Hi John

On Tue, 04 Sep 2007 12:58:59 +0100, John Rumm
wrote:

Adrian wrote:

Not directly on topic - but a bit of advice, please...

We have a new heat-pump installation for hot water and central heating
purposes - basically a giant fridge that sucks heat out of a
ground-loop and pushes it into the circulating water.....

...anyway....

The pump is installed in my 'studio' (=big timber shed !) -
and the electrical connection is via a sub-consumer unit in the
studio. The power consumption of the pump is (allegedly!- can't find a
rating plate on it) 15A - and it's fed from the CU with a spur of
2.5mm cable, clipped to the surface of the OSB that forms the inside
wall of the studio..

How do you "know" its 15A?

Not wishing to be pedantic (but one needs to be in this case, for
reasons I will cover), 15A is a current loading and not a power
consumption.

All works fine - but the cable is warm to the touch - not hot - just
warm - maybe 50C - will measure it later if anybody's interested.

It would be interesting to know what the actual temperature is.

Should I be worried that the cable's warm ?

Depends on how warm, and probably not for the reasons you expect! [1]

Is it worth doubling-up the cable (2 x 2.5mm cables) - or is a
warm cable acceptable ??

There is a popular misconception that something must be wrong when a
cable gets warm. The reality is that the way we deduce the cables
current carrying capacity, is based on how much current it will carry at
a maximum conductor temperature.

With PVC clad cables the maximum operating conductor temperature is 70
deg C. Cables running at up to this temperature should still enjoy a
reasonable and safe productive life. For a 2.5mm^2 cable clipped direct
to a surface as you describe, the current that would cause a rise to
this temperature is 27A. (it is this notion of a temperature "budget"
that means when you install cables in ways that prevent them losing heat
as quickly, you need to "derate" the current carrying capacity to take
account of the faster rate of temperature rise that will result).

[1] Unless the cable is dangerously hot then there is no direct problem
with the cable being warm, however it may suggest that your equipment is
drawing far more current than you previously thought.

Given that we are talking about a heat pump which probably has a
compressor driven by an induction motor, you may also have to consider
the implications of a poor power factor here. i.e. the cable may have to
carry a higher current than the power consumption alone would suggest -
but that will not necessarily correspond to a higher energy consumption.

OK - will look out the clamp meter and a digi thermometer and see if I
can't catch the thing when it's running (control is by means of a
_really clever_ micro with an instruction manual in a mix of Engish
and German... so it's not as simple as just throwing a switch g

Still - gives me time to look for that clamp meter !

Thanks
Adrian