NM Cable sizing temp column

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mstrlucky74

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Thought the insulation was rated for 90 degrees? I saw on an online video that you must use the 60 degree Column to size? Is that true and if so why? Thanks
 
mstrlucky74 said:
Thought the insulation was rated for 90 degrees? I saw on an online video that you must use the 60 degree Column to size? Is that true and if so why? Thanks
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Yes modern NM is rated 90 C which is great as many light fixtures require 90 C conductors.

Yes we must rate the ampacity based on 60 C because the NEC says so. Sorry I don't know why that is just that it is.
 
iwire said:
...Yes we must rate the ampacity based on 60 C because the NEC says so. ...
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I surmise it is because the jacket impedes dissipation of heat generated by current through the conductor. Where the 90°C rating is pertinent to wiring lighting fixtures, the jacket is normally removed... but not required to be removed. :blink:
 
iwire said:
Yet not an issue for MC, AC, jacketed MC etc?
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Metal jacket does not impede heat dissipation (as much). I'll claim jacketed MC is because it has a larger diameter. Additionally, I'll claim it is based on beliefs more than physics. :happyyes:
 
iwire said:
Yes modern NM is rated 90 C which is great as many light fixtures require 90 C conductors.

Yes we must rate the ampacity based on 60 C because the NEC says so. Sorry I don't know why that is just that it is.
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In in a way you stated the reason why. Light fixtures.

Reading the 1983 TCRA pretty much says that even though the new "NM-B" had 90C conductors, the old NM wiring to incandescent/filament light fixtures with thermal insulation covering that wiring and fixtures was a hazard. We have all seen such fried connections.

Devices were holding up fine, but the consensus, 8-2 votes, was to just derate all NM-B to a final ampacity of the 60C rating.

At some point light manufacturors said cool and said their fixtures need 90C NM-B.

The TCC did not feel like adding an exception allowing one to use the 90C for certain installations, derating only.
 
jumper said:
In in a way you stated the reason why. Light fixtures.

Reading the 1983 TCRA pretty much says that even though the new "NM-B" had 90C conductors, the old NM wiring to incandescent/filament light fixtures with thermal insulation covering that wiring and fixtures was a hazard. We have all seen such fried connections.

Devices were holding up fine, but the consensus, 8-2 votes, was to just derate all NM-B to a final ampacity of the 60C rating.

At some point light manufacturors said cool and said their fixtures need 90C NM-B.

The TCC did not feel like adding an exception allowing one to use the 90C for certain installations, derating only.
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:?
 
iwire said:
:?
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Shoot I tried to explain the best I could on why NM has a 60C final ampacity even though the conductors are 90C.

Why not at least 75C if the terminals on both ends of the cable are 75C? Like SER.

It is basically a safety factor because of all connections that have burned up in incandescent and such light fixtures. Device connections did not have the same failure rate.

Heat from the bulbs and thermal insulation were the main factors.
 
jumper said:
Shoot I tried to explain the best I could on why NM has a 60C final ampacity even though the conductors are 90C.

Why not at least 75C if the terminals on both ends of the cable are 75C? Like SER.

It is basically a safety factor because all of connections that have burned up in incandescent and such light fixtures.
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This is not making sense or MC and any other cable that feeds light fixtures needs to be used at 60 C.
 
iwire said:
This is not making sense or MC and any other cable that feeds light fixtures needs to be used at 60 C.
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I see your point about jumper's explanation... but the reality of it is no explanation will make complete sense if you are looking for one based entirely on the physics involved. This one has the human element written all over it. :D
 
iwire said:
This is not making sense or MC and any other cable that feeds light fixtures needs to be used at 60 C.
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Smart $ said:
I see your point about jumper's explanation... but the reality of it is no explanation will make complete sense if you are looking for one based entirely on the physics involved. This one has the human element written all over it. :D
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I hate trying to write coherently.:)

Yes, when hooking up lights MC and NM are both limited to the 60C column or more specifically by 240.4(D).

Let us go to a 240.(G) application say motors/HVAC.

MC is not limited to the 60C column for this application like NM is still bound to 60C by 334.80. Only the terminal rating limits MC per 110.14(C) to a specific column.

No matter the application, the CMP limited NM in total unlike MC. SE(R) and AC only get limited if installed in thermal insulation.

Why this limit for NM only?

The gist of what I read is that so many NM connections in lights burned up because of heat build up and that NM is more often than not in thermal insulation that a blanket limitation was passed. Regular devices such as receptacles and switches nor fluourscent lights had the same failure rate, but the CMP just said forget it and make the final ampacity of NM-B at 60C no matter what.
 
I always looked at this like limiting a #12 conductor to 20 amps even when the conductors and the terminations are listed for 75 degrees C. It's just the way that it is. :roll:
 
334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. #6 AL is good for 40A in the 60 deg column
 
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