On 24/04/2017 14:00, Jeff Layman wrote:
On 23/04/17 15:29, John Rumm wrote:
On 23/04/2017 07:59, Jeff Layman wrote:
On 23/04/17 03:18, John Rumm wrote:

Its not usually a problem with lighting circuits since the cable is
significantly over specified in the first place.

Do you know if this is a hangover from the 20s and 30s when "lighting"
circuits were often used for devices with a moderate power consumption?

Not so much that, more a case of 1mm^2 is the smallest T&E used and
anything smaller would start to get a bit fragile, not to mention the
resistance would creep up - hence more brightness variation and longer
disconnect times in the event of a fault.

Also there is a (not often used) standard lighting circuit protected
with a 10A MCB, and that is somewhat closer to the 1mm^2 11.5A "in
conduit" rating for the cable.

I can most certainly remember in the 50s when my mother used to iron by
plugging it (bayonet plug) into a two-way connector in the ceiling
light. As far as I remember, the only time the 15 amp wall socket was
used was for a 2 or 3-bar heater.

Indeed... A reflection of reality that in many properties, the only
electrical provision installed was for lighting. Sockets etc came later.

With even lower power needed for lighting in these days of LEDs (and
CFLs), isn't rather a lot of metal being wasted unnecessarily in the
wire in lighting circuits?

I suppose you could install a 0.75mm^2 T&E if there were such a thing,
but I expect in most cases the savings would be marginal.

Well, although not twin and earth, there is 0.5mm^2 cable:
https://www.tlc-direct.co.uk/Products/CA0dot5F3B.html

That's flex and not really suitable for fixed wiring.

I see this is rated at 3 amps, which is much more than is needed for LED
lighting. I reckon you'd be hard-pushed to use even 1 amp for LED
domestic lighting.

Unless they house is wired with low energy fittings that preclude the
use of other lamp types, then you still need to provision assuming 100W
at each location (or the actual wattage if greater).

The price is £19.00 for 100 metres. Compare that to
the 0.75mm^2 equivalent which is £29.00 per 100 metres

Quite a lot of savings to be made if you were wiring up hundreds of new
homes.

Unfortunately, as far as I can see 0.5mm^2 T&E is not made - or at least
is not available in the UK.

Not very practical though is it?

If we use a B3 MCB and assume a maximum circuit load of 3A that means we
will drop around 264mV per metre of cable. We are allowed 3% of 230V or
6.9V, hence we have a maximum circuit length of only 26m (and note that
includes the drops to the switch wire as well)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/