I’m getting some quotes to run power to my shed which is 25m from the house.

Depending on final routing, the cable run will be between 35 – 40m which appears to push it towards using 10mm SWA for capacity and voltage drop.

I also brought a plastic CU a couple of years ago which 1 sparks is adamant he can’t use due to the new metal clad CU regs. 2 sparks haven’t been worried by that and also reckon 6mm SWA will be OK.

I want to do this properly but not go totally overboard because of cost and all the ground work that will be required.

Anyone any experience of the recent reg changes?

TIA

Just run the cable along the fence – much easier and cheaper than doing any groundwork…

see http://singletrackworld.com/forum/topic/power-to-shed-what-piping-hose-etc

The regs change has been hotly debated on electrical forums. You think we can argue! The requirement is for them to be non combustible which as every pedant knows is a bit limiting. Steel will happily combust under the right conditions. It seems that steel is considered compliant though, and plastic not. That said if you can get away with plastic and your spark is happy to sign it off I can’t see it being a big deal. There’s millions of a plastic consumer units out there not spontaneously combusting. As for the cable that’ll be down to some maths and a bit of interpretation. Depending on your load requirments I’m sure 6mm will be adequate but wouldn’t be surprised if the calcs showed 10mm over that distance.

@ Nickjb

I’ve been grazing some sparks forums and yes I’ve seen some of the comments!

According to that one sparks, 6mm is apparently marginal because of the voltage drop –

…..but BS7671:2008(2015) Amendment 3 states that no circuit can be designed with more than 3% drop. (3% 230v = 6.9v. Therefore minimum voltage = 223.1v

Footflaps

Nice job there and bigger than what I will be running. I’m not keen on fence mounting though I’ve also got 8M of stepped patio with mains water and a foul drain in them to contend with.

(Catenary poles aren’t an option either)

BTW what prog did you use for your diagrams? Visio doesn’t seem to have any decent symbols?

BTW what prog did you use for your diagrams?

Just did it manually in Power Point. Actually I think I nicked some pics out of the regs documents and butchered those in PP.

There are tables in the regs for max runs for each cable diameter, do you have a copy? If not, buy the on site guide, very handy:

According to that one sparks, 6mm is apparently marginal because of the voltage drop

Sounds about right. Depends how marginal. Is anything in the shed really going to be impacted by a 3.1% or even 4% voltage drop? And that’s only when there is maximum power draw which is probably never. I think our old workshop had a 50% drop when the welder started up the light noticeably dimmed. Never seemed a big enough issue to actually dig the old cable up. 🙂

Additions to existing stuff doesn’t need require bringing the old stuff up to regs. Otherwise we’d be rewiring our houses every time we wanted an extra socket and there had been a regs change since the last rewire.

Adding a circuit for a shed to your existing CU shouldn’t be a problem assuming there is a circuit they can use on the consumer unit. Wanting to change the whole lot sounds either like looking for work or being very cautious about interpreting the regs.

I’m not a sparky, but I’ve just looked into pretty much the same thing with a CU that was fitted last year.

Nickjb

I’ve measured 210v at the extension reel I’ve been using and that’s coped with running everything I’m likely to want connected.

We have spare capacity on the house CU and tron, good point about refreshing everything for any additional work. Chap seemed conscientious and I guess wants to do things ‘properly’

Footflaps

Cheers for the info. Haven’t fired up PP in yonks so will have a nosey

In my opinion, the shed is far enough away from the dwelling that a plastic consumer unit would be ok.

Cable sizes – 6mm carries 46 amps buried, 10mm 60 amps buried, so protected by 40A mcb at the house or 50A mcb/60A 1361 for 10mm.

Voltage drop is 7.3mA/m for 6mm, 4.4mA/m for 10mm.

Assume 30m = 30*7.3/1000*40 = 8.76v for 6mm
30*4.4/1000/60= 7.92v for 10mm. or 6.6v if you use a 50A MCB.

Lighting 3% of 230v = 6.9v so 6mm and 10 with a 60A fuse are technically underspecified due to voltage drop. However, the regulation is actually that the drop should result in voltage not being less than the minimum operating voltage for the equipment being run and you’ll not be pulling 60A in a shed unless you’ve got some serious stuff going on in there.

If it was mine? 10mm with a 50A MCB at the source. Do it once, do it properly. The extra cost of 10mm over 6 is negligible.

Rich.

If it was mine? 10mm with a 50A MCB at the source. Do it once, do it properly. The extra cost of 10mm over 6 is negligible.

Most of the cost is running the cable in a conduit or trench, so the cable size isn’t that significant in the overall cost.

Another sparks and yet another interpretation!

