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.

 

[ImpededLoop]

1mm in a small domestic property. Easier to terminate and never seen thermal damage due to insulation. Installations should be designed to use materials without waste so 1.5mm in this scenario is wasteful in my opinion. Some will no doubt disagree

 

[bagpuss_24]

1mm in a small domestic property. Easier to terminate and never seen thermal damage due to insulation. Installations should be designed to use materials without waste so 1.5mm in this scenario is wasteful in my opinion. Some will no doubt disagree

Totally agree with ImpededLoop.
I've never seen thermal damage due to insulation either.

 

[telectrix]

1.0mm here for most applications. however,est to snap endof cable at termination esp. if screw on cable as opposed to cage

 

[Dustydazzler]

1mm for 99% of domestic jobs

Can't see any real justification to use 1.5 on domestic lighting these days unless it is a massive fooking house extremely long cable runs

 

[pc1966]

With LED lights it would have to be a massive house to reach the VD limit.

Yes, you might get to the Zs limit of a MCB and I'm not very keen on relying on the RCD part to meet disconnection, but for your typical 6A B-curve it is quite a lot (don't have OSG to hand to check but will look it up later).

Hmm, an RFC for lights in 1mm....

 

[Dustydazzler]

With LED lights it would have to be a massive house to reach the VD limit.

Yes, you might get to the Zs limit of a MCB and I'm not very keen on relying on the RCD part to meet disconnection, but for your typical 6A B-curve it is quite a lot (don't have OSG to hand to check but will look it up later).

Hmm, an RFC for lights in 1mm....

Nah...

Combined lighting and power circuits on 16a breakers all in 1.5

 

[pc1966]

Just checked, for 6A B-curve with 1mm (at full distributed load, very unlikely in LED case!) the OSG gives length limit as 68m, pretty big for most homes.

 

[Calebp43]

I would love for someone to make a case for 1.5mm cable as I have post this on a few places and haven't had anyone really make a good case for using 1.5 other than futureproofing.
Maybe it's inevitable that I will be using 1mm when Its my decision of course I will be mindful of all necessary factors though.
Thanks everyone who took the time too look things up and comment on this post. Really love that I came across this website.

 

[James]

I would love for someone to make a case for 1.5mm cable as I have post this on a few places and haven't had anyone really make a good case for using 1.5 other than futureproofing.
Maybe it's inevitable that I will be using 1mm when Its my decision of course I will be mindful of all necessary factors though.
Thanks everyone who took the time too look things up and comment on this post. Really love that I came across this website.

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.

 

[nicebutdim]

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.

It's the norm in NI - can't explain why, but a lot of people in the trade would have a fit at the idea of wiring domestic lighting in 1mm.

 

[WallaceP]

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.

 

[catdog1121]

I would love for someone to make a case for 1.5mm cable as I have post this on a few places and haven't had anyone really make a good case for using 1.5 other than futureproofing.
Maybe it's inevitable that I will be using 1mm when Its my decision of course I will be mindful of all necessary factors though.
Thanks everyone who took the time too look things up and comment on this post. Really love that I came across this website.

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

 

[Calebp43]

You have some decent points there i must say.

 

[pc1966]

If looking at 10A of lighting in some methods then yes 1.5mm makes sense and it is physically stronger.

But I would be astonished to see that load level in any domestic house with LED lights. You would expect to see different floors of any multi-storey building on separate circuits just to reduce the impact of a fault taking out ALL lights, so really 6A is more than enough unless its a massive hotel-size floor, etc (for LED at ~3A total that is equivalent to about 4kW of old filament lamps so about 40 traditional 100W points, say 15 rooms with two lights and a corridor with 10?).

 

[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?

 

[nicebutdim]

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

I'd guessed it did relate to domestic, but added that comment as a response to points raised in the quoted post.

 

[SJD]

Since I've not seen it mentioned (domestic context), how about 1.5mm2 feed from the consumer unit to one light switch to the next. Then 1.0mm2 from each switch to the light(s). And none of this loop in at the lights nonsense!

 

[brianmoooore]

Since I've not seen it mentioned (domestic context), how about 1.5mm2 feed from the consumer unit to one light switch to the next. Then 1.0mm2 from each switch to the light(s). And none of this loop in at the lights nonsense!

You haven't read post 18, then.
It all depends on the calculations. I use 1.5 if it helps meet the regs, but otherwise 1.0,

 

[nicebutdim]

Since I've not seen it mentioned (domestic context), how about 1.5mm2 feed from the consumer unit to one light switch to the next. Then 1.0mm2 from each switch to the light(s). And none of this loop in at the lights nonsense!

Funnily enough I do see similar in NI - 1.5mm from one point to the next, with switch cables in 1mm. I think this house was done that way, but looped at lights.

 

[SJD]

You haven't read post 18, then.
It all depends on the calculations. I use 1.5 if it helps meet the regs, but otherwise 1.0,

Sorry, I'd missed that post. I've done this on a couple of large domestic new builds (3-storey mansion, CUs in basement plant rooms). I've quite got to like it even for more normal sized houses, and indeed it does mean you always know the feed in/out at a switch, though I'm sure someone will tell me I'm contributing to the rise in copper prices.

 

[brianmoooore]

Feed in and out at switch is the new normal way of wiring lights, if just one or so lights per room on a single switch point. Where things get more complicated, I favour the multi way junction box, fitted somewhere reasonably accessible, with sometimes as many as 20 cables coming to it.
Nothing to do with 1.0 or 1.5 though.

 

[ian.settle1]

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

When I was at college many years ago one of the other apprentices told us that an electrician he worked with said when he wired lighting the minimum size was 2.5mm and sockets 4mm

 

[SJD]

When I was at college many years ago one of the other apprentices told us that an electrician he worked with said when he wired lighting the minimum size was 2.5mm and sockets 4mm

Lights in 2.5mm2 - are you sure this electrician wasn't really a builder?

 

[MJPD29]

1.5mm every single time. 1mm twin and earth is extremely rare in the north of Ireland and I'm yet to come across a property wired in it. Most wholesalers don't even stock it because no one buys it. Commercial and industrial applications I've used up to 10mm and street lighting applications 16mm.

 

[Pretty Mouth]

1mm for me, unless I'm wiring a palace.

• Cheaper
• Thinner, so can cram more into a busy backbox
• Easier to terminate: You get some tiny terminals on things these days - dimmer modules being a prime example. Getting more than 1 X 1.5mm in them can be impossible.

 

[LukeD]

1mm for me, unless I'm wiring a palace.

• Cheaper
• Thinner, so can cram more into a busy backbox
• Easier to terminate: You get some tiny terminals on things these days - dimmer modules being a prime example. Getting more than 1 X 1.5mm in them can be impossible.

THis is a issue .. terminal sizes are rubbish now

 

[UNG]

I hate fault finding on a lighting circuit wired in 1mm² cable you invariably end up spending more time fixing broken / snapped cables than finding the fault
Another argument for using 1.5mm² especially on the upstairs lighting is that it is quite often used to supply the aerial amp in the loft, although some of the newer aerial amps can be supplied though the co-ax using an inline power injector from a socket behind the TV