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I am installing a 50 amp 120/240 volt RV outlet, the distance from my main panel to the RV outlet is 200’ what size wire will I need to run? What size breaker should I use in my main panel? What size duct should I run the wire through?

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    If you're installing a 50a outlet & cable to match, you'd better install a breaker no larger than 50a, otherwise you risk things catching fire. Also, "duct" is called conduit. Make sure you get the stuff from the electrical aisle, not the plumbing aisle. It's a touch more expensive, they're both made of PVC & both called "Schedule 40", but the plumbing stuff isn't NEC approved for electrical use.
    – FreeMan
    Feb 2, 2023 at 15:13
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    Schedule 80 from a real electrcial supply can be well worth it if using PVC. It's actually required anywhere the conduit is considered to be "exposed to damage." Then again, more expensive metallic conduit can save a lot on the cost of the trench, and replace the need for a ground wire.That can make the whole job less expensive, depending on circumstances.
    – Ecnerwal
    Feb 2, 2023 at 15:18
  • Conduit is also called duct - that's why the gray putty to prevent air and bug traffic is "duct seal"
    – Ecnerwal
    Feb 2, 2023 at 15:43
  • What is the purpose of this RV outlet? Is it to actually plug in an RV? Or is it for electrical vehicle charging? In either case, a small subpanel (they make them specifically for RV hookups with a 50A 240V, 30A 120V and 20A 120V) may make sense. But if it is for EV charging and not for RV use then a hardwired EVSE may make more sense (though still possibly with a subpanel so you can have a 120V convenience receptacle). Also GFCI is a Must. Feb 2, 2023 at 16:02

2 Answers 2

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50A breaker

4 AWG aluminum conductors (voltage drop might be an issue on 6 AWG - it's marginal, but not outright unthinkable. 4 is also the size at or above which you can buy all black and just tape the ends to indicate function - 6, you have to buy proper colors for. So you can get bulk deals on a roll of 4 that you won't get on 6.)

1 inch (or larger) conduit (duct)

Depending on local pricing, 2-2-2-4 aluminum mobile home feeder might even cost less, though it's overkill for a 50A feeder and requires 1-1/4" conduit, (unless you opt to direct-bury it - which needs a deep trench and is also not something I would recommend, in general, due to maintenance issues - digging up a damaged line is expensive) which might make it cost more.

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    If using 2-2-2-4 mhf and a subpanel at receptacle location then in the US you could feed it with up to a 90A breaker. 90A load would result in Voltage drop near 5% on feeder and would exceed NEC feeder recommendation of 3%. The 3% recommendation is pretty conservative considering NEMA equipment standards call out acceptable operation at 10% loss. Feb 2, 2023 at 19:41
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40A or 50A breaker is mandatory per NEC 210.21. If you are powering an RV, it must be a 50A breaker.

If this is for an electric vehicle charger, talk to us - you're following the lemmings and doing it the hard way! It's easier and cheaper than you know.

Most NEMA 14-50 RV outlets are rated for aluminum wire and 75 degrees C thermal. I advise people take full advantage of that. But look closely at the wire size limit of the socket - some limit you to 4 AWG wire.

As an aside - this has nothing to do with aluminum wire - it is vitally important to torque the screws to specification. If you don't, the terminals can burn up whether they are aluminum or copper - not aluminum's fault obviously. (this was NOT understood in the 1980s when aluminum got blamed for problems that were actually misuse. Heavy feeder has always been reliable.)

Below about 170' I don't even bother crunching the numbers. However at your distance I would consider one size bump, to 6 AWG copper or 4 AWG aluminum. If you're using UF-B cable this is the minimum size anyway due to UF-B's low temperature rating. NM cable cannot be used underground or outdoors, not even in conduit. Outdoor conduit is presumed to be full of water all the time; the protection is in the wire.

2 AWG aluminum may be at a pricing advantage, since biblical quantities of it are used in wiring 100A distribution around mobile home parks. However make sure it fits on your socket.

Aluminum is a particularly good choice if you are going to an "RV Subpanel" which typically provides you a 50A NEMA 14-50 and a standard 120V/20A socket, and often a 30A NEMA TT30 for smaller RVs. That is because subpanels have lugs which are made of aluminum. Aluminum lugs play well with both aluminum and copper due to the differences in thermal expansion working favorably. Torque still matters, as always.

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