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I am running a 10 awg wire for a 30 amp camper outlet to a carport. Can I add an outside receptacle to the same line or do I have to run a separate 12/2 cable for the receptacle?

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15A/20A can't be on a 30A circuit. However, it might make sense to put in a subpanel. You could even feed it with much larger aluminum wire for not much more cost than the 10 AWG copper wire, which would give you more flexibility.

For example, you could use 2 AWG aluminum and a 60A breaker to feed a small (perhaps 12 spaces) subpanel. Put in the 30A camper receptacle as one circuit, the 20A convenience receptacle as another circuit, and you can later add a 20A or 30A hardwired EVSE, additional 20A circuits for tools, etc. You can't exceed the total capacity of the subpanel (60A can provide 48A continuous) but the breaker values can add up to far more than the feed size without a problem (like they already do in your main panel).

As noted, all 15A and 20A receptacles need to be GFCI protected. In many areas, depending on NEC adoption and local amendments, other receptacles (30A or larger, 120V or 240V) require GFCI protection as well. A hardwired EVSE (a.k.a., electric car charger) might not need GFCI protection added as it may include that built in (check the installation manual), but if it is plug/cord connected to a receptacle then it does need GFCI because you could plug in something else.

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    That #2 aluminum will also be about the same price as the 10/3 they might otherwise run. Commented Nov 25, 2022 at 20:34
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First, any outlet to a carport should think about EV charging. Why? Because home buyers make better offers on cars with EV ports! (If your actual intent was EV charging, a 30A RV outlet is the wrong thing to do. It has the same overload risk as a 30A/240V outlet, but is half the speed at best and on many vehicles will crank down to plain old 1.4 or 1.9kW level 1 charging).

30A Travel Trailer outlets are 120V. That means thinking extra hard about voltage drop without wasting money on #8 wire, preferably.

GFCI

The first issue we must confront is GFCI. It's always a good idea in a garage outlet to have GFCI protection. If your state has adopted NEC 2020 and not repealed this part, GFCI protection is a requirement.

Now, GFCI is a hot mess on a 30A/120V outlet. First, there is no such thing as a 30A GFCI receptacle, and the various GFCI outlets and deadfronts have a 20A max, so you can't pass GFCI protection through them. Your only option is a 30A GFCI circuit breaker.

And the problem is, 30A/120V single-pole GFCI breakers are a black swan. Even if they're made, they'll be difficult to find and costly when you find them. (because there is no use whatsoever for a 30A/120V breaker, except travel trailer outlets. I've looked.)

So you are actually better off in the end buying a 30A/240V double-pole GFCI breaker. Not least, you're more likely to find other uses for that breaker in the future, instead of having a single-use throwaway.

30A and 20A outlets at once

That's a problem. 20A outlets aren't allowed on 30A circuits because a 30A breaker won't adequately protect a 15-20A appliance. (UL requires that "normal socket" plug-in appliances be designed to trip a 20A breaker if they fail).

However, one way to solve this is a subpanel at the garage. In fact there's a special type of subpanel called an "RV panel". It provides a 20A GFCI and a 30A RV socket in the same enclosure. Some of them even provide a 50A 240V RV socket as well. These can be connected to the main panel using inexpensive 2 AWG or even 6 AWG aluminum feeder.

Another way to solve it, particularly if you need GFCI, is to install two outlets next to each other using 10/3 cable from the panel: A NEMA 14-30 (4-prong dryer outlet) and a NEMA TT30 wired into the same circuit. Then, adapters are readily available which plug into the 14-30 and provide 15-20A sockets. These adapters, which are UL Listed, have internal fuses to protect the 20A outlets.

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Standard 15 and 20 amp outlets can't be protected by a 30 amp breaker. Play it safe and run a separate 12/2 run. Outdoor outlets need to be GFCI protected so either a GFCI outlet or a GFCI breaker. GFCI breakers are three times the cost of a GFCI outlet last time I checked.

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    But keep in mind that the 15A and 20A can be protected by a GFCI/receptacle. I've never seen a 30A receptacle with a GFCI, so if code requires it (which it is starting to, jurisdiction dependent) then that may require GFCI/breaker. Commented Nov 25, 2022 at 18:47

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