What size cable do you use for new lighting circuits?

[Calebp43]

So im a third year apprentice and should be qualified next year (whoopee). One thing has always made me curious and that is what cable size should we be using to rewire or in a new build for lighting in 2021? OF COURSE this depends on the load and how it's installed etc but in general what size cable would you use?
Out of the three people I worked with only one of them used 1mm. I kind of levitate towards that side as well and I'll explain why right now -
1mm cable can handle 8amps when surrounded all sides with thick insulation. That's 8x230 so 1804 Watts. 16amps clipped direct, that's 3680 watts. If anything we are all gonna be seeing more and more LED's with low power consumption and even if we don't... Well we could still have 26, 70 watt bulbs in one circuit if the cable is surrounded in insulation... Which it rarely is. Usually it's ran on top of the insulation anyway. That doubles too 52 when clipped direct! I know the price difference isn't that much but it all adds up.
I would love to know the cable size you qualified electricians use and maybe if you have time a little comment explaining why. Like I said I should be qualified next year and want to evaluate everyone's opinions before I go ahead and make my own decisions in the future.

 

[pc1966]

Just to add to the cable length debate, 1mm T&E has R1+R2 = 36.2 ohm/km (OSG Table I1) and a 6A B-curve MCB has Zs limit of 5.87 ohm (OSG Table B6) so if we allow 0.8 ohm as your typical max TN-S Ze value the disconnection length limit is 140m (the 68m in OSG Table 7.1(i) seems to be on volt-drop).

Working backwards, the OSG gives the drop for 1mm at 44 V/A/km, if we keep the silly 3% requirement for lights even for LEDs, then it is 6.9V. If we draw the line at 140m Zs limit, then our max distributed load is 2.24A or about 500W of LEDs which is quite a lot for most domestic floor sizes.

If we decided to draw the line at a 100m reel of 1mm T&E (simple on the job test of length) then obviously Zs is fine on 6A B-curve, and the max distributed lighting load load is 3.1A or about 720W total.

Of course outside of domestic cases you might have a lot of simultaneously switched lights so your MCB limits might have to be adjusted to allow for in-rush, etc.

 

[Dustydazzler]

The only people that I have ever known to make a case for 1.5 have been “electricians” who say all lighting is done in 1.5 it’s the way I was shown and it is the only way.

Going back to my apprentice days we used to fit a lot of 500w flood security lights , sometimes 3 on the same lighting circuit , couple that with a dozen or more 50w halogen downlights etc. We started using 1.5 and 10amp mcbs because sometimes when the floods all got activated it would knock out a 6a mcb.
Other than that there was no reason to pull in 1.5 , also back then I guess the price difference was so small you could throw in 1.5 at very little extra expense.

Today with LED floods and LED lights most circuits will barely pull 1 amp , so 1mm will cover 99.9999% of modern day homes

 

[littlespark]

You will get the odd member of the public who buys a new house, finds that some LEDs lamps have failed and replaces them with what they have brought from old home…. Your nicely calculated maximum load has suddenly trebled!
What if they do it 2 or 3 times, and the supply is only 1mm?
I know, extreme situation, but it might happen.

Not right, I know, but I’ve seen some new builds where it’s 1.5 looped to switches, and 1mm from switch to light.

You would know an unmarked switch wire when it came to second fix

 

[HappyHippyDad]

As I write this, 18 people have replied to the poll, 7 of whom have voted for 1.5mm. I would love to know their reasons for using 1.5mm as standard?

 

[Dustydazzler]

As I write this, 18 people have replied to the poll, 7 of whom have voted for 1.5mm. I would love to know their reasons for using 1.5mm as standard?

