Lighting circuits are wired into a 10amp MCB

[Jupiter01]

During some routine maintenance, I noticed that my lights are wired into a 10amp MCB and also, I have two lighting circuits in the same MCB. Lighting is using 1.5mm T&E. One of the circuits powers two ceiling pendants and the other is used for an external light.

I assume that the MCB needs to be downsized to 6amp. Is it permissible to have two lighting circuits running off the same MCB?

Thanks in advance.

 

[JohnW2]

During some routine maintenance, I noticed that my lights are wired into a 10amp MCB .... Lighting is using 1.5mm T&E.

Nothing wrong with that (albeit totally unnecessary). You don't mention installation Method but, if it's Method C, then 1.5 mm² cable is good for 20A, so ....

I assume that the MCB needs to be downsized to 6amp.

As above, not really necessary (although some people might fuss about things like ceiling roses sometimes being 'rated at only 6A''), but you could if you wanted.

... and also, I have two lighting circuits in the same MCB. ... I assume that the MCB needs to be downsized to 6amp. Is it permissible to have two lighting circuits running off the same MCB?

It's not "two lighting circuits on one MCB" but, rather, one lighting circuit that branches at the MCB - which is no different from if it branched anywhere else. [ By BS7671 definition, everything protected by one MCB is "one circuit" ]

Kind Regards, John

 

[Jupiter01]

[ By BS7671 definition, everything protected by one MCB is "one circuit" ]

I had a similar question recently where two RFC’s would be protected by the same MCB and some on this group suggested that was not allowed and also, it would cause problems with the test sheet. Is the lighting situation not affected by the same concerns?

 

[JohnW2]

I had a similar question recently where two RFC’s would be protected by the same MCB and some on this group suggested that was not allowed and also, it would cause problems with the test sheet. ....

They did but, although it would be a pretty strange and confusing thing to do, as I said at the time ...

(a) I can't think of anything in BS7671 which 'disallows' it, and ...
(b) The issue with the 'test sheet' is pretty trivial, and can be remedied by completing two rows, one for each ring, for one MCB

The same people would probably (hopefully!) have no problem with one or more 'spurs' from the ring (essentially 'radials') originating from the MCB protecting the ring, so why should it be different if the extra bit(s) originating from the MCB is another ring?

I'm not saying that I would 'do it', but that's not the point!

.... Is the lighting situation not affected by the same concerns?

I suppose you would have to ask 'them' about 'their concerns' . However, I doubt that they would have a problem with that, since there's nothing wrong with having two branches of a radial originating from the MCB. After all, if there were a single cable coming from the MCB (i.e. 'a radial') which, after 6 inches or so, split into two branches (which is virtually the same as 'splitting at the MCB'), then I don't think anyone would have any problem with that - and lighting circuit radials often 'branch' all over the place.

As I implied before, in some senses this is semantic since, by BS7671 definition, everything protected by one MCB is "a [i.e. 'one'] circuit", no matter how many conductors are connected to the MCB - so, as far as BS7671 is concerned, you cannot have more than one circuit protected by one MCB!.

All that matters to BS7671 is that each and every cable connected to an MCB is adequately protected by that MCB, regardless of whether the cable is 'a radial', 'a branch of a radial', 'one end of a ring' or 'a spur from a ring' (the last being no different from a radial).

That's how I see it, anyway!

Kind Regards, John

 

[ericmark]

In theory a bulb should have a built in fuse, this was from the days of tungsten, and designed to rupture should the bulb ionise the gases when it blows, often seen as bright flash as it blows, which would otherwise take out the main fuse.

When MCB's replace the fuse, the built in fuse in bulb would not rupture fast enough, and it would take out the MCB, but most BA22d and ES27 lamp holders are rated 2 amp, so even with a 6 amp MCB without the built in fuse, the bulb holder can be overloaded.

