Published ampacity assumes 30 °C ambient and no more than three current-carrying conductors together. Six conductors in a 45 °C roof space leaves about 65% of the printed figure — enough to turn a correct 12 AWG answer into an undersized one.
A conductor is rated by how much heat it can shed before its insulation cooks, so both corrections are about the same thing: how hard it is to get that heat away. Hot surroundings shrink the temperature difference driving the heat out; neighbouring conductors each add heat and block the path. The factors multiply, and 0.82 × 0.80 is already a third of the capacity gone before anything unusual has happened.
It is not a safety margin you can spend — the derated figure IS the rating, and the table figure never applied to your installation in the first place. Going the other way, a run in a cool basement on its own genuinely earns slightly more than the table says, which most people never claim.
Estimate with the rule, then check it against the calculator that models it properly.
Open Wire Gauge & Voltage Drop Calculator →Published ampacity assumes 30 °C ambient and no more than three current-carrying conductors together. Six conductors in a 45 °C roof space leaves about 65% of the printed figure — enough to turn a correct 12 AWG answer into an undersized one. A conductor is rated by how much heat it can shed before its insulation cooks, so both corrections are about the same thing: how hard it is to get that heat away.