Hypothetical. I am hooking up a range/oven that requires 50A. Its terminations are rated 60C, and there's no way around it.

The 100' wiring run is all in EMT conduit and I am running THHN wires, so I am entitled to use the 75C column. #8 copper wire is allowed 50A@75C.

My circuit breaker has 75C terminations.

I have these lovely MAC Block splice connectors, rated to 90C. They take #8 and #6 wire just fine.

So I get the idea to take the #8 wire to the Mac Block splices (90C), splice to a short #6 copper pigtail, and take the #6 to the range terminations.

Have I successfully dodged the 60C rule, or is my plan thwarted for some reason?

  • I would certainly think so! Your connectors are all rated for it. I would think you'd want 75C or 90C 6ga. for the short run from the connectors to the range, just to be sure, but that should be fine. IMHO, again, I'm not in the trades, but do have a good understanding of this stuff. Great question. Mar 22 '20 at 20:08
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    So I would ask how long would the #6 have to be to insure that the splice reaching 75°C wouldn't heat the #6 to above 60°C at the device end? Mar 22 '20 at 21:51
  • " Its terminations are rated 60C, and there's no way around it." Does that mean that 75C rated wire is impossible to connect mechanically? Mar 23 '20 at 6:10

If you use a j box with terminals rated at 75 you can use the higher 75deg table or even 90 for the main run then the last few feet with the larger wire at the 60 deg table we do this using the 90 deg table running large +500mcm for hundreds of feet then the 75 degree temp for the last 10 feet or so at both ends same thing works for 60 degree table.

  • Note see 215.2 for a code reference and a pretty picture in exhibit 215.1.
    – Ed Beal
    Mar 23 '20 at 13:08

The long length of wire which could be at 75C would be a heat reservoir which could heat a short pigtail above 60C. I would say that this arrangement would not be code compliant.

[EDIT] But a longer pigtail (e.g., 2 ft) might reasonably be assumed to dissipate enough heat to keep the temperature at the appliance end below 60C.

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    So it depend on the length of the pigtail? Would there be a minimum length above which you'd say it's fine? 10 feet? 5? 2? Mar 23 '20 at 6:15
  • I have never heard of something so ridiculous in my years as an electrician. Please review exhibit 215.1 a feeder installed in accordance with 215.2.A.1.A this example specifically shows 90 degree wire used for a long length With standard 75 degree terminations. No minimum length is specified just the temp ratings. The housing must be rated for the higher temp this is why we use J boxes external to a mcc (motor control center)
    – Ed Beal
    Mar 23 '20 at 13:06
  • @Ed Beal, I am sure that my answer based on elementary physics could be the wrong in the real world and even ridiculous. It could well be that any practical 6 or 8 inch pigtail would be long enough to dissipate the heat from the long length of wire at 75C so that the end of the pigtail (at device termination?) would be below 60C, but really this should be determined experimentally. Surely there would be a lower limit on the length of the pigtail, right? Mar 23 '20 at 16:23
  • @Peter - Reinstate Monica , I would guess that 2 ft would be more than enough length to allow cooling of the pigtail. Anyway in practice would the long wire ever be a 75C? Mar 23 '20 at 16:31
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    It depends in some cases a closed nipple directly into the next box I have some that the wall were removed (not by me) and it passed inspection with 3 parallel 500 mcm into load-centers (2 places) I think this is a violation but it was passed, that same inspector challenged me on mounting Strut & nipples vs direct mount and I said it’s within 3’ and he said oh you are right so he expected it to be mounted directly on the mcc. I have several that are close to 10’ because that was easier to mount. My belief is there should be at least a nipple not 1/2 the side of the J box cut out to the MCC.
    – Ed Beal
    Mar 23 '20 at 18:36

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