How should NM-B wire (romex) be run from an exterior panel to comply with California Electric Code 2019 (based on NEC 2017)?

Here are two ideas:

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Option 1: run THWN through an LB into an 8x8 box in the crawlspace, where the THWN is spliced (wirenuts) to NM-B romex. This avoids running the NM-B in exterior conduit, which is not allowed as it a 'wet location'

Option 2: run THWN through a nipple directly out the back of the panel box, into an 8x8 box in the crawlspace with same splicing to NM-B as Option 1.


  1. Do both of these comply with California Electric Code 2019 (based on NEC 2017)?
  2. For Option 2, what fitting would be used between the nipple and the back of the panel to prevent water ingress? (see orange mark for location). Would that require extra siding to be removed to accommodate the fitting to allow the panel to sit flush?
  3. Is there a code compliant 'Option 2b' where no splicing is needed, and the the NM-B runs directly into the subpanel?
  4. Any better ideas?

I'm assuming I don't need to derate the ampacity of the THWN in either case, as the distance from panel to the box is <24", so no derating is required.

I'm curious - if the nipple/conduit in these designs is considered a 'wet location', then why is the 8x8 box not? Is it because any water in the conduit is assumed to drain out the bottom of the box without mixing with the wires? Just curious.

  • I presume you're actually on the 2017 NEC? Commented Oct 11, 2022 at 0:16
  • 1
    @ThreePhaseEel 2019 CA Electrical code, which adopts the 2017 NEC with amendments - good catch
    – tom
    Commented Oct 11, 2022 at 0:27
  • How many wires/circuits will be running through this, just the one indicated or all wiring for the house or...?
    – FreeMan
    Commented Oct 11, 2022 at 17:12
  • @FreeMan approximately 10x 20A branch circuits. I believe the ampacity of the wires does not need to be derated as the conduit/nipple is less than 24". I'll look up the code reference for that and add it. If necessary, I can add a second nipple from the back of the box.
    – tom
    Commented Oct 11, 2022 at 22:07

1 Answer 1


Both of these options are fine. The concept of a few feet of NM-B ran through a nipple and/or LB into exterior pipe due to the fact it is a WET location is nit-picking. IMO. If you run into a building inspector that calls you on 12 inches of Romex in a exterior conduit, your concern should not be THAT issue. Your concern should be dealing with a prick like this inspector.

If it were me, I would drop a pipe down to an LB, nipple from the LB into the crawl space leaving an inch or two of nipple sticking into the crawl space, then spin a pipe bushing onto the end of the nipple. Get my non metallic cable to that nipple with 8 feet of an extra length to extend into the LB and up into the panel. THEN, just strip the sheathing from the Romex and fish the conductors up into the panel. Silicone where the nipple penetrates your wall.

I have installed hundreds, maybe over one thousand service upgrades in my 36 years as an electrician and this is a minor hair splitting issue. My advice to you, don't get nervous about small details like these. Electrical theory and practice will present REAL issues that require REAL consideration regarding code and safety. THIS issue is not one of those issues.

  • The inspector may be nit picking, but since he's the one with all the cards, they're his nits to pick. You as an experienced, licensed electrician may know how to deal with an inspector like that and be able to come to an agreement to get it ignored, but your average DIYer who has little to no experience likely will not be able to get away with it and, frankly, should not be allowed to do so. Part of the inspector's job is to protect well intentioned idiots from themselves.
    – FreeMan
    Commented Dec 6, 2022 at 19:12
  • The inspector's job is also to protect the home owner from licensed, experienced contractors who are looking for short cuts. I had a very experienced (30+ years at the time, I'd think) electrician who was a friend of the family install a sub panel for me in a very non-code-compliant manner. I ass-u-med that he'd done it properly, but only learned about a year ago just how wrong the job was that he'd done. That work was done off the books and not inspected. But then, I got what I paid for, I guess...
    – FreeMan
    Commented Dec 6, 2022 at 19:14
  • 1
    Also remember that people tend to do about 50% of the right things you tell them and about 200% of the wrong things you tell them. We try to make all answers here as code compliant as possible knowing that some people will take short cuts. At least we're not to blame.
    – FreeMan
    Commented Dec 6, 2022 at 19:16
  • Thanks @David Brown - I ended up doing something pretty similar. I ran a 2.5" by 6" RMC nipple straight out the back of the box and through the siding into my crawl space, put a bushing on that, then ran my NMC through it. I'm getting my rough inspection on Friday - I'll let you know how it goes.
    – tom
    Commented Dec 7, 2022 at 6:50
  • @ FreeMan - It sounds like you really believe building inspectors are the repository of all knowledge. Or, as you say, "he's the one with all the cards." The cold hard truth is this...like most bureaucrats, building inspectors end up in that line of work because they washed out of the private sector. The best and the brightest DO NOT end up in government positions. No, the best and brightest are usually business owners or highly compensated employees. Commented Dec 7, 2022 at 22:16

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