Lighting circuit taken off 13A cabling

[Jeff Layman]

Some time ago I had a problem with one of the lighting circuits tripping an MCB and then the RCD. This was traced to the garage, and to save further time and money the lighting cable to the garage was disconnected from the junction box which supplied its power. It's not a simple matter to find the wiring fault and remedy it, as the garage roof has insulation under it, which is covered by plasterboard nailed to the garage roof rafters, and the cabling runs within it.

So I thought I'd run new wiring to the lights externally in trunking fixed to the plasterboard, and then reuse the lighting switch pattress (but with a new twin switch and only two of the lights connected, as I don't need the outside light reconnected). To save a lot of time and effort, I thought of taking power via a spur off the 13A ring main in the garage. As it happens, there is a convenient double-socket less than

50cm from the lighting switch. I'd like to protect the lighting circuit with a 6A MCB, or perhaps RCBO, but all the spur units seem to only have fuses. I could fit a garage-type consumer unit (such as ), but it seems a bit of an overkill as the power supply to the garage already has RCD protection.

Any suggestions as to ways to do this, or is not allowed under Part P anyway?

[Theo]

I don't think Part P would apply, since you aren't adding an additional circuit at the consumer unit.

IANA electrician but I can't see a reason why this would be problematic, as long as there's a FCU to isolate the lights in case of failure or servicing. They become effectively fixed appliances in the same way as say a towel rail or plinth heater.

Just be sure to fit a fuse in the FCU to match the wiring for the lights - so that a short doesn't melt the wiring before it blows the fuse. You may well find that a 1A is fine if they're all LEDs.

Theo

[nothanks]

If you want an RCD, rather than the usual fused spur, then howsabout one of these:

or similar.

[ARW]

What do you have against fuses?

It's a 13A fuse that allows you to use a 32A MCB on your "13A ring main"

- ie the 13A fuse in the plug.

[Animal]

A 3A or 5A fuse in a plug or fcu is fine. Unless you're running a lathe or similar, in which case you won't want to plunge into darkness while it spins down.

[John Rumm]

I would go for a bog standard FCU (unswitched) with a 5A fuse in it. It is unlikely to ever blow with modern lamps - so it is fit and forget.

(much less hassle than trying to use a MCB which will offer no real advantage)

It is find - just modifying an existing circuit and not in a special location.

[Jeff Layman]

Thanks for all the replies. I'll go for the simplest solution - a 3A fuse.

[Andrew]

That requires two single pattresses, one for the FCU, one for a switch. Wouldn't it be simpler to just fit a switched FCU for protection and switching (unless the switch is required to be located further away from the fuse) ?.

This is a garage, so presumably it will only be used occasionally ?

[Theo]

One downside of switched FCUs is they're often rated for higher current than a light switch, meaning they need more force to press them. Most light switches are fairly light touch.

Not really a problem in a garage, but in a house it could be annoying that 'flicking a switch' takes a bit more effort.

Theo

[ARW]

Not with Click Mode and MiniGrid.

[John Rumm]

Indeed - I was assuming that it would be a typical lighting setup where the feed from the supply may not be in a convenient place for the switch.

You could do that. FCUs usually have heavier switching action when compared to a light switch, and there are no two way switching options, which may or may not matter...

[Jeff Layman]

I thought I'd have a look at the fluorescent lamps in the garage as a start to replacing the wiring. There are three lamps - A and B are on one switch, and C is on a different switch (an outside filament lamp, on a third switch, will be removed).

Of course, nothing is simple here where wiring is concerned. When I removed the cover of lamp A, instead of the expected single cable with switched live, I found four cables and two connector blocks! See

. The partially-hidden block on the left appears to be the original one which connects to the choke and starter. I think the other one is being used as a loop-in "junction box" to take power to the wall switch and back to lamp A and lamp B (which has only a single cable to it), and perhaps unswitched power to lamp C and the outside lamp. All the wiring is red and black.

All three black wires at the wall switch are sleeved with red, but none of it is sleeved at lamp A to indicate neutral used as switched live. Interestingly, the connections to the partially hidden block include a black wire connected to a red wire (see top left of block in the photo); I assume that went to the wall switch as a switched live.

Anyway, all the wires will be disconnected and new wiring put in. I probably won't use loop-in, but what would look like single and earth, although the live and neutral wires will run in the same T/E cable to the fluo lamps. All wiring will run in external trunking.

[The Natural Philosopher]

Sometimes it's a lot quicker to install to 'standard practice' than figure out WTF the previous amateur was trying to do.

[Jeff Layman]

Absolutely. I just took the cover off lamp C, and found two cables. Its connector was being used as a "junction box" for a loop-in. Interestingly, though, this time the black wire did have a red sleeve indicating it was the switched live. There's nothing like consistency...