I'm installing an RV pedestal at my home. Out-of-the-box pedestals are not available. I'm using a 70 AMP outdoor main lug load center (HOM24L880RBCP), and a quad 2-20A 1P, 1-50A 1P breaker to feed two receptacles. Namely a 20 AMP GFCI and 50A 14-50R.

My question, Does NEC allow, and is it kosher, to install two separate circuits that are 50A and 20A in the same 2 gang box? If so, do they need to be de-rated?

I'll be using 6 AWG THHN and 12 AWG THHN between the load center and the box ~ 6 inches away.

Thanks for the help.

  • Personally, I don't think a 14-50R belongs in anything smaller than a 4x4 box of its very own. I'd have to take a look but I'm not sure the thing even physically fits in a single gang space... – Ecnerwal Aug 29 at 13:06
  • That's an awfully small box. I'd consider a 6-space. Since 1x50A and 1x20A plain breakers are cheaper than that quadplex, you'll net back some money on the breaker, so the cost difference is - normally I'm only joking about "the cost of a latté" as in "when you wish you had the extra spaces, will you still remember the taste of that latté?".. but in this case, yeah, actually. The 6-space will allow you to support stuff like an additional TT30 if that comes up, a main disconnect if the AHJ flags you for that, or even chain another subpanel off it. – Harper - Reinstate Monica Aug 29 at 15:09
  • Also if you like saving money, remember feeders should be aluminum wire. Some people get in their heads some poppycock about aluminum wire being "unsafe", but that never applied to heavy feeder like to this kind of subpanel. Use aluminum not copper, just use -2 numerical sizes (e.g. 4Al instead of 6Cu). The savings are a heck of a lot more than a latté. – Harper - Reinstate Monica Aug 29 at 15:12
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    Have you looked into wall-mount RV receptacle boxes? Nothing prohibits you from sticking one of those on a 4x4 post...also, what makes you say that prefab pedestals are unavailable? Have you tried an actual electrical supply house and not just a bigbox store? – ThreePhaseEel Aug 29 at 16:31
  • @Harper-ReinstateMonica Thanks for the comments. Honestly I didn't really think about using aluminum, because I was originally just going to run a single circuit, but then I decided to through an OCP at the point of use. I've already burried the 1" conduit, but this should still work. I will re-evaluate this. I plan to install an upstream OCP to limit to 50A. I don't intend on using both the 50A and 20A circuits simultaneously. The 50A is for RV hookup ,and the 20 for occasional use in that area of the yard. – cueball Aug 30 at 13:18

Box fill requirements basically make that a hard no.

#6 wires need 5 cubic inches each. The device (receptacle) they connect to adds two more, so you need 30 cubic inches just for that.

#12 wires need 2.25 cubic inches each, and the device they connect to adds 2 more, but you get to skip the ground (or count only 1/4 of it under 2020 rules) so 9.56 cubic inches for that.

A 4x4x2-1/8 would just fit the 14-50R. The 20 amp outlet is easy in most boxes so long as it's separate from the 14-50R (and then you count the ground as it'a not a shared box - so you need 11.25 cubic inches, though you'll appreciate more room since GFCIs are big.)

Depending on how you have protected the panel/outlets from the weather in your home-built pedestal will affect whether you also need to deal with exterior boxes and in-use covers. Since you mention that it's an outdoor load center, it may be the case that you will need exterior boxes (which will vary somewhat from the cubic inch sizes I've called out above for interior stamped steel boxes - just pick ones that are at least as large or larger than required) of appropriate volume and in-use covers for proper weather protection.

If using a quad breaker, you might as well put in an additional 20A GFCI on the other 20A feed.

If the terminals on both breaker and receptacle are rated for 75C (and they usually are for a 14-50R) you could reduce the cubic inch requirement quite a bit by using #8 THWN or THWN-2 (3 cubic inches per count) since you are running wires in conduit, not cable, so you can use the 75C column in the table rather than the 60C column. Then you'd only need 18 cubic inches for that, rather than 30.

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    A 4-11/16" (120mm) square box (normally 2-1/8 deep) has enough cubic inches. Finish up with a domed cover or 2-gang mud ring (which are available 0" rise if you need that). – Harper - Reinstate Monica Aug 29 at 15:05
  • @Ecnerwal - Thanks. I didn't think about down sizing the cable, just because I had 500' of 6 AWG, but this is a good point. I can probably pick up a couple of feet of it. Plus 8 AWG will be that much easier to work with. – cueball Aug 30 at 13:21

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