At 55°C ambient, the factor is .
At 20°C, that 12 AWG wire’s insulation can shed heat easily. But at 55°C, the wire starts hotter. It heats up more under load. Now the copper and insulation approach — the insulation’s long-term limit. wire derating chart
Multiply the wire’s original 90°C ampacity (30A for 12 AWG) by 0.71: At 55°C ambient, the factor is