Number of lights per circuit

[EvilDonut]

I'm doing first fix for a rewire in my house and I've run a single 1.5mm cable from the CU to upstairs to serve all the lights on the floor.

The circuit will serve 9 LED bulbs in the hallway/landing/loft plus 7-8 LED down lights in the bathroom.

Total demand, even if they're all switched on at once, about 80 watts. So virtually nothing!

However, whilst doing some more research I've uncovered a troublesome regulation that all light sockets must be assumed to consume 100 Watts (can you even purchase these any more?) and no more than 12 per circuit total.

So my question is - do I really, really, have to run another circuit upstairs to allow for this outdated reg? How much of an issue is it if I serve all 17 outlets on one 1.5mm circuit?

I'm getting a qualified electrician to test whole installation and fit the CU btw.

 

[EFLImpudence]

No, you don't. Ignore it.

You wouldn't anyway? Did you see anything about only counting 66% of the total as they wouldn't all be on at once?

1700W x 66% would only be 4.72A. Easily within the 20A your cable can carry.

 

[JohnW2]

However, whilst doing some more research I've uncovered a troublesome regulation that all light sockets must be assumed to consume 100 Watts (can you even purchase these any more?) and no more than 12 per circuit total.

Forget that!

How much of an issue is it if I serve all 17 outlets on one 1.5mm circuit?

No issue, IMO, but 1.0mm² cable would be more than adequate.

I'm getting a qualified electrician to test whole installation and fit the CU btw.

It doesn't work quite like that. You are talking about a 'new circuit', which is notifiable work, and you can't avoid the need to notify (and pay substantial notification fees) by getting an electrician (who hasn't installed the circuit) to just test it.

Kind Regards, John

 

[winston1]

I'm doing first fix for a rewire in my house and I've run a single 1.5mm cable from the CU to upstairs to serve all the lights on the floor.

Why? Lighting circuits only need 1.0 mm cable.

 

[EvilDonut]

Did you see anything about only counting 66% of the total as they wouldn't all be on at once?.

Yep, I knew about the diversity - but my understanding was that the 12-light limit was already taking that into account. Sounds like that 'rule' is not actually a rule though, so doesn't matter.

Forget that!

Excellent Upon reading the most recent on site guide I can't find any reference to this rule, so it must have been and old rule-of-thumb or something.

Why? Lighting circuits only need 1.0 mm cable.

I (pointlessly?) over-specced the cable so that I'd get less voltage-drop and decrease the chance that I'd stray outside the allowable 3%.

 

[JohnW2]

I (pointlessly?) over-specced the cable so that I'd get less voltage-drop and decrease the chance that I'd stray outside the allowable 3%.

Fair enough - but that 3% is only a guideline, not a regulation - and, in any event, you're probably going to be nowhere near even 3% (even with 1mm² cable) with your modest current requirements. To work out what the voltage drop might be IF you filled your house with 100W incandescent lamps/bulbs would, IMO, be just plain daft.

Kind Regards, John

 

[JohnW2]

Excellent Upon reading the most recent on site guide I can't find any reference to this rule, so it must have been and old rule-of-thumb or something.

I'm not at all sure that it has ever been a regulation - certainly not for a long time. I have a feeling that it is mentioned in my (old) OSG - but, as you say, even if it was, that would only have been a guideline, not a rule/regulation. In the days when most lights were 60W or 100W, it probably made some sense - but not today.

Kind Regards, John

 

[winston1]

Fair enough - but that 3% is only a guideline, not a regulation - and, in any event, you're probably going to be nowhere near even 3% (even with 1mm² cable) with your modest current requirements. To work out what the voltage drop might be IF you filled your house with 100W incandescent lamps/bulbs would, IMO, be just plain daft.

Kind Regards, John

Even 3% or 30% would not matter with mode lights using switch mode techniques.

 

[JohnW2]

Even 3% or 30% would not matter with mode lights using switch mode techniques.

I agree totally. Although there are some who will undoubtedly say that "regulations are regulations" (or, in this case, "guidelines are guidelines"), I find it very hard to get particularly excited about VD in nearly all situations. In general, if a circuit satisfies the Zs requirements, I'm happy that VD is not going to be a significant issue.

Kind Regards, John

 

[securespark]

The 12 light limit came about because the 100W x 12 = 1200W and 1200/240 (back then) = 5A, the usual lighting fusewire for the old 3036's.

However, in the green OSG (don't have the latest), it still advises the following:

Current demand to be assumed for lighting outlet: Current equivalent to the connected load, with a minimum of 100W per lampholder.

In other words, if the lampholder is < 100W, treat it as a 100W load, if it is more, use the higher figure.
I know the OSG is not regs and there are no reg numbers alongside this advice.
It seems archaic, but this advice may well have been left in so that lighting circuits are not extended hopelessly. Not necessarily from an overload POV, but to consider the "inconvenience" of a huge string of lights extinguishing in the event of a fault.

 

[securespark]

In general, if a circuit satisfies the Zs requirements, I'm happy that VD is not going to be a significant issue.

Do you include TT installations in that?

 

[JohnW2]

Do you include TT installations in that?

Obviously not - but I would still think in terms of a corresponding rule of thumb - i.e. if the R1+R2 of the circuit was such that it would satisfy the Zs requirement were it a TN installation with a typical Ze, then I would not usually be concerned about VD issues.

Having said that, thanks to some bonded metal water pipes (which may also be bonded to a neighbour's TN earth), I have a TT installation with a Ze of about 0.32Ω - so I suppose I could adopt the previously-stated approach with my TT installation

Kind Regards, John

 

[JohnW2]

It seems archaic, but this advice may well have been left in so that lighting circuits are not extended hopelessly. Not necessarily from an overload POV, but to consider the "inconvenience" of a huge string of lights extinguishing in the event of a fault.

Possibly, but I personally doubt it. I think it more likely that the OSG (as we've both said, this has never, at least not in my time, been in the regs) simply has not 'kept up with the times'! In these days of (as some would have called it!) "multiple ceiling torches", I would imagine that there are a good few rooms which would, just in themself, exceed a 12-lamp limit!

Kind Regards, John

 

[securespark]

To be fair to the OSG, some of what is contained in its pages is cross-referenced with a regulation number.

 

[JohnW2]

To be fair to the OSG, some of what is contained in its pages is cross-referenced with a regulation number.

Indeed so - and the fact that there has never been any regulation number referenced by the bit of guidance we're talking about underlines the fact that it is just 'guidance', unrelated to any regulation.

Kind Regards, John