You can see that the cable at 20 amps 2cicuits will rise about 30 degrees above ambient.
So if the roof space was already 40 degree, or 50 degrees and you have HWS circuit and Air Cond circuit tgether??
Cable Temperature Rise part 2.
Rodney Chant
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I leant that you can not put two circuits in an insulation especially when all the circuits draws more current like HWT and Air con .