Goal: Replace existing 10-30 (8/3 Al on 30A breaker) receptacle with a 14-50 (6/3 Cu on 50A breaker). Receptacle will be used to charge an EV (at 32A, but I think its only right to actually make sure the 14-50 is capable of doing what its supposed to, who knows what next owner of the house may use it for).

Reasons I want to avoid using THHN, location is a 2 car garage, the run starts in wall (behind drywall, about 10 feet), run ends in a wall (behind drywall about 20 feet). The space between these two walls is an existing 3/4in metal flex conduit (length is width of 2 car garage). To use THHN Id have to rip up two walls to install conduit. So NM-B is much more convenient for me. I have to use some sort of conduit (i think) because the ceiling of the garage is unfinished (exposed beams), so there is no drywall or anything to put the NM-B behind.

The conduit is existing (its what the 8/3 Al on 30A breaker is running to the existing NEMA 10-30 receptacle).

Now the questions. Please correct me if I am reading something incorrectly.

From reading NEC it appears that NM-B is not allowed in wet locations. Conduits outdoors are considered wet locations. My entire run is in a dry location, all indoors and more than 8ft off the ground.

Two other concerns that may come up.

  1. Derating - being NM-B it is automatically derated in NEC table to 60C and 50A (individual conductors are rated 90C but dertaed as a whole to 60C). So since I am putting it on a 50A breaker and using 14-50, it seems like derating is a moot debate (since its already done automatically).
  2. Fill table calculation. NEC specifically says to treat NM-B as one conductor when calculating fill table. I measured my NM-B (it is round not oval) and it is 0.5in diameter. I used Southwire's fill calculator - https://www.southwire.com/calculator-conduit with the following settings:
  • Conduit - Flex FMC 3/4
  • Conductors - 0.5in x 1

Calculation gives 37% fill rate. Within NEC spec.

So, what am I missing? Based on this it should be to code for me to run NM-B 6/3 through a 3/4in metal Flex conduit across the ceiling from one wall to another?

Thanks for any feedback!

  • Someone correct me if I’m wrong but I believe if your EVSE specifies a 40 amp circuit, you need to use a 40 amp breaker, even with the 50 amp outlet.
    – DoxyLover
    Jul 31, 2020 at 17:31
  • 1
    Perhaps for hard wired solutions, but in this particular case its a mobile charger that calls for 14-50 because its the plug on it. I doubt that it expects you to change the breaker on your 14-50 for it. The idea is that you can use it on ANY 14-50, because it assumes that the 14-50 is wired correctly (i.e. 50A breaker and appropriate gauge wire)
    – Duxa
    Jul 31, 2020 at 19:26

2 Answers 2


While my measurements showed 0.5 inch diameter, the Southwire website suggests that NM-B is 0.67inches, and with using that as calculation it comes out as overfilled for 3/4 inch FMC.

So I ended up replacing the FMC with 1 inch Schedule 40 (gray) electrical PVC conduit.

All is well. Thanks everyone for feedback/help.

  • Glad you got it worked out! Please give yourself a check-mark so that others know this issue is resolved.
    – FreeMan
    Aug 2, 2020 at 18:15
  • Checking the accepted answer check mark shows it is resolved and you won’t get questions asking if you figured out how to do it sometimes we don’t read all the comments and answers before commenting, I have left comments on my own answers without realizing it was my answer (adding additional supporting info for the answer).
    – Ed Beal
    Aug 3, 2020 at 14:49

More than 8’ off the ground you do not need to encase nmb unless required by local code 334.10 allows 2 conductor 6 awg or 3 conductor 8 awg to be surface mounted exposed On the surface with no “rat run” or backing so I would not mess with the conduit. Smaller cables need a backer when surface mounted there is even a nice photo in the code book exhibit 334.1. The article 334.15 covers exposed work and examples I provided are of basements and crawl spaces but inspectors in every state I have worked uses this as the standard if not run through bored holes.

  • Well, the conduit is already there, so I might as well use it right? Cant hurt by having extra protection. I just want to make sure this doesnt break NEC, and it appears that the conduit is appropriately sized based on my calcs above. Im just hoping someone stops by this post and answers definitively if my math is correct.
    – Duxa
    Jul 31, 2020 at 19:28
  • 1
    Yes the calculations looked correct a single conductor is right, but it will be a bugger to pull even over sized conduit nm is a pain. The only thing is protect the cable from the leading edge Of the conduit, that’s where inspectors look for damage if the outer covering is damaged it is a re do because of the damage got hit on that a long time back.
    – Ed Beal
    Jul 31, 2020 at 20:24
  • @Duxa -- thing is, FMC doesn't provide any additional mechanical protection at least from a Code standpoint (this isn't like EMT or Sch80 which'd be a legal protective sleeve for the NM) Aug 1, 2020 at 0:10
  • I ended up replacing the FMC with 1 inch Sch 40 PVC. You are all right, the FMC was too much of a bugger to pull. Thank got for 10 feet of the PVC being only like $3.50. I would have done Shc 80, but Home Depot doesnt seem to carry it.
    – Duxa
    Aug 1, 2020 at 6:37
  • Metal Flex is the hardest To pull through it bunches up when you pull, liquid tight is easier but still tougher than rigid or emt, when I do put it in conduit up walls I do upsize like you did but I run exposed in cases like you have where running conduit the entire run is when I use conduit with nmb or UF . Thanks for adding your final solution but you may want to put it as an answer do others can find what you did. I don’t use a lot of pvc it pulls harder and I have had the wires burn through a sweep even with lube but in your case it sounded like a good option.
    – Ed Beal
    Aug 1, 2020 at 23:53

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