Fireproofing in cable calculations

Yes, it is a bit more physics-ish than perhaps one is used to. If you take a bit of copper wire and heat one end in a flame, the thermal conductivity quickly heats quite a length to an uncomfortable holding temperature, moving the heat along the wire by thermal conduction. The same applies to conduit or trunking, we do not get an immediate "hot spot", just a spread out temperature which may be slightly higher than the rest, but not excessive. The wall also has a property called thermal capacity, a bit like the bricks in a storage heater which are made of a material with a very high value. This means that it takes a long time for this heating of the wall to happen, and of course more spread out, losing the heat to the atmosphere by convection and to a slight degree radiation.


If the wall is very thick and the expected temperature rise high, one might use somewhat thicker cables, which both reduce the resistive loss and provide more conduction along the length of the cable, to the outside of the wall. All this is the theory, in reality, one does not find damaged cables in short lengths through walls which is why the condition is not discussed very much in textbooks or lectures. Cables buried in insulation are a rather different thing, because the length is too long for much conduction outside the insulation, and in general cables surrounded by serious insulation are a bad idea (because they end up very large and therefore expensive) and it is wise to make a route without long lengths buried. This does not apply of course to lightly loaded cables, or short lengths dropping through for say a light drop in the roof space.


David

Yes, it is a bit more physics-ish than perhaps one is used to. If you take a bit of copper wire and heat one end in a flame, the thermal conductivity quickly heats quite a length to an uncomfortable holding temperature, moving the heat along the wire by thermal conduction. The same applies to conduit or trunking, we do not get an immediate "hot spot", just a spread out temperature which may be slightly higher than the rest, but not excessive. The wall also has a property called thermal capacity, a bit like the bricks in a storage heater which are made of a material with a very high value. This means that it takes a long time for this heating of the wall to happen, and of course more spread out, losing the heat to the atmosphere by convection and to a slight degree radiation.


If the wall is very thick and the expected temperature rise high, one might use somewhat thicker cables, which both reduce the resistive loss and provide more conduction along the length of the cable, to the outside of the wall. All this is the theory, in reality, one does not find damaged cables in short lengths through walls which is why the condition is not discussed very much in textbooks or lectures. Cables buried in insulation are a rather different thing, because the length is too long for much conduction outside the insulation, and in general cables surrounded by serious insulation are a bad idea (because they end up very large and therefore expensive) and it is wise to make a route without long lengths buried. This does not apply of course to lightly loaded cables, or short lengths dropping through for say a light drop in the roof space.


David