garage electrics: in conduit or not?

[Fred]

Hi,

As you may remember, I have been giving my garage a facelift and lined the walls with insulation and plywood. So it's time to fit the electrics back onto the walls. previously the wires were clipped direct to the brickwork. I was wondering whether it might look neater if I used conduit (the plastic variety) instead? OTOH would conduit take up more space and possibly look more unsightly?

I've not used conduit before: any tips? How many wires could I fit into a conduit? Am I limited by size or by regs? I can't see any problems with light duty light circuits but what about the sockets? It's more of a storage area than workshop, so there's only going to be a couple of sockets and a couple of light switches; nothing heavy duty. Even so, if I wire supply > socket 1 > socket 2 either as a 32A ring or a 20A radial, I will need two cables going to socket 1. Can these be dropped down one conduit or does derating apply and prevent that?

I was thinking of adding an outside socket. IIRC the MK masterseal are the best. Is it worth thinking about a cheaper clone, as seen in CPC, or are they not as good?

If I added an outdoor socket, I would connect this via a FCU. This way it would be switched off when we are not lawn mowing, to stop burglars using it to power their tools to break in! Potentially this would mean three wires in a conduit: the two sides of the ring/radial plus the cable for the socket. Would three cables get a bit too warm packed in there and pulling 20A?

TIA

[robgraham]

I recommend you use this from Screwfix - 19624 - easy to install and looks really neat.

Rob

[Malcolm]

Surely in a radial circuit, the sockets are daisy chaine?

Malcolm

[ARWadsworth]

There is no problem with you using conduit for either your ring or radial. You can get two 2.5 T&E cables through straight runs of 20mm conduit without too much difficulty.

Is the install going to be all in conduit or are you intending to just use conduit for the drops?

[Spamlet]

I suppose a lot depends on where you want to route the cables and whether you might bash into them or not. My old garage had an isolator box at near ground level, with thick iron pipes as conduit. My newer garage has a consumer unit above head height and the cables run high, out of the way through spare bolt holes in the concrete sections.

It's probably worth getting a decent waterproof outside socket, as an outside switch I bought did tend to collect water behind it's flip cover, until I sealed it up completely.

S

[Ronald Raygun]

Yes, that's why he said he'd wire supply to socket 1 to socket 2. That's two cables going to socket 1, one from the supply, the other onwards to socket 2.

I presume the regs don't allow a radial circuit to fork. But just because it's not allowed to fork electrically doesn't mean it can't fork physically. For example he might want to run his conduit "spine" at one particular height (say 2.5m above the floor) and have a junction box with a vertical length of "spur" conduit to a socket say 1.2 m above the floor.

That would be perfectly OK, but it would mean two cables sharing that vertical section of conduit, one going to the socket from the supply, the other carrying on to the next socket.

[chris French]

It's a bit ambiguous, but no I think that the OP did mean he would need to run 2 cables between the supply and socket 1 (as you would do if you were running a ring)

Incorrect, the sockets can be daisy chained, or can spur off as and when ). The circuit is protected by a 20A MCB so there is no chance of overloading the cable even if you were to hang lots of big loads off the sockets.

So no need to run the twin cables down. A singlr spur down would do. There is no need really, other than neatness to un the cable in conduit or trunking though if up high oput of the way. It's not uncommon to see cable fixed bare up high and conduit just used for the drops where it is more likely to get damaged.

For the likely use the OP is going to put these sockets to, then daisy chaining one socket on from the other seems the easy route, which just requires running one cable from the supply, first to one then the other.

[John Rumm]

Leaving them clipped to the brickwork and insulating over them, or laying them in a grove in the insulation so they are in contact with the back of the ply would have been neatest! ;-)

In which case you could just surface wire - it does not sound like there is much chance of a cable getting damaged.

If you go for conduit - then trunking is actually easier to wire. If conduit, then thick wall plastic in preference to thin wall (easier to bend successfully).

De-rating will apply, but it does not have enough effect to cause a problem assuming no other factors also apply (high temperature for example).

