Stephen wrote:
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


"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.

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?

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)

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

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

http://wiki.diyfaq.org.uk/index.php?title=Cables#T.26E

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.

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?

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.

Is it commonly used for radials or do pros use 2.5mm^2 for everything?

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)

At least that way you are protected if someone decides to do something
wrong in the future (e.g. spur off a spur).

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

--
Cheers,

John.

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