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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?

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  • So, you think your tv is drawing 35A ? That must be HUGE... why not use a breaker to limit it to 10A? Or 5A that would refuce the heating effect - label the socket as 10A etc...
    – Solar Mike
    Jan 30, 2019 at 5:19
  • The outlet I am installing for the TV is an extension to an existing circuit with 8 other devices.
    – Bill Barnes
    Jan 30, 2019 at 5:27

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So, 35 amps x .91 = 31.85 amps (albeit 14 guage 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?

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.

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Why not just use one size larger gauge wire? But keep the outlet and the breaker the same amperage. What does the code say about running cables through plenums? I think any cables running in plenums should be rated FT4 (fire test, 4). Then, no need to derate. But my larger, overall question is why is there so much lost heat in the wall cavity? Shouldn't it mostly be trapped in the chimney flue?

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