Fred coughed up some electrons that declared:


My understanding of cables is to derate them to 0% at 70degC. The belief
is
that below 70degC PVC won't creep sufficiently and above, it will. You're
going to have to assume that the joists and air around the joists will be
whatever temperature your boiler is set to unless you use a mixer to
reduce temperature.

I will have a mixer - but it's not necessarily the case that the air will
reach pipe surface temperature, except in a later very close to the pipe.

Calculations based on IEE regs data and other factors limit my design to max
50, preferably 45C ambient (edge case, 2.5mm2 T+E cable, required current
capacity of =20A)

Floorboards at or above 50degC are going to hurt!!

Don't be daft, the *air* below them is at X degrees, where X might be 35,
40, 45 or 50 (the calculation of which is non trivial and certainly beyond
me).

The heat has to transfer through air then wood, where the surface of the
wood is being cooled by losing heat into the room, so it stands to reason
that the air under the floor may be considerably hotter than the upper
surface. But it could be 10 or 20 degrees below pipe temp, especially below
the pipes: In Andrew's scheme, the pipes are about 1" below the floor
surface.

Cables must be 2" below, generally the might be near the mid point, so
3-5" below.

It's all going to be speculation other that a real life observation or a
complicated set of calculations.

Cheers

Tim