Cable for 150W Floodlight

[petesouthwest]

The wiring instructions for the 150W floodlight I have just bought say to use 3 core 1.5mm cable. I thought that size cable was rated up to

3600W and 15A, why so much?

Pete

[Tim S]

So it's adequately protected by the last/smallest fuse/MCB which supplies that circuit, which is *likely* to be 6A minimum and might be higher depending on your wiring.

If you spec the cable to be good for 1A, just because that's good enough for the 150W lamp, either the lamp or the wiring will (Sod's Law) develop an esoteric fault causing many amps (but less than that required to trip the circuit protection in any sane time) to be drawn, or more likely, someone else comes along and adds a couple more big lamps to your bit of wire later.

Either way, the fuse doesn't blow and your cable gets hot and catches your house on fire.

You might be able to argue for a smaller capacity cable if you supplied the cable and lamp off an FCU but as someone can trivially re-fuse those to

13A, best to assume the worst case, and cable is cheap.

There's volt drops too, but the first problem is the one to really worry about...

That's *an* explanation anyway,

Cheers

Tim

[Andrew Gabriel]

[...]

Another might be that anything smaller won't make a waterproof seal in the cable gland.

[meow2222]

1mm is fine for limited runs on 5/6A circuits. If its on a lead and mains plug, 0.5mm is fine.

NT

[Christian McArdle]

These things are likely to be on the end of a long run of cable. Although

1mm will carry the current and even have acceptable voltage drop for the 150W load, it will soon have issues with earth loop impedence once the length gets past a certain point.

If you know the length of the cable, it is easy to calculate the required cable. Go to:

type in the details. For external cabling I would use SWA/XLPE.

Christian.