Lighting - 1.5mm2 or 1mm2 for switched line?

[echoes]

I'm starting to plan my rewire, and am thinking about my lighting circuits.
I will have upstairs and a downstairs circuits (+ garage and external), each protected by RCB and 6A MCBs. (3 bed house).
Please take LABC notification as read.

Now she-who-must-be-obeyed likes her lighting, well.., solar, so I was going to use 1.5mm2 T & E for both circuits to provide above-ample capacity.

Nothing controversial so far I hope, but would it be acceptable to use 1mm2 cable for switched lines to each luminaire? Not only would this help reduce crowding when looping in/out at the switches, but it would also conveniently help distinguish between the loop and switched cable runs.

Your comments gratefully received as always...

 

[electronicsuk]

There is no reason you can't do this on a 6A protective device, although it isn't particularly good practice to mix different conductor sizes like this IMO, and it seems especially daft to do so on a rewire.

Many on here take the opposite view in that 1.5mmsq provides plenty of capacity even with insulation derating factors, but I prefer 1.0mmsq in that it's easier to work with, especially given the very small amount of space for cable termination inside modern light fittings.

 

[echoes]

That was quick! - I was uncomfortable myself with mixing sizes on the same circuit, hence the post.

On fag-packet calcs so far, 1mm2 would actually suffice for the lot, but I do like a job done properly and a rewire seems the ideal opportunity to do it in 1.5mm2.

 

[ban-all-sheds]

1.5mm² 3C+E is almost as big as 4mm twin & earth....

 

[echoes]

1.5mm² 3C+E is almost as big as 4mm twin & earth....

Don't I know it!

I like the idea of an easier rewire, but then I also like the idea of spare capacity. But which is best....?

What does 'sheds think about smaller conductor sizes for switched line?

 

[LoveRocket]

The protective device must be sized to protect the smallest size cable used in the circuit - so if you're going to use 1.0mm for the switch drops, you may as well do the whole lot in 1.0mm. (I know what you're thinking - there'll only be one luminaire on the end of each 1.0mm, but the rules don't work like that!)

A 6amp MCB gives you nearly 1.5kW to play with, so unless the missus wants to go on a crazy downlighter binge, then you shouldn't have a problem.

 

[echoes]

...so unless the missus wants to go on a crazy downlighter binge

That's exactly the danger that may constantly haunt me for the next 20 years, and I'm just thinking that 1.5mm2 and deeper switch boxes (at least it's breeze block) might be a price worth paying...

 

[ban-all-sheds]

The protective device must be sized to protect the smallest size cable used in the circuit - so if you're going to use 1.0mm for the switch drops, you may as well do the whole lot in 1.0mm. (I know what you're thinking - there'll only be one luminaire on the end of each 1.0mm, but the rules don't work like that!).

But they do for unfused spurs on socket circuits.

 

[LoveRocket]

1.0mm will be fine for 10amps as long as it's not completely surrounded by thermal insulation. But if you really want to go to town then by all means, use 1.5mm

 

[LoveRocket]

The protective device must be sized to protect the smallest size cable used in the circuit - so if you're going to use 1.0mm for the switch drops, you may as well do the whole lot in 1.0mm. (I know what you're thinking - there'll only be one luminaire on the end of each 1.0mm, but the rules don't work like that!).

But they do for unfused spurs on socket circuits.

That's a very good point, Ban. It's one of the anomalies that results from the regs still permitting these daft ring circuits - hopefully when the 18th edition comes along someone at the IET will have the balls (or ovaries) to finally ditch ring finals.

As an aside, would it be deemed compliant to install a circuit consisting of one DSO on 2.5mm cable on a 30/32a OPD?

 

[echoes]

OK - no strong opinions coming over, so I'll likely give my cold chisel an extra sharpen and do the job in 1.5mm2 throughout and feel inwardly wholesome. Then I can have the opportunity and pleasure of changing halogen bulbs with monotonous regularity.

As an aside, would it be deemed compliant to install a circuit consisting of one DSO on 2.5mm cable on a 30/32a OPD?

Ring would be OK, but for a radial the OPD would be rated higher than the current rating of the cable, so I would say NO. Even if it did technically comply by assuming the max load would be 2 x 13A. I would use a 20A OPD.

 

[echoes]

...these daft ring circuits...

You're preaching to the converted!

 

[aptsys]

You could install an additional circuit where there may be a greater load - e.g. the kitchen. That way you can keep everything in 1mm.

 

[echoes]

You could install an additional circuit where there may be a greater load - e.g. the kitchen. That way you can keep everything in 1mm.

Could do, but it's easy to get carried away with separate circuits for this, that and the other. If I took all 'sheds's suggestions (from another posting) I'd be creeping towards a 20+ way CU, and that's not going to happen. Reasonable suggestion though!

 

[ban-all-sheds]

cold chisel