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I'm replacing some light fixtures in my house, and the existing cable coming out is very old. I was able to make out the stamp along the length, and it's 12/2 NM cable, rated to 60C. It's got a neutral, hot, and a ground.

Newer cable, I've come to find out, is NM-B, which has a 90C rating. The light fixtures I've picked up are labeled that they need a 90C minimum rating.

So, I take it I've got to replace at least the cable that connects to the light. My plan is to pull out the existing NM cable in the attic, and splice in some NM-B, which I will run to the fixture.

Is there anything wrong with doing this? Do I need to run a new NM-B cable from the breaker?

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    Oh dear. This is a bigger problem...can you name the make and model of light fixture that's requiring 90degC cable? – ThreePhaseEel Feb 18 '18 at 2:26
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    Since when new fixtures dictate cable ratings? This sounds like a manufacturer simply regurgitating current standards for cable. – isherwood Feb 18 '18 at 3:27
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    Replace with socketless LED fixtures and they won't require 90C cable. The reason to want such cable is the high heat of the fixture, meaning it is either incandescent, or high-intensity discharge (sodium, mercury, halide). – Harper - Reinstate Monica Feb 18 '18 at 5:05
  • The fixure is socketless LED. A warning label on the fixture itself reads: CAUTION. RISK OF FIRE. CONSULT A QUALIFIED ELECTRICIAN TO ENSURE CORRECT BRANCH CIRCUIT CONDUCTOR. MIN 90C SUPPLY CONDUCTOR. The old wiring was probably put in around '82 or '85. – Bill Feb 18 '18 at 14:29
  • Weird. If an LED fixture gets that hot, it's got problems. – Harper - Reinstate Monica Feb 18 '18 at 22:05
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You can splice diferent sizes, temperatures or types (NM, UF, THHN, SJOOW) cable anywhere it's legal to splice cable.

  • It needs to be inside a junction box.
  • The boxes need to be someplaces boxes aren't disallowed.
  • The cables need to have proper cable clamps entering the box.
  • The boxes need to forever be accessible without disassembling any part of the building with any tool (aside from the box covers, obviously :)
  • There needs to be enough room in the box for the splices.

"room in the box" works like this: All ground wires count as 1 wire. All cable clamps count as 1 wire. All other wires count as 1 wire each. Multiply your number of "wires" by 2.0 cubic inches for 14AWG.... 2.25 cubic inches for 12AWG..... or 2.5 for 10AWG. For you, 6 wires = 13.5 cubic inches = a deep handy-box will suffice, or my go-to, the 4x4x1-1/2 square box (21 cu.in.)

I'm very fond of steel boxes because if there's any sort of arcing or shorting in there, the metal chassis will help assure a breaker trip and also dissipate the heat to keep things below combustion temperatures. Also plastic boxes are stupid overpriced for what they are.

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  • I've strayed away from metal boxes because it seems like you need to buy separate clamps, and I heard you need to ground the box. I'm not sure what that looks like or how to do it. My real question though, is whether or not it's OK to splice two cables of two different temp ratings. I can't find any words of caution about that, so I guess it's OK? – Bill Feb 18 '18 at 14:32
  • @Bill yes, clarified. Some metal boxes have integral clamps or you can just fit clamps in a knockout. Grounding is easy, there's a hole tapped for a #10-32 screw. Put that size screw there and pigtail, they even sell green screws and premade pigtails, though any bared copper wire will do. – Harper - Reinstate Monica Feb 18 '18 at 17:03
  • I also prefer metal boxes, most of the 4.0 boxes I purchase have a premade hole for a standard ground screw. – Ed Beal Feb 18 '18 at 20:45

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