Wiring for a Shed

[RJH]

A friend has build a fairly elaborate shed - about 5m x 5m, insulated. An electrician has installed a 63A feed from the main supply, using a consumer unit and an MCB, to the shed (which is 75m from the house), terminated with a 13A socket, with lighting spurred off the socket.

He's asked me to help him install 6 double sockets and wiring/switching to 4 x 4' 50W LED batten lights.

I'm not sure yet whether I'm going to do it - I'd do it for myself, but he's quite impatient, and I don't think he'd be happy with the 3 months I'd give myself to exercise excessive caution. Anyway, a couple of Qs:

1) Would you use a consumer unit in the shed? If so, what's the MCB and RCD situation? I's assume the CU and MCB/RCD arrangements at the main supply have some bearing on this.

2) Ring or radial for the sockets? I've had a look at the DIY wiki:

I think it's going to need a ring because he's factoring in an oven (3kW, for pottery) at a later date. But it'd be a lot easier to do a dedicated oven socket on a separate feed from however (1) is organised, and a radial for the remaining 5 sockets. I don't follow where the '20A' in the radial comes from in the wiki diagram - how is a radial socket circuit designed for a particular use? The only anticipated high current device is a 2kW heater. But I'd guess contingencies have to be designed in.

3) Routing the wiring. He's intending to use 20mm plastic tube conduit, which for layout reasons has to go up, round, and down to the sockets/lights/switches. It'd obviously be neater to route all the cables in one run of conduit - is there some guidance relating to stuffing conduit with cable?

The fact that I'm asking these questions tends towards telling him to get an electrician - I can see this being a bit of a nightmare. But answers will help me understand it all better, and give him something to think about.

[Tim Watts]

Yes - there's the fill factor:

That's purely on space. In essence, if it fits easily, it's OK.

Are you proposing to use singles?

[RJH]

Excellent, thanks.

As in a single run of cable? I need to draw it all out, but at the very least there'll be the 13A socket and the lighting cable running through the conduit, to keep things neat.

The lighting cable could in theory be quite skinny - 3A, for the 160W load?

[tabbypurr]

You'll want a local earth rod with everything RCDed.

Ring has 2 slight advantages over radial: usually a mite cheaper on cable & a bit safer. But either works fine.

see the wiki page on Cable.

Should be no nightmares once you've found out what to do.

NT

[RJH]

Thanks. I'm loosely assuming there's a local earth rod fitted by the sparks - but will check.

Thanks - got that now.

It's more the 'client' - he's quite impatient and has a habit of looking for fault when it isn't necessarily there. A sort of irritatingly enquiring mind looking for shortcuts.

[John Rumm]

Well if you take it step by step, this is quite doable. The hard work part is usually getting the submain there in the first place, and that bit has been done.

In this case, definitely.

Yup they do, you will need to find out how that end is configured since it will limit what you can do later.

32A Ring probably

If you have read through of :

its a bit more focussed on the particular job in hand.

I would tend to lay on a dedicated radial for the kiln (especially as many are >3kW)

Yup

A radial socket circuit is just a general purpose socket circuit, so it needs overload protection provided at the origin of the circuit by a MCB (or RCBO). The 20A comes from the size of MCB typically used if the circuit is wired entirely in 2.5mm^2 T&E (its maximum current rating os less than 32A so would not have overload protection with the normal 32A MCB used on a ring). You can however wire a radial in 4.00mm^2 T&E and use a 32A MCB if you prefer (rarely seen in practice since wiring in the larger cable is harder work than having the extra return cable in the smaller size for a full ring).

Basically it needs to go in ;-)

Conduit de-rates the cable current carrying capacity a bit, but not normally enough to have a significant impact on the design.

It ought to be relatively straight forward. You will likely want to terminate the incoming feed (probably a SWA cable) in an insulated box, and the connect that to the CU. That way you can have a local TT earthing system for the shed with and earth spike.

If you are lucky and the sparks has not wired the whole submain from a RCD protected supply, then you can have a split load CU in the shed to keep the lights on a separate RCD to the sockets.

Indeed. It will be a case of having a closer look at the starting point (like how power was taken from the house supply, what type and sixe of cable was used, and what the earthing arrangements are for the house supply.

[Bill Wright]

Set ground rules. If he doesn't obey them just don't go for a few weeks til he rings up. Then warn him. Be as awkward as he is. Don't give him an inch. It's the only way. I'm speaking from 45 years' experience installing for private customers.

Bill

[Tim Watts]

No - as in single insulated cores - if you are running 100% in trunking or conduit, it makes sense - more manageable.

You can buy that by the metre in some electrical wholesales (think TLC do).

If you run T+E in conduit/trunking with multiple circuits you need to apply a grouping factor (a <1 multiplier to the current carrying caapcity of the cable).

I am not sure of the calculation with singles for multiple circuits - did look at my Regs but couldn't find a table for that case.

[Brian Gaff]

Hi, In the past in a shed I used to work on equipment with a live chassis by having it on the other side of an isolating transformer and earthing the non live side, if you get my drift. After all this is just what replacing an auto transformer would do to mains gear of that age. However I used to fly by the seat of my pants in those days, and so what I'd want to do with my new shed when its built is also to protect that circuit with more than a 5 amp fuse.

Now I'm probably well outside of the law here on this one but two problems come to mind. First what sort of breaker and also of course you would need any gear connected to have a plug on it that could not, accidentally be put into a standard socket as that would be disaster on many levels. are there special plugs or sockets for this kind of use?

Brian

[Tim Lamb]

In message <q0j921$vfa$ [e-mail], RJH [e-mail] writes

I'll leave others to comment on split load consumer units but the length of supply run should push you to a local earth rod.

You mention layout reasons for going up and down with 20mm conduit. If practical, it can be much simpler to use 50mm metal duct at the outlet level. I mount twin socket outlets directly above or below the duct using 20mm metal conduit connectors and brass bushes. The duct has ample space for using 4mm singles and allows easy access for afterthoughts:-) Best to trap the cables away from the cover (sharp edges). I use short lengths of flexible plastic duct.

>

[Roger Hayter]

I used to do the same, but I haven't seen any live chassis equipment I wanted to repair for about fifty years. And getting the series heater replacement valves might prove expensive.

Having said that, I can't think of any way an isolated supply with one side connected to local earth could be protected in any way that doesn't make it more hazardous to the operator rather than less.

[Robin]

It's a very detailed point but given it's a shed with an oven (and other tools?) I'd provide 2 (at least) independent light sources so tripping one MCB/RCD doesn't leave you in the dark. If the whole supply might be cut off by a trip in the house then a non-maintained emergency light could kill 2 birds with one pony*.

• 25 pounds that is - or guineas if he is a professional :)

[tabbypurr]

under an hour's pay each

RCD after the iso transformer.

NT

[newshound]

I was going to say that too

Surely he's going to need a reel of each

[Dave Plowman (News)]

If you earth one leg of an isolation transformer, you are back to how mains is without one. Leaving the secondary floating means you have to touch both legs to get a 'shock'

But these days, you'd use an RCD for safety purposes.

[ARW]

+1

And all RCBOs.

[Roger Hayter]

Fascinated to know you can still get them. Though I know some LOP valves are still in use.

That only works if you reference the secondary voltage to earth before the RCD. Otherwise there is nowhere for unbalanced leakage to go. And I am not sure that is actually safer, as it does not protect against a shock from live to chassis, the most dangerous kind.

[tabbypurr]

What can't you get on ebay?

Isos make a shock require 2 touch points instead of one - an improvement in safety. But be a bit wary of isos, one can sometimes still get shocks from them. All is not as it first appears.

NT