I'd like to build a cord with a an 14-30P plug on one end and a 14-30R receptacle on the other end. I'll be using 8/4 SOOW cord.

Since there are no 14-30R receptacles designed to attach to the end of a cord that I can find, I believe I'll need a 2-gang electrical box with the appropriate cord connector. I've been reading that the usual electrical boxes with knockouts are not code compliant because they are only listed for permanent installation.

If I want to do this in a code compliant way, how would I find an electrical box that is listed for non-permanent installation, and then lookup it's listing to verify it is listed for that?

I'm having a hard enough time finding the UL listing for something to start with, let along finding something with a listing like I need.

I read read that electrical boxes with threaded hubs are what I need for non-permanent installation. I found one item, Greenfield Industries B34PS (wrong width though), and I think I found the applicable UL listing.

How can I tell if this would be code compliant to use?

  • 1
    Are you just needing to extend NEMA 14-30 to NEMA 14-30, or are you looking to build some sort of adapter cord? Aug 17, 2022 at 1:58
  • Sorry, I simplified the project a bit for the description. I was actually looking at making a cable that plugs in to a wall NEMA 14-30 receptacle and has an output of one NEMA 14-30 receptacle and one NEMA L14-30 receptacle. Essentially a splitter. Once I saw the cost of cord grips that can take the 0.76" 8/4 cord, I decided it was better to just buy an NEMA 14-30 splitter off of Amazon (unfortunately not UL listed) since I already have a NEMA 14-30P to NEMA L14-30R cable anyways. I'm still curious what the right way to find what something is listed for though.
    – tinfever
    Aug 17, 2022 at 3:17
  • Funny that you're worried about using a box (UL listed for permanent installation only) in a non-code compliant way and are looking for a code compliant alternative (good on ya' and well done!), yet you're satisfied with using a non-UL listed gadget from Amazon built who knows where by who knows who, just because one component of your compliant assembly was too much money.
    – FreeMan
    Aug 18, 2022 at 11:57
  • What exactly "listing" or rules are we referring to when designing home made extension cords? If you are not making it part of a building and not selling it, I do not know what compliance issues might exist. A lot of things are not UL listed because it's not commercially viable to get them listed, not because they are inherently unsafe. That doesn't mean you can't use common sense in building something safe for your own use. I think the (one so far) answer here gives good advice to that effect and I wonder whether the parts of the question about compliance are irrelevant?
    – jay613
    Aug 18, 2022 at 15:11
  • @FreeMan I see the irony you point out. Time also came into consideration. Same/lower cost and more time might be OK, but more cost and more time is hard to swallow. This is what I ended up buying (amazon.com/dp/B07CVSSYQL). It says it uses 10/4 wire but who knows if that's correct. You pays your money and you takes your chances.
    – tinfever
    Aug 19, 2022 at 3:36

1 Answer 1


Since there are no 14-30R receptacles designed to attach to the end of a cord that I can find

Use the "box" trick discussed in your link.

You don't need 8/4 cordage unless you have length well over 200' and loads near max amps a lot and loads intolerant of voltage drop. Remember, don't calculate voltage drop on 30A. Calculate it on actual realistic loads.

10/4 cordage will be fine. You made trouble for yourself by insisting on 8/4 as that is larger cord, and the strain reliefs on inline plugs/sockets aren't designed to fit it.

What isn't going to work is putting two 10/4 cords into one socket. They're not made for that.

I read that electrical boxes with threaded hubs are what I need for non-permanent installation.

I completely concur with that writer's assessment of steel boxes with pre-punched knockouts. When abused as a extension cord box, yes - the KO's inevitably get KO'd lol. But their idea of using non-KO-equipped boxes is fine IMO. For your use with 2 sockets, 2 of those boxes "back to back" linked with short nipple(s) is the way to go. If I were using metal boxes, I'd use 4 short nipples simply for structural strength. The wires would go through one of them (NEC 300.3).

Their fear of metal boxes might be valid on a 30A adapter, given the huge number of ungrounded NEMA 10-30 sockets out there.

Costwise if there's a particular route this cord will always be found in, you may be better off hardwiring this to a stand/post with 6-6-6-6 aluminum cable (which is not available in flexible cordage) or #6Al individual conductors in conduit. Note that FMC and IMC conduit, while costly, only needs 6" of cover burial depth.

While the homemade junction-box-cord idea may seem hokey, it's a hell of a lot more safe than any of that "cheap Cheese from overseas". You're lucky if they use #16 wire in that.

  • The OP's frustrations are understandable -- it doesn't seem that any of the "big names" (I checked Hubbell, Eaton/Arrow-Hart, Legrand, and Leviton) even make a female 14-30 cord connector for some odd reason, although L14-30 is fine...) Aug 18, 2022 at 6:27
  • @ThreePhaseEel ouch... thanks. I wonder why. Aug 18, 2022 at 8:11
  • It's baffling, especially since 10-30 cord connectors exist, too... Aug 18, 2022 at 8:15
  • Guessing, cord connectors are meant to be plugged in easily and conveniently with one hand (or with a male and female in both respective hands). From memory, the barrel of a locking 14-30 is much smaller than the non-locking one would be if it existed in a barrel. It could not be easily gripped in one hand in the way that you do when plugging cords in. A guess.
    – jay613
    Aug 18, 2022 at 15:05
  • Also, a 14-30 cord end benefits so much from locking that maybe it's just a no brainer to use a locking one. As opposed to one fixed to a wall behind a dryer, where locking may not solve any problem.
    – jay613
    Aug 18, 2022 at 15:08

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