Cable temperature ratings – it’s all about the insulation

Cables can be purchased that are rated for different temperatures. The three most common conductor temperature ratings are 60ºC, 75ºC, and 90ºC . . . .

The benefit of a cable with a higher rating is that cables of the same size can carry more current. For example, a jacketed three-conductor, #8 AWG copper cable, in raceway in free air, rated at 60ºC has an ampacity of 36A, but the same cable rated at 90ºC has an ampacity of 48 amps [NEC 2014 Table B.310.15(B)(2)(1)].

That’s an increase of 33%. The cable rated at 90ºC has better insulation.

You’ll frequently specify cable that is rated at 90ºC because many plants prefer to use Class 1E-rated cable even in non-safety applications. Class 1E cable is usually rated at 90ºC (if not higher).

But be careful. There are some limitations here that you need to be aware of. Two big ones are ambient temperature variations along the cable route and equipment terminations.

As it so often goes in life, a chain is only as strong as its weakest link. If a cable passes through a room that can be as hot as 50ºC (ambient), then even though the majority of the cable’s route is in ambient temperatures of 40ºC, the cable will have to be derated based on the 50ºC ambient temperature.

If a 90ºC cable terminates to a breaker that is rated less than that — say, 75ºC — then a cable’s ampacity must not exceed its 75ºC ampacity values.