twin and earth 1.5mm^2

[Stephen]

Hello,

I was reading the excellent wiki faq and I noticed it says that

1.5mm^2 T&E can be used for high power lighting circuits and 16A radials.

What is the definition of "high power" lighting: several 500W outdoor PIRs? Before anyone shouts about using 500W bulbs, I use 150w myself ;)

I think I've always used 1.5mm^2 rather than 1.0mm^2 for all lighting but then I've only ever used a bit. If I was wiring several hundred metres a day, I'm sure I would quickly use the cheapest and safest cable. Is 1.0mm^2 commonly used?

Can I ask about the 16A radial bit? I know that 1.5mm^2 T&E is rated for IIRC 16A (provided it is clipped direct to the wall and not in conduit, insulation, etc.) but doesn't it have a smaller earth conductor? I thought it had 1.0mm^2 "earth" whereas 2.5mm^2 T^E has a

1.5mm^2 central conductor doesn't it? Doesn't the smaller earth wire have implications? I remember reading the "taking electric outdoors" and I am sure earth impedance was an issue there. Why isn't the smaller earth an issue on 16A radials?

Is it commonly used for radials or do pros use 2.5mm^2 for everything? At least that way you are protected if someone decides to do something wrong in the future (e.g. spur off a spur).

Thanks, Stephen.

[Owain]

usually any circuit above 6A for lighting is regarded as high power. There is (was?) a prohibition on using SES and SBS lampholders on such circuits.

Wiring to outbuildings etc is often longer distance than circuits inside the house. Even using extension leads can be sufficient to have an excessively high impedance.

The smaller earth conductor is not /usually/ an issue because it will only carry large currents in the event of a fault, and a high current fault will be cleared by the cutout fairly quickly *if* the impedance isn't too high. The energy the cable has to carry is a function of current x time.

Not protected, because a spur off a spur could be carrying 2 x 13A =

26A, or more if 2 double sockets are spurred, whereas 2.5mm is usually good for about 20A (depending on installation method etc).

I don't think I'd use 1.5mm for a 16A radial but then the only application for a 16A radial would likely be using ceeform industrial connectors. The two standard circuit configurations are 2.5mm / 20A or

4mm / 32A, with floor area limits.

Owain

[John Rumm]

"normal" lighting circuits are typically protected at 6A, however you can also use a 10A MCB in some cases. Both 1.0mm^2 and 1.5mm^2 are able to take the current, however the voltage drop is larger on the smaller cable. Hence if you have a 10A circuit and longish cable runs you would probably be better with 1.5mm^2 cable.

Yes. Its current carrying capacity in a "clipped direct" mode of installation (that includes buried in plaster) is 16A which gives plenty of headroom. You would need quite a number of de-rating factors to run into current carrying capacity problems. Circuit length can be more of a problem if you are pulling higher currents though - especially as the

17th edition has reduced the allowable voltage drop for lighting circuits to 3% (from 4% in the 16th edition)

20A in fact, 1.0mm^2 can do 16A clipped direct.

conductor? I thought it had 1.0mm^2 "earth" whereas 2.5mm^2 T^E has a Yes, 1.0mm^2 typically has a 1.0mm^2 CPC, 1.5 also has a 1.0mm^ CPC.

The time that the CPC size matters is in the case of a fault - it does not need to carry the full circuit current in the way the main conductors do. Under fault conditions it will need to carry the full fault current - but only for a very short time. So basically as long as it can open the protective device before vaporising then its ok.

I presume you mean radials feeding sockets etc, rather than lights?

IME, not that often. Often radials for power circuits are protected at

20A, which does not really leave much margin for error with 1.5mm^2. It might turn up on things like immersion circuits where the load is only 3kW.

2.5mm^2 is not much more difficult to route, or that much cheaper to make it desirable to avoid using it where possible. (compared to say a

32A radial where 4.0mm^2 cable is much harder to work with)

(personally I very rarely buy 1.5mm^2 T&E)

Even with 2.5mm^2 you are not fully protected from that, but the chances of cable damage occurring are much lower.

[Stephen]

Sorry for the late reply (holidays etc.).

Thanks for the info. BTW, why should ses and sbs lamp holders be prohibited?

[John Rumm]

because they don't have adequate current carrying capacity to be protected at more than 6A.

[Stephen]

Thanks, I see. A light bulb in an SES fitting that took 6A would be pretty bright, wouldn't it!

Thanks, Stephen.

[John Rumm]

Or it could be the lamp fitting was being shorted by someone changing the bulb with the lamp switched on (trying to stuff a SES lamp into a mini BC fitting tends to do it quite nicely).