Workshop Wiring

[Andy M0FYA]

At what point does the mains wiring in a shed/workshop become subject to the ‘Part P’ regulations?

I am fitting out my shed which is to be used as a workshop. It has a floor area just less than 15 square metres, so isn’t subject to Building Regulations. Presently I am providing power via an extension lead which is plugged into a waterproof socket on the outside of the house. This in turn is supplied via a wire through the house wall which is plugged in inside the house using an RCD plug-top. The extension wire goes into the shed under the eaves and terminates in a multiple socket-strip into which I plug lights, heater, power tools etc as needed. So everything falls into the category of ‘portable appliances’ and Part P isn’t relevant.

It’s my intention to retain the RCD-protected ‘flying lead’ plugged into the waterproof socket as the connection to the mains when the workshop is completed. This obviously means the total load can’t exceed 13A, but I don’t see that as a limitation.

What can I do inside the shed before Part P becomes applicable? Does everything have to remain ‘portable’, plugged into the multiple socket strip? For lighting I’m going to add a number (probably four) of strings of small ‘link light’ fluorescent fittings, so terminating each string in a suitably fused 13A plugtop is no hardship. The multiple socket strip outlets are all individually switched. Similarly supplying a lathe or drill-press via an extension lead is perfectly feasible. Not exceeding the total load limit won’t be a problem as I can’t use more than one machine at a time.

But what if I replace the multiple socket strip with a distribution panel and hard-wire to lights and sockets? Could I claim that the whole shed is a ‘portable appliance’, as it is plugged into a 13A socket in the house? Or is there any acceptable compromise between these two extremes?

I want to end up with a safe installation, which complies with safety regulations from a technical viewpoint, but I just don’t want to get involved with ‘Part P’ compliance if I can avoid it.

Any guidance would be appreciated, please.

Many thanks,
Andy

[Mark Hennessy]

Hi Andy,

A can of worms there

I looked into this a few years back. The short answer is that officially, providing a supply to a shed is notifiable work under Part P.

However, what you propose is essentially "running an extension cable". So as it's not a permanent installation, you might be able to argue that it doesn't need notification?

In short, Part P is a mess. You only have to look at the discussions on the various electrical forums to see how confused the professionals are - even on the IET forums - so what chance do we stand? The regulations get revised frequently as well, so it's really hard for us to keep up to date.

There are two main issues from my point of view. First, safety - needless to say. Second, what happens when you need to move house?

It seems that since the recession started all those years back, the whole business of moving house has got more complicated, expensive and litigious. So depending on your long-terms plans, you might want to keep half an eye on that. In your case, the shed might have rotted away before you plan to sell up? Or you might take the shed with you? Either way, if you simply unplug your shed from your external socket, then that might be enough to solve the problem from the legal point of view. Of course, the outside socket might need looking at to ensure it complies with the rules applicable at the time of installation. From what I've read, I think that today, a "new circuit" needs notification - and anything downstream of a protective device is classed as a "new circuit".

Back to safety.

I think it's good that the supply is RCD protected at the house. You're using an RCD plug-top, but are the sockets RCD-protected as well? If so, you might suffer the mild inconvenience of the RCD in the CU tripping first. However, you might want to consider fitting an emergency light fitting out there, as obviously you'll lose lights when the RCD trips. I bought an LED one from CPC for about a tenner recently, and that seems really quite reasonable for the money.

Once the cable is outside, there are lots of rules that ought to apply to it. Again, you could argue that it's no different to running out an extension lead temporarily, but as much as you regard this as "temporary" in the eyes of the law, it will still pay to think about safety - so protection from physical damage is obvious. And making sure it's suitable rated in terms of voltage drop, etc, needless to say. In theory, it ought to be SWA, but that doesn't fit in a 13A BS1363 plug-top especially easily

What about the earth? This is a really complex topic, that starts with determining how your house is fed (PME, TNC-S, etc). An earth spike might be prudent. If there is any plumbing nearby, perhaps that needs bonding?

Ultimately, it might be worth involving a professional. They will take care of the notification process, which would otherwise cost a few £hundred if you were doing it yourself, and if you find one who is "friendly", they might be happy to let you do the actual work - so in effect you're just paying them to design, inspect, test and certify. In the current climate, there are plenty who are happy to work that way, because they know that otherwise folk will just do the work anyway (perhaps unsafely) and not have it inspected, so at least they know that the work is being done properly, and they get something out of the bargain. It's a pragmatic compromise

I never did build the shed, but I need to revisit this because I really need to sort out the feed to my garage. It's just a 4mm T&E that runs the length of the garden with no physical protection whatsoever, and it's simply picked up from behind a socket in the kitchen. Really nasty. That said, we've been here for 10 years, and it hasn't caused a problem. I plan to run SWA of decent CSA because I want to run woodworking machinery out there. But so many other jobs need to be finished first.

Jeffrey (pppenguin) is more up to date with wiring regs than I am, so will hopefully have some more specific thoughts...

Hope this helps,

Mark

[Andy M0FYA]

Hi Mark,
Many thanks for your reply. Firstly, I’ve no intention of moving house; I’ll be carried out in a box! If you saw the way I previously converted my garage to a radio display area and workshop (before Part P etc came into force), now followed by my new shed/workshop, you’d see there’s no way, I’ve just got too much time invested in them!

