I am based in Canada. I am planning to install an electrical outlet above a gas fireplace in which to mount a TV. I plan to fish 14/2 NMD90 to that outlet which means the cable will be routed directly above the gas fireplace and radiating heat inside the finished wall cavity. The cable has a temperature rating of 90 degrees. The circuit would be 15 amp circuit breaker.
I have dropped a thermometer into the wall cavity at the approximate cable location to measure the ambient temperature after fireplace running for 2 hours. It is averaging 36 degrees.
From googling; I am learning that if the ambient temperature is above 30 degrees Celsius, I must derate the cable ampacity using ambient temperature table.
Using the CEC and NEC tables; I have concluded the following but hoping someone can validate my work and understanding to advise if I have factored this correctly.
Romex 14/2 NMD90 rated for 90 degrees
CEC table shows 90 degree 14 gauge cable as capable of carrying 35 amps at 30 degrees Celsius ambient temperature. (with a caveat that circuit breaker cannot exceed 15 amps).
NEC table shows that a 36-40 degree ambient temperature when using a 90 degree rated cable should be derated at .91 of its ampacity.
So, 35 amps x .91 = 31.85 amps (albeit 14 gauge must be restricted to 15 amp circuit breaker). I would assume based on this, that it would be safe to install the 14/2 NMD90 as planned?
Best Answer
You are correct, the ampacities of #14 NM for 15A circuits and #12 NM for 20A circuits are conservative. Even after derating for ambient temperature, you wind up using the same cables. For this same reason #14 and #12 NM are typically used in hot attics without derating.