Why 1.5mm cable for lighting circuits?

aesmith · 21 Dec 2014

Hi,

Our house was rewired in 2011. For some reason the lighting circuits are all done in 1.5mm rather than 1mm. Any ideas as to why that might be?

Tony S

ericmark · 21 Dec 2014

Likely nothing more than what the electrical whole sale outlet had in stock. Because 1.5mm² has more uses than lighting more of it is made and so very little difference in price.

It could be volt drop but in real terms not normally a problem. It could be that the builder intended to use larger than the standard B6 MCB but really there are just too many items rated at 6A and to use more than 6A is unusual.

With 1.5mm² around 44 meters is the limit with 1mm² around 29 meters is the limit assuming using a B6 MCB it would need to be a large house to need more than 29 meters. Move up to a 10A MCB and the length of 1mm² drops to 16 meters.

Many electricians just don't want to be bothered in the calculation required. Simpler to play safe and use 1.5mm² Imagen the work involved if on one house when it came to inspection and testing it failed. Just not worth the chance use 1.5mm² and really no worries.

RF Lighting · 21 Dec 2014

Nope. Some electricians seem to think it's better. I worked for a firm for years who only used 1.5 for lighting.

Completely unnecessary. Nothing but a waste of money.

EddieM · 21 Dec 2014

Reasons as stated above, for the sake of £5 per 100m doubt that they really CBA.

Lectrician · 21 Dec 2014

1mm for me too. 1.5mm is pointless, and fills a pendent and switch back box.

Some seem to think 1mm is "against regs" in their words.

aesmith · 21 Dec 2014

ericmark said:
With 1.5mm² around 44 meters is the limit with 1mm² around 29 meters is the limit assuming using a B6 MCB it would need to be a large house to need more than 29 meters. Move up to a 10A MCB and the length of 1mm² drops to 16 meters.

Would that limit arise from disconnection times, ie loop impedance? The certificate shows plenty of headroom on all three circuits (which reminds me, I wonder why they split the upstairs lights over two circuits).

The reason for suddenly wondering is that I'm adding a second light fitting in the kitchen, so adding about 2.5m to the length. However that which will still be easily within spec using 1mm for that last bit. (After all, the CPC's the same size in any case).

flameport · 21 Dec 2014

The ~29m limit is due to volt drop which should not exceed 3% on lighting circuits.
However that also assumes a full load of 6A located at the end of the cable.

In reality, lighting circuits have the load distributed at various points along the cable and often are nowhere near 6A even with every light switched on.

aesmith · 21 Dec 2014

flameport said:
The ~29m limit is due to volt drop which should not exceed 3% on lighting circuits.

Cheers. Interestingly if I take the circuit's R1+R2 of 0.91 and divide by the cable resistance for 1.5 T&E then that comes out at 30m - ie just over your spec for 6A load on 1mm. Are there accepted figures for resistance from screw terminals, or is that negligible?

In any case we'd draw nothing like 6A on the circuit with everything on.

ericmark · 21 Dec 2014

flameport said:
The ~29m limit is due to volt drop which should not exceed 3% on lighting circuits.
However that also assumes a full load of 6A located at the end of the cable.

In reality, lighting circuits have the load distributed at various points along the cable and often are nowhere near 6A even with every light switched on.

I would agree unlikely to have max current at end of cable with lights of course it is fixed unlike a ring final so one would have to work it out. There was a consideration that every light was 100W at one time but today even 60W would be over egging it. I would expect around 40W max per fitting today.

However people do go daft I saw one living room with ten 50mm recessed ceiling lamps at 50W each that could mean 2A just for one room.

With LED lighting with a voltage range of 150 - 250 volt really there is no volt drop problem. But with old 58W fluorescent tubes I can see why 6.9 volt limit.

But personally I would not worry about volt drop if the loop impedance passes then good enough if a light fails to work swap the fitting to one able to take the volt drop.

I have never tested volt drop on an emergency fluorescent fitting not sure if there is a dead area? Clearly no volts it works and 230 volts it works I would expect it would still work at 180 volt but not sure what would happen with 100 volts.

But in real terms only seen 100 volts when the fan belts failed on the generator and it closed down on over heat. Because the load increase was so fast on the remaining generator it did not trip but ran slow but can't really see this happening to a domestic supply.

OwainDIYer · 21 Dec 2014

ericmark said:
With 1.5mm² around 44 meters is the limit with 1mm² around 29 meters is the limit assuming using a B6 MCB it would need to be a large house to need more than 29 meters.

