My mother in law recently replaced her 4 burner Frigidaire range and is often popping her breaker when she is cooking for a large party - ie 4 burners going and oven.

I checked the specs for the range and they suggest a 50A breaker. The electrical circuit however is a 40A breaker. The cable is a cu Phillips NMD7 8/3 Nylon 900 300V. The run is about 40-50ft, not in conduit - parts are in unfinished ceiling, other parts behind acoustic tile. But either way, plenty of ventilation.

From what I've read,I can put a 50A breaker on a 8/3 THHN cable but I don't see that rating (THHN) printed anywhere.

Is the NMD7 nylon 900 THHN? Or does it need to be specifically indicated? What does the 900 stand for?

Is the only choice to have a new 6/3 cable run?

  • What is the wire distance from panel to the range outlet? – wallyk Apr 24 '16 at 0:34
  • Are you in Canada? ....... In the US #8 NM cable is only good for 40A. #8cu THHN/THWN conductors in conduit would be good for 50A. – Speedy Petey Apr 24 '16 at 0:36
  • Yes. I'm in Canada. It's 8/3 cu, no conduit, run of about 40-50ft. Partly exposed (unfinished ceiling), partly behind acoustic ceiling tiles – Eric B. Apr 24 '16 at 3:03
  • @speedypetey How do I know if it is THHN? Does it have to be stamped on the sheath as well? If it is not marked, assume it is not? – Eric B. Apr 24 '16 at 3:13
  • @EricB., if it's in cable that's all that matters. The cable will be marked, not the inside conductors. – Speedy Petey Apr 24 '16 at 12:17

In Canada, NMD 8/3 is allowed for up to a 40A circuit, NMD 6/3 is required for 50 A (Actually NMD 6/3 has an ampacity up to 55A, but try to find a 55A breaker…). Sorry, Eric.

Additional info: You may find some ampacity tables that list NMD8/3 as having an ampacity of 55A, but these tables assume a 90˚C conductor temp, and the code says you need to use the 75˚C ampacity when connecting to terminals rated to 75˚C, which includes all domestic circuit breakers and the NEMA 14-50 receptacles used for ranges and stoves.

  • There maybe a way around your problem: Terminate each end of the NMD8/3 with a minimum 1m (3 ft) length of NMD 6/3. You will need to use connectors (lugs, probably) that are rated to 90˚C to make the splice from the 8/3 to the 6/3 in a recognized junction box, but then you will have NMD 6/3 (which does have a 55A ampacity at 75˚C) terminating on the breaker and NEMA 14-50 receptacle. Check that your inspector will accept this before you implement, but it is a code-legal way of increasing your circuit to 50A without ripping out the existing NMD 8/3. – AndyW Jun 22 '16 at 2:57

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