Although ceiling rose rated at 6 amp, regulations say 16 amp is the limit, however I did have a Ikea bulb blow and weld its self in the lamp holder with a 16 amp MCB, and clearly Ikea does not fit internal fuses in the CFL bulbs, but there was no danger as such, just a pain having to renew the fitting.

As a result I would always use a 6 amp overload, in my case RCBO's, but there is nothing to really force you to do that, OK ceiling rose rated 6 amp and they double as junction boxes, so should use 6 amp, but looking at them, they can clearly take a lot more.

I did consider using a larger MCB to stop it opening when tungsten bulbs ruptured, but as the tungsten went out, the problem went away, so I have three 6 amp RCBO's for the lights. Really should change them to type A rather than type AC, but think the risk is very low, so not bothered.

 

[winston1]

During some routine maintenance, I noticed that my lights are wired into a 10amp MCB

Perfectly OK.

also, I have two lighting circuits in the same MCB.

No you don’t. Two branches of one circuit.

Lighting is using 1.5mm T&E.

Actually oversized and wasteful of copper when 1.00 mm will do.

I assume that the MCB needs to be downsized to 6amp.

No.

Is it permissible to have two lighting circuits running off the same MCB?

Thanks in advance.

That is not what you have.

 

[JohnW2]

<various things>

Thanks for essentially repeating everything I wrote last night

 

[DetlefSchmitz]

 

[ericmark]

I assume that the MCB needs to be downsized to 6amp.

It does not NEED to be down sized, but personally I would down size if easy enough to get a 6 amp MCB, but if the CU is no longer supported I would not worry.

As said a BA22d lamp holder unless ceramic is only rated at 2 amp. But should be fuse built into bulb. And although you can still get these which were common in the 50's to do the ironing with, one would hope you would not use one. So only with multi-lamps can 6 amp be exceeded.

 

[JohnW2]

In theory a bulb should have a built in fuse, this was from the days of tungsten, and designed to rupture should the bulb ionise the gases when it blows, often seen as bright flash as it blows, which would otherwise take out the main fuse.

Indeed, and it is my experience that even the cheapest and nastiest of LED lamps/bubs commonly have a 'fuse', usually in the form of a flimsy bit of foil.

When MCB's replace the fuse, the built in fuse in bulb would not rupture fast enough, and it would take out the MCB, but most BA22d and ES27 lamp holders are rated 2 amp, so even with a 6 amp MCB without the built in fuse, the bulb holder can be overloaded. ... Although ceiling rose rated at 6 amp, regulations say 16 amp is the limit, however I did have a Ikea bulb blow and weld its self in the lamp holder with a 16 amp MCB, ...

You are the only person I have ever heard of experiencing such a problem as a result of a bulb dying, so it is presumably a very rare occurrence, and probably almost unknown with LEDs. If it ever does happen, I would imagine that it would only be in relation to the lampholder and, even then, probably a consequence of the quality (or otherwise) of electrical contact between bulb and holder, rather than anything to do with the 'rating' of the holder - and I really don't think the 'rating' of a ceiling rose (or even a light switch) is of any relevance.

As a result I would always use a 6 amp overload, in my case RCBO's, but there is nothing to really force you to do that, OK ceiling rose rated 6 amp and they double as junction boxes, so should use 6 amp, but looking at them, they can clearly take a lot more.

Quite so. With LED (or even CFL) bulbs/lamps, there is clearly no need for a lighting circuit to be more than 6A (1A or 2A would probably usually be more than adequate!) but, as above, even if it's protected by a 16A device, I imagine that the chances of any consequent ';problem' are probably incredibly small.

Kind Regards, John

 

[ericmark]

Quite so. With LED (or even CFL) bulbs/lamps, there is clearly no need for a lighting circuit to be more than 6A (1A or 2A would probably usually be more than adequate!) but, as above, even if it's protected by a 16A device, I imagine that the chances of any consequent ';problem' are probably incredibly small.

I agree.