I'm already on my second 50m reel of 3+e in a 3-bedroom flat (and I've used 1mm throughout). But then I have 10 lighting 'points' in the lounge ...

Owain

aesmith · 21 Dec 2014

Our last house had 8 x 50W halogens in the kitchen which is a pretty shocking amount of power just for one room, but nowadays that would be 8 x 4W LEDs or similar.

Here I reckon our downstairs lights total we have 11 light fittings downstairs which currently total something like 125W. Is 100W per fitting the standard sizing used? If we actually maxed ours out, 150W bulbs in each normal lamp holder and halogen GU10s in place of LEDs, we'd total just over 6A for the downstairs including the new light I fitted today.

I don't have a decent low ohm meter at the moment, so have to take loop impedance on trust by making an estimate. Starting from the test results at time of rewire and assuming my new wiring is on the end of the longest branch (which I think it is) then it should be around 1.0 ohm.

JohnW2 · 21 Dec 2014

aesmith said:
Interestingly if I take the circuit's R1+R2 of 0.91 and divide by the cable resistance for 1.5 T&E then that comes out at 30m ...

That's not correct, because you have assumed that the 1.5mm² T+E has a 1.5mm² CPC, whereas it actually has a 1mm² CPC. The R1+R2 of 1.5mm² T+E is about 36.5 mΩ/metre (14.5 for R1, 22 for R2), hence a calculated answer of about 24.9 metres (not 30 metres) for your circuit. Is this a very large house?

Kind Regards, John

aesmith · 21 Dec 2014

Actually I think I've used duff data that doesn't apply the uplift for temperature rise. My figures were 0.0121/m for 1.5mm and 0.0181 for 1.0mm, but those are at 20 deg C.

Which leads me to ask, when an electrician records R1+R2, or any other variable relating to earth look, is the record the actual measured value or is it corrected for temperature rise? I can't quite see how it would work either way without the temperature at time of measurement been known.

Either 24 or 30m could make sense in terms of the size of the house, depending on cable routes. Straight line for the consumer unit to the kitchen lights will be around 15m, and it's looped at the switches which adds to the overall length.

JohnW2 · 21 Dec 2014

aesmith said:
Actually I think I've used duff data that doesn't apply the uplift for temperature rise. My figures were 0.0121/m for 1.5mm and 0.0181 for 1.0mm, but those are at 20 deg C.

Fair enough. I was quoting BGB figures, which relate to 70°C. I guess it was just a co-incidence that you ended up with a figure very close to what you would have got with both R1 and R2 relating to 1.5mm² conductors (at 70°C).

aesmith said:
Which leads me to ask, when an electrician records R1+R2, or any other variable relating to earth look, is the record the actual measured value or is it corrected for temperature rise? I can't quite see how it would work either way without the temperature at time of measurement been known.

You'd have to ask an electrician, but I would imagine that they probably simply record what they measure, usually on unloaded circuits, hence with conductors around ambient temperature. If that is what they do, as you imply, it could be a somewhat iffy practice, since a Zs which was fine under those 'cold' conditions could rise to an unacceptably high figure if the circuit ever got heavily loaded (hence much hotter). Maybe some of them can explain this to us!

Kind Regards, John

ericmark · 21 Dec 2014

Your question is very valid as I know my boss never calculated volt drop it was me as the site electrician who would suggest maybe he should use thicker cable and until the loop impedance meter was used really we had only a guess at the result.

In the main it was the limits for the MCB which we looked at rather than volt drop but this was before Part P.

When Part P arrived I for one got worried that something I fit today may come back and bit me in 5 years time and some one could take my installation certificate and use that data to demand we correct an error in 5 years time.

So with the advent of Part P I started to work out the volt drop. But only really with customers with in-house installations the volt drop problem could be solved by simply swapping lamp type.

Pre-2004 I am sure there were many installations where little regard was given to volt drop.

In my own house I would not be worried at all. If volt drop proves a problem I would simply change the light fitting. For a customer one has to consider the chances that some one will come back to you in 5 years time and so more care is required.

We on this forum had a discussion about volt drop and as pointed out much depends on the DNO supply. If your supply voltage is 230 - 240 then excessive volt it not a problem but if your supply drops to 216.2 volts then the further 3% is likely a problem.

Personally I would not worry if it works and is safe then that's good enough if loop impedance is under 7.6Ω and light works then no worries.

diynot

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Why 1.5mm cable for lighting circuits?

EddieM

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