Have also been asking about seperately upgrading the house CU and this one said ditch the current MCBs + 1 RCD switch unit and RCBO the lot.

Would RCBOs be a bit overkill for a typical domestic install or is this the ultimate way of doing things?

RCBos are good in that they will not take out circuits that are not affected by the fault. The problem with an RCD main switch will be that everything goes off and is not now considered good practice.
I would consider either splitting the incomers such that the shed doesn’t go through the main consumer unit or failing that, put it on a non rcd way in a replacement consumer unit which will have 2x RCDs at a minimum protecting the house. An RCD in the shed will limit tripping to local circuits only and you’ll not have to go to the house to reset.

If your earthing system is TT (ie uses an earth rod), you’ll need to have an RCD at the front end to ensure disconnection if an earth fault occurs. This should be 100mA and, with 30mA rcds downstream of it, be what is known as an “S curve” type which will delay tripping. This will prevent it tripping before the 30mA RCDs which is a possibility even though it’s less sensitive.

If your house earthing system is what’s known as TNC-S, your electrician should ideally fit a rod at the shed rather than use the suppliers earth. This is not so important if there’s no earth potential being introduced into the shed but if there’s water or structural steel it’s best practice to use a rod.

My interpretation of the use of a plastic CU being ok comes from the “on site guide” which says that:
“Where the consumer unit is to be located in an external non-habitable building eg garage or shed which is not in close proximity to a dwelling, consideration could be given to installing a consumer unit of non-ferrous construction”.

25m to me is far enough awy to be considered not in close proximity and if it did combust it would not be a significant risk to the house.
Obviously just my view and ultimately it’s down to the bloke signing the certificate who’s got to justify it.

Rich.

@ rwamartin

Thanks very much Rich

We currently have an earth rod at the house, and the supply is fed by overhead cables. This latest sparks suggested ditching that rod and RCBOing the house (plus some other stuff I didn’t fully take in), using 2 core SWA to the shed and adding a separate earth there.

Two others were proposing 3 core SWA and the 1st guy just said yeah it’ll be about £400 with no technical explanation! 😐

Current house CU doesn’t have any fault isolation and we have had the very odd occasion when a bulb goes it’s taken everything with it. We’re mulling over a loft conversion which will require at the very minimum, mains powered smoke detectors, so I’m minded to update and expand what we’ve got.

Plastic enclosures need to be “self extinguishing” and not “Non combustible” Big difference!

Take a look around, most mains plugs and sockets are made out of “plastic” and have been for years, it’s just they use a self extinguishing glass re-enforced plastic.

There is no such thing as “Non combustible”, provide enough heat and an O2 rich atmosphere and just about anything will burn…

We currently have an earth rod at the house, and the supply is fed by overhead cables. This latest sparks suggested ditching that rod and RCBOing the house (plus some other stuff I didn’t fully take in), using 2 core SWA to the shed and adding a separate earth there.

Obviously not being there I can’t give definitive (well, my opinion as to what’s definitive!) advice. However, this seems to be along the lines of what I’d be looking to do.

RCBOs will be expensive, particularly if there are a lot of circuits. If cost was an issue I’d look to having a new “17th edition board” ie 2 RCDs with the existing circuits split between them. The board change is sort of not required if you can split the incoming cables before the consumer unit but there are definitely benefits from a new CU.

He sounds like he knows what he’s talking about but check the quote carefully as he might be upselling a bit and making some extra on the RCBOs.

Not local to Pembrokeshire by any chance? Happy to look at it for you. (Not after the work, I specialize in inspection and test rather than bigger jobs).

Rich.

Plastic enclosures need to be “self extinguishing” and not “Non combustible” Big difference!

Not since the regs change, now says this:

421.1.201 Within domestic (household) premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439-3 and shall:
(i) have their enclosure manufactured from non-combustible material, or
(ii) be enclosed in a cabinet or enclosure constructed of non-combustible material and complying with Regulation 132.12.

Although they have added this note:

NOTE 1: Ferrous metal, e.g. steel, is deemed to be an example of a non-combustible material.

I’ve got my consumer as Rich describes with two RCDs each protecting 4 or so circuits. I then have a couple of non RCD circuits. One for the fridge and another for the shed which has its own RCD locally. A lot cheaper than all RCBO but I understand a lot of sparks like the all RCBO arrangement

I can see the attraction of RCBOS, well at least with my limited understanding of such things! However at £25 a lump they certainly aren’t cheap and we presently have 10 MCBs!

Replacement house CU quotes are currently between £380 and £500

Rich, thanks for the offer but we’re in Sussex so probably not that convenient for you 🙂