There is absolutely no reason to use 1.5 for a standard lighting circuit , it is just old habit

you would only ever use 1.5 for NON standard lighting , like as I mention before a massive fooking house with crazy long lighting circuits or if you picked up a load of old 500w halogen lights from the boot sale and were hell bent on using them

 

[Moley]

Depends if the house owner has an 'attic garden'. If they have then possible 6mm to cover all the lights

 

[pc1966]

You will get the odd member of the public who buys a new house, finds that some LEDs lamps have failed and replaces them with what they have brought from old home…. Your nicely calculated maximum load has suddenly trebled!
What if they do it 2 or 3 times, and the supply is only 1mm?

On a 6A MCB then worst case they trip it, or more likely see a bit more dimming/brightening than expected when switching other lights.

Still safe.

 

[brianmoooore]

AFAIK, we're still supposed to design lighting circuits with BC or ES lamps assuming 100W per point, so the above shouldn't arise.

 

[pc1966]

AFAIK, we're still supposed to design lighting circuits with BC or ES lamps assuming 100W per point, so the above shouldn't arise.

At some point the regs will catch up with LED lighting so the 100W/light and 3% VD will be revised, but until then even if you assume 100W per fitting most flats or building floors are still well within a 1mm / 6A circuit.

I have used 1.5mm and it has the physical strength advantage, but often LED lights with internal connectors are just more of a pain to get the 1.5mm in to place so really now would look to 1mm first and foremost, unless something very unusual applied.

 

[pc1966]

1mm FTW, there is a stupid regulation about not using 1mm for 'power' circuits but you can still use it for lights and fans which seems to come from the bigger is better mentality.

I have never understood the rational for that, as the larger T&E still has 1mm CPCs.

The only thing I can guess is it comes down to the risk that DIY folks would start add 13A sockets on to a 1mm circuit for loads assumed to be much lower, etc. But trying to legislate against rubbish workmanship and poor/no design is not really something for those following the regs.

 

[brianmoooore]

As I've mentioned on here before, I tended to specialise on larger domestic properties, and have occasionally used 1.5mm2 from the CU to the first point in a lighting circuit 'cluster' some distance away.

 

[nicebutdim]

Ok here is one, I work both domestic and commercial, I often find commercial with 10A lighting circuits (Also see it in domestic) With more and more people piling more and more insulation into their lofts the derating factor of 0.5 could be achieved that would take it below 10A. Because of this I use 1.5mm when dealing with circuits with 10A lighting just to be safe..

Having said the bullc**p above which 1mm would be fine for most scenarios, here is the two real reasons I only use 1.5mm..

1. I don't have the space to carry around 1mm and 1.5mm T+E as well as 1mm 3core and earth and 1.5mm 3core and earth and then there is singles... so I just carry 1.5mm
2. I find 1.5mm is more resilient in a terminal, have a number of times with 1mm when you do the tug test the copper breaks because of the intents from possibly being over tightened. I don't seem to get this issue with 1.5mm

maybe if copper prices keep going up then I might change my mind but for now mainly really for space and the advantage that I don't have as much stock which is more efficient. The cost difference between 1mm and 1.5mm is not yet enough that I would bother carrying both..

Also although its seems like 68m is a long distance its surprising on a larger house how it can start to get close, 1.5mm you don't have to think about it, its also really difficult to judge how long the run is (Probably me being lazy though)... Would not dis anyone for using 1mm for lighting its just personal preference for the 2 above reasons why I use 1.5mm

Most lighting circuits I work on are 1.5 or occasionally 2.5mm

 

[James]

Most lighting circuits I work on are 1.5 or occasionally 2.5mm

2.5mm, often the correct choice for a warehouse with rows of 300 to 800w lights.

 

[pc1966]

2.5mm, often the correct choice for a warehouse with rows of 300 to 800w lights.

Good point! The assumption that I (and probable most) made was the poll was about domestic lights.

 

[brianmoooore]

Good point! The assumption that I (and probable most) made was the poll was about domestic lights.

I consider that to be a reasonable assumption. If we're including commercial, then does that include floodlit stadia with towers of several kW each, or the lights around the circuit at yesterday's GP?