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Finishing a basement and I need to add a subpanel. Main panel is outside. Subpanel will be in the basement. 70 Amp breaker in the main to feed the subpanel. I was thinking THHN but would this mean I need to run the conduit all the way to the subpanel?

Would NM-B be ok?

Edit: I live in Colorado if that makes any difference and I think the wire size is #6 for THHN and #4 for NM-B.

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The wiring will have to be in some form of raceway the entire run. If you're using Non-metallic sheathed cable, the jacket fulfills this requirement. If you're running individual conductors, you'll have to use conduit or some other approved raceway.

As for conductor size. If you can meet all the requirements for using the 90°C column, you'll be able to use 6 AWG copper for the run. However, since you're less than 100 amperes, you're likely going to be using the 60°C column. This means whether you use single conductors or a cable assembly, you're still going to have to use 4 AWG conductors.

NOTES:
- More information can be found in my answer to this question: What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run?

- After further research, it appears 75°C is common for load center terminals (check your specific equipment to be sure).

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When figuring circuit ampacity you can never use the 90 °C column. There is no way that every item on the circuit will meet or have 90 °C ratings. The best you'll do for conductors in conduit it the 75 °C column.

In my opinion 70A is an oddball sub-feed size. There's no reason for it. Either go with #6cu cable and a 60A feeder, or #4cu/#2al and go with a 90A feeder. The sub-panel can be the same in either case.

  • "You cannot protect wire rated for 85A with a 90A breaker" You most certainly can. This is a very typical installation. NEC 240.6 & 240.4(B) – Speedy Petey Feb 14 '14 at 2:00
  • Seems you're right again, however, I'm not sure I'd be comfortable upsizing the breaker when feeding a subpanel. – Tester101 Feb 14 '14 at 13:45
  • It's really not an upsize. It is the appropriate size according to code. The NEC tends to be pretty conservative, so I have no problem at all following the NEC in cases like this. Of course there are times when the NEC is safe, but not very convenient, so code minimum is not the best route to take. You also have to consider, how often is a residential sub-panel loaded to the max? Hardly ever in my experience. – Speedy Petey Feb 14 '14 at 16:45
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I know this is an old question, but there was one aspect the other answers did not address: You cannot use NM for the whole run because of this comment in the question "Main panel is outside." NM cannot be used in wet or damp locations (see 334.12(B)(4) of the NEC quoted below).

You could potentially use UF (similar to NM wire, but constructed to handle the outdoors) provided the cable assembly is not subject to damage as defined by the NEC, or conduit/THWN outdoors and switch wire types at a junction box once the wiring reaches interior spaces.

From the 2011 and 2014 NEC (wording is the same in both for this section):

334.12(B) Types NM and NMS. Types NM and NMS cables shall not be used under the following conditions or in the following locations:

(4) In wet or damp locations

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