.

That depends on what your earthing system is. What have you got now? Is the 10sq 3core and one of the cores being used as a cpc/combined bonding conductor? or is the swa being used?
You'd best get your new electrician to assess best method to be used.

 

[HappyHippyDad]

Do double check with the electrician that they are happy with you running the cable. It's just never as simple as it sounds. Correct depth, cable warning tape, plenty of excess left rather than exactly the right amount so no room for error/complications.. etc etc. These are things you may discuss with the electrician. Just be aware that every little bit of help and advice they give you, in order for you to do the work yourself is basically them giving you free work (as it is still their time). I would personally take this in to account when being asked to quote for a job, as I know that every day the customer will have questions on how to proceed. This is absolutely fine if it's been well communicated before the job starts, and both parties are happy with it.

I'm actually about to turn down a job where the customer wants to run all his own cables, as it will be me signing off the work, and I just don't get a good feeling about them. Of course I will do it very politely, and simply explain that I like to do the whole job myself seeing as though I am signing it off. In your case, if it is just a single piece of underground cable, then I would probably be fine with that.