I quite like the Aquatec ones. Cheaper than MK, and easier to open - especially in the cold, where getting the lid to lift on the MKs can be hard IME.

If I added an outdoor socket, I would connect this via a FCU. This way

I doubt many go limiting themselves to houses that have laid on power!(bricks, jemmys etc work cordless!)

Can't see you would need three - the drop wire to the socket could go straight through the wall.

[Fred]

I'm not sure yet. I'm considering all my options before starting. What do you suggest about using conduit for everything/just the drops? My thoughts are that if only for the drops, then the wiring at the top might look messy. OTOH if I have conduit for lighting and conduit for sockets, one is going to have to bend over the other. Unless I can get these ready made, I don't fancy spending all day with a spring making them, so perhaps just the drops would be enough?

[Fred]

I don't like the idea of cables behind insulation and I may want to upgrade the wiring at a later date and having the cables on this side of the wall will make that much easier.

I was worried about the derating, so I'm glad to hear it is not an issue. High temperatures do not apply as this is in the UK. That's why I've had to insulate it ;)

I don't think it would go straight through, as I'd probably want the socket lower than the switch (or does the 450mm rule apply outside too)? More than likely two wires would go one way and one wire another but thought I'd ask everything at the same time.

Thanks again.

[Fred]

I think I meant I would need two cables at socket one: one from the supply and one going to socket 2.

AFAICS the only difference between a ring and a radial is the inclusion of an extra cable from socket 2 "back" to the supply. For the sake of a small length of T&E, is there any reason not to do this?

Was the suggestion that the sockets spurred off from junction boxes above them? I just thought this would add extra work and unnecessary joins. Wouldn't it be best to have just one break/join in the cables at the back of the sockets?

I know there are only two sockets at the moment but in future I may wish to upgrade things, so I'd like to be able to future proof it.

Thanks.

[robgraham]

I didn't read your post right through I'm afraid so I missed the bit about an external socket !!

I use mains power for a number of activities outside and have never felt the particular urge to fit an external,socket - like most sockets it's never going to be in quite the right place anyway. With so many inexpensive extension leads around I'm quite happy putting a socket inside near a door and just leaving the shed/garage/house door open for a short period.

Rob

[ARWadsworth]

If all the cables are in trunking or conduit then you would use singles cable instead of T&E.

It might be easier and look nicer to run trunking around the room between the ceiling and wall. Make it large enough trunking to accomodate all your cables You then drop down (or across the ceiling to the lights) in either conduit or smaller trunking to your switches and sockets.

[ARWadsworth]

You don't have an outside fridge then:-)

[Andy Wade]

Radial circuits which feed several points can fork and branch however you like, provided you obey all the rules relating to cable current rating (including all the derating factors), voltage drop and earth fault loop impedance. Lighting circuits are a common example.

The on-site guide gives guidelines for two radials feeding 13 A sockets and other BS 1363 accessories:

- the 20 A radial using 2.5 mm^2 cable can feed any number of points, subject to a floor area limit of 50 m^2;

- ditto for the 32 A radial using 4 mm^2 cable, with the floor area limit increased to 75 m^2. This circuit can have 'spurs' in 2.5 mm^2 cable subject to the same rules as for ring circuits. (In practice though the terminal capacity of the wiring accessories tends to limit what is practical - three 4 mm^2 conductors into one terminal won't go!)

For socket radials in a home workshop, a design current of 16 A is often more than adequate, especially if you put any electric space heating on its own separate circuit. With a 16 A MCB and 2.5 mm^2 cable you have plenty in hand for conduit grouping and thermal insulation factors, etc.

[John Rumm]

While not ideal, having a wire covered in insulation on only one side and being clipped to a conductive building material is not actually that bad - there is still an escape route for the heat. Its when a wire is buried in the middle of the stuff it get to be a real problem.

The 450mm rule probably does not apply here anyway - that's for newbuild etc.

[Doctor Drivel]

The days of MK being good are now gone since being bought out. Most is quite tatty.