Adding a 100amp RV sub panel outside of garage wall opposite 200amp main panel on other side of wall. 50/30/20 setup inside sub panel. What type/size wire is needed to feed it? 50amp breaker in main panel? Sleeve through wall?


2 Answers 2


We can't begin to guess. We need to know the loads that will be fed by the subpanel.

Now. If your subpanel will only be feeding RVs, then it's a simple matter. With one RV stand, you simply take the largest socket and that's it. 50A = 12,000 VA = that's what you provision.

What if there are two RV stands? HAHA! We take the two largest sockets and we assume the RVs are using those. So a 50A socket is 12,000 VA and a 30A socket is 3600 VA*. We add them up (15,600 VA). Now with multiple RVs it's vanishingly unlikely both will be maxed out at once, so we get a "favorable derate" per NEC 551.73. 2 stands = 90%. 90% of 15,600 = 14040 VA. Divide by 240V and that's 58.5 amps. A 60A circuit will suffice.

There is no such thing as 60A wire, however. So you'd be forced to 65A wire. NM isn't allowed outdoors, UF isn't made above 55A, so that forces you to MH feeder or XHHW in conduit. #6 copper or #4 aluminum will suffice. #2 aluminum may be value-priced, as it's widely used for mobile home feeders. That is 90A. That's enough for two 50A RV stands. ( 50A x 2 x 90% = 90A).

* Now you have two questions. First why did we switch from amps to VA, and second how is the 30A socket about 1/4 of the 50A socket? Because they are different voltages. The USA is not a 120V country but the 30A RV socket is 120V indeed. (as is the 20A GFCI socket). In a mixed-voltage situation like this, you convert everything to VA (Volts x Amps).**

So actually, your 50A+30A+20A isn't a 100A panel at all, because the 30A and 20A are 120V loads. It's a 12,000 VA + 3600 VA + 2400 VA panel, total 18,000 VA = 75A panel.

**. Wait, isn't that VA thing just Watts? Yeah, for these purposes. The distinction matters when dealing with motor loads and other things that load the AC sinewave in a funny way, forcing you to deliver more amps than the watts would indicate.


A single pedestal with multiple RV receptacles are normally considered non-coincidental loads so are sized for largest receptacle. Some people would recommend aluminum, but for such short distance cost may not be critical. #8 copper THWN in conduit would be adequate. #6 aluminum THWN or XHHW would also be acceptable.

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