I’ll have rotted away long before the new shed, and it would be physically impossible to move. (Indeed it would be very difficult to demolish without the assistance of a JCB or explosives.........!). The timber-frame construction and cladding is based heavily on the advice and design ideas I got from the knowledgeable Lawrence of this forum.

My house distribution unit is an old-style box with wired fuses, so no RCD upstream of the RCD plug-top. The outside waterproof double socket and RCD plug-top were bought together as a ‘kit’ from B&Q, and I regard that as a ‘portable appliance’ in its own right as it can be unplugged, not formally a ‘new circuit’.

The cable from the waterproof socket to the shed is a length of 16A ‘Arctic’ cable, intended for outdoor use. At the moment it is hanging under its own weight over a two metre span above head height, but I intend to put a (stainless steel or nylon) rope in place to support it from. So I think that should be ‘fit for purpose’.

The earth is simply that which is provided via the cable back to the house wiring – hopefully continuous, but I confess I have no measured proof of that. Adding an earth spike makes technical sense, but maybe it would move the installation away from simply “running an extension cable”! I don’t know how the house is fed in terms of earthing philosophy, it’s however it was done in the late 70’s when the house was built (hence wired fuses and no RCDs).

Unfortunately I don’t know a friendly qualified electrician to ask for advice.

I just want to identify a pragmatic but safe approach which doesn’t require notification if at all possible, to avoid hassle and cost.

Andy

[ppppenguin]

Simple pragmatic but not necessarily fully compliant solutions:

1: If your main supply is not PME. Armoured cable from house to shed. fed from its own way on the consumer unit. RCD at one end or other but not both. If at shed end then less ofa nuisance if it trips. If at house it protects cable too.

2: If your main supply is PME. More or less the same but earthing is more of a problem. Exporting the earth from a PME setup has problems so use earth spike at shed, RCD defintiely at shed. Cable armour earthed at house end, isolated at shed.

This is not a full guide to the subject, I'm not 100% sure myslef on some finer points, but it will keep you safe enough. As for the Part P police, they don't come knocking int he night.

[Andy M0FYA]

Jeffrey, I really don't want to have to go down that route!
Simple in principle, but not in practice. It would probably have to start with a new updated consumer unit in the house. Routing the cable inside the house would be decidedly problematic, underground difficult, time consuming and/or expensive. So any cable would have to go externally on the house wall round two sides of the house.

Is there anything a) technically unacceptable, or b) offensive to the part P police, regarding my simplistic 'extension cable' approach?

From a personal safety point of view, I have no intention to work inside mains-powered live equipment in this shed - that activity will be reserved for my existing garage-conversion. This shed will be partitioned into two areas, one for metal-bashing, including a lathe, drill-press etc with mains motors, and one for a display of aircraft systems powered indirectly from the mains either via 28v DC supply units or a mains-motor driven 80v AC generator.

Andy

[Mark Hennessy]

I must admit that the more I search around the subject, the more confusing it becomes. As I said, I'd done quite a bit a few years back, and I did a fair bit before my earlier reply. The lack of consensus was the overriding conclusion, which is why I'd be tempted to "outsource" the decision to someone willing and able to sign off the work. But I agree that can be easier said that done

With a 13A fuse in the BS1363 plugtop, feeding 16A cable, and also being RCD-protected just inside the house, I think that you've covered most bases. SWA cable obviously offers much-improved mechanical protection, but is a PITA to terminate. And gets away from the "extension cable" philosophy.

While it probably doesn't meet the regs, I can't think of an obvious problem with it. Should you slice through the cable with a spade or loppers, the fuse and RCD should offer decent protection. Should the roof get ripped from your shed during a freak storm, any water ingress will trip your RCD plugtop in the house. If you include at least one emergency light fitting, then that makes things much safer after dark.

I've had to study "temporary installations" used by OB vehicles in the past, and these often run from 13A BS1363 outlets at a venue. So, not much different to what you are doing. The complication arises with the earth connection, and the fact that like you, many venues don't know about their incoming earth. Measuring the earth-loop impedance normally gives a decent indication of the earth type. It might be worth measuring the earth-loop impedance at the outlets in your shed, but even if it's a bit high, the plugtop RCD should take care of that.

Interestingly, the mains cables on some of these vehicles are 13A BS1363 to 16A "C-form". Once inside the vehicle, there is a full and comprehensive installation. I can't help wondering if you could do a similar thing - mount one of those on a wall of the shed, so the cable can be disconnected at either end. Inside the shed, I don't see why you shouldn't install as many sockets and lights as you want. If you wanted to be smart, how about including a cheap voltage meter: http://golbornevintageradio.co.uk/forum/...p?tid=3299

[pwdrive]

Wire up the shed however you like, so long as it is safe as regards no fire risk and no shock hazzard..publish and be damned..argue the case afterwards if the planning/regs police turn up, if you ever sell the house just don't tick most of the boxes on those stupid questionairs and sign everything to "the best of my knowledge" lot's of forms omit that quote these days, I always write it in in my own hand just before the signature, that's what I've always done.

Our new cabin will be battery and maybe micro hydro powered...stuff the tax man.

Good luck and enjoy.

Lawrence