How to tell NM vs. NM-B, without seeing the cable jacket?

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Old 10-04-18, 08:19 AM
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How to tell NM vs. NM-B, without seeing the cable jacket?

I want to install a light fixture that is marked as requiring 90C conductors. The house is old, and I know it has both NM and NM-B wiring in it. The problem is, at both the switch and the light fixture box, there isn't enough of the cable jacket (or at least, the right part of the jacket) to see if it's marked NM or NM-B.

Is there a way to tell the difference, based only on looking at the individual conductors? Thanks.
 
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Old 10-04-18, 08:58 AM
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Is there a way to tell the difference, based only on looking at the individual conductors?


Just by looking at the conductors? Not that I am aware of. What color is the jacket you can see in the boxes? Are you looking at 14 AWG or 12 AWG conductors?
 
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Old 10-04-18, 09:39 AM
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The transition was around 1985, so you might get a hint based on the age of construction of that area. Another hint would be if it has any fabric or cloth braid material in it, then it's definitely regular NM. Other than that I'm not aware of a method to identify just by looking at the conductors.
 
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Old 10-04-18, 09:51 AM
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Not that this is a foolproof way to tell but if the insulation on the conductors is kind of thick and spongy/soft it's most likely old NM. If the insulation looks thinner and slick and is tough it's most likely NM-b. But I make no guarantee.
 
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Old 10-04-18, 10:14 AM
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The house was built pre-1900, and has everything from the old cloth-covered NM, to post-2000's NM-B. This wiring is 14 AWG, in a white plastic jacket. Unfortunately, I have found plastic jacket cable marked both NM and NM-B coming from the breaker box. In this case, the wiring from the breaker box to the switch is the older cloth-covered NM, and then from the switch to the fixture is the newer plastic cable. It makes me think that somebody may have updated the last part of the run to meet the 90C requirement in the past, but I can't be sure.

The fixture itself screams "1990s" in terms of its design. I could take a closer look for a manufacturing date, but that wouldn't prove that the wiring to it dates from the same period.

ETA: On closer inspection, the insulation on the conductors does appear thin, and is shiny. Where it bends, you can what looks like clear plastic wrinkles, like there is a clear coating over the black insulation.
 

Last edited by SuperSquirrel; 10-04-18 at 10:26 AM. Reason: more info
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Old 10-04-18, 12:02 PM
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I believe the wiring that you describe, which is a clear plastic coating over plastic insulation, is always NM-B.

Also, NM-B usually (always?) is a thinner plastic insulation. The thicker plastic insulation was a different type of plastic and is NM. Fabric insulation is always 60deg NM. My understanding is if you compare the insulation thickness of NM and NM-B, it's an obvious difference - NM-B is always thinner.

(This is all from discussion and reading on the subject... I unfortunately don't have a definitive source)
 
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Old 10-04-18, 01:17 PM
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I agree that if the conductors have the nylon overcoating it's probably NM-B, but unfortunately not conclusive. There was low temp nylon coated TN (thermoplastic + nylon) wire prior to high temp THHN (thermoplastic + high heat + nylon) so a manufacturer could have been using the low temp version in NM the 1970s and 1980s.
 
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Old 10-04-18, 02:19 PM
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That's what I was afraid of, but also the answer I expected.

My wife wants to put up big-box store, $20 flush mount fixtures. I'd like to do something a little nicer, and not flush mount -- which, as an added bonus, would hopefully not have the 90C restriction (although the way they slap that warning on everything now who knows...) But I have a feeling we're going to be rewiring just to accommodate the cheap ones.
 
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Old 10-10-18, 08:21 PM
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I found a fixture marked as requiring 75C conductors. Was plastic-jacket NM rated at 75C? I want to say yes, but I can't remember for sure.
 
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