On 07/11/2016 01:23, Bill Wright wrote:
On 06/11/2016 09:03, Roger Hayter wrote:

Unless you have an installation method that results in a big de-rating
on the cable, 1mm^2 is usually more than adequately protected by a B10
MCB. So I would not worry.

OK. Suppose a load of 1kW, and 3kW. How hot would 1mm cable get in each
case, assuming it wasn't in an airless place?

Those are impossible questions to answer without exquisitely detailed
knowledge of the actual installation - i.e. the length of the circuit,
and what de-rating factors apply all the way along it.

However you can look at it the other way.

If you start with the tabulated ratings for the cable:

http://wiki.diyfaq.org.uk/index.php/Cables#Cable_Sizes

You will note there are a number of different maximum current ratings
specified for 1mm^2 T&E. They change depending on the installation method.

Installation methods that allow for good cooling of the cable, like
being buried directly in masonry, or being clipped to a surface in free
air (both method C), will allow the fastest rate of heat loss from the
cable and hence the highest current.

The current limits are set empirically such that the temperature of the
cables conductors at that load will not exceed the temperature at which
the insulation would either be damaged, or at least start to experience
accelerated ageing. For normal PVC clad cables the limit temperature is
70 deg C.

The temperature of the cable actually achieves will be proportional to
the power being dissipated in it. That will be dictated by I^2 r. The
squared term in there will mean that the temperature rise will be non
linear with the lions share of the rise happening near to maximum
current load.

--
Cheers,

John.

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