30 Amp or 50 Amp?

[rmkrum]

I paid attention during the electrical power classes. AC and DC...

That knowledge is circa 1966-1967. Hasn't changed much.

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[TG Twinkie]

I misspoke in my comment above. Meant to type one leg as having a load of 10 amps and the other 20 amps. In this case the load is not balanced and the neutral carries the difference.
In a perfectly balanced system there would be no current flow on the neutral.
Unfortunately this is not the case when we look at an RV. Since most only use one hot leg and one neutral.
The reason the neutral wire doesn't have to be larger than either hot leg is it never has to handle more current than the maximum current of one leg.
Typically RV's that require a 4 wire 240 volt system are not wired to balance the load. They are wired so one can run 1 A/C unit and all other AC devices on 1 hot leg and 1 neutral. The second leg is used for the second A/C unit.
If this were not the case you could not use adaptors and have all systems working except for the second A/C unit.

[HiJoeSilver]

As a price point for those thinking it's not that big a difference.

Assuming say 125' run. <3% voltage drop. Meeting Table 310.16 Allowable Ampacities. Copper wire. (Aluminum can be used but requires larger wire)

For 30 amp you could get away with direct bury UF-B cable, though I'd still put it in conduit, the UF-B in 6/2 would cost about $155.
For 50 amp you'd end up needing to use 4 gauge cable USE-2. You'd need 4-4 cable equivalent, since you need an extra conductor, and would cost around $415. You could save a small amount by using a smaller ground, which is allowed if installed properly. This is 4 separate cables though so will require a little more labor.

That's a $260 difference.

But a $260 diff vs doing it twice!?

If you can square these numbers with the electrician, buy the wire yourself, rent a trencher, you could probably save some money having him make the final connections.
USE-2 and UF-B can both be direct buried but I'd put them in conduit. <$100 in materials. The 4 gauge will be fun pulling so go with a larger conduit(1-1/2 for more pulling room, 30 degree bends to make it easier).

Don't forget a permit! Hate to have to rip it all out!
Don't forget 811 call before digging.

[BAB]

I have two 50 amp boxes at home. One next to the shed I had built for the trailer. I keep the trailer plugged in at all times since I've got a high quality (non Airstream) Magnum inverter/charger, as well as a (redundant) PowerMax PM4B 60 AMP 4 stage Converter/Charger. I installed an additional Jamestown box adjacent to my driveway that covers the waterfront...50 amp, 30 amp, and 20 amp with GFCI. I use this box when I'm getting ready to launch for a trip, and it gives overnighters full electric as well. The biggest hassle in the installs is pulling that big cable from my main panel. The pain was worth it.

[xrvr]

Neat setup!