A 50A circuit would be better as you have less restriction on what you can run if you need it.

A 30A RV receptacle is a 30A-120v.
A 50A RV receptacle is a 50A-120/240v.

Are you running the wire and is someone else doing to other work?

The question is.. What size and type of cable do I need to run that distance and achieve the desired 50 amps? I have had two recommendations from electricians and, after checking, apparently neither recommendation keeps it under a 5% voltage drop. Would a 3 pole, 4 wire, 4 gauge copper UF-B (no conduit) work, or do I need to go to 2 gauge?

The plug that I will use will be 14-50R for 50 amp or 10-50R for 30 amp. An unknown electrician in the Austin area will do the hookups.

Thanks

First of all, you figure your voltage drop for the load you will be drawing, not the total circuit ampacity. With #4,(copper) even if you were to draw 50 amps, the voltage drop would only be 4.4%. At 40 amps it drops to 3.5%.

You could use #6 and still draw 36 amps at a 5% voltage drop. So check and see what your max load will be.

If you are running conduit, you would use THWN conductors. If you are using UF, no need for the conduit. Of course there are minimum burial requirements for both.

Exactly Sherm. And how can we tell you what wire when you say it will be either 30A @ 120v or 50A @ 120/240v???
BIG difference my friend.

Personally I would recommend #4 for that distance.
Even at 45 amps, @ 240v, @ 350', you are at about 3.75% drop. Certainly reasonable considering you will likely never even draw 45 amps.

There's that "hookups" term again. LOL
I would definitely have someone come out and assist you before you run anything. Most real electricians do not like "hooking up" someone else's work. In fact, the "hookups" are the easiest part of the job.

Good point petey has brought up, your voltage drop at 240 will be half of what it is at 120 for the same amp draw.

Actual, I forget the number but it is more like 1/3 to 1/4 less VD for 240v compared to 120v. It is not a direct relationship. It is compounded.

The "voltage drop" for a given amperage is independent of the the line voltage. But it's not a "given amperage" you are concerned about, it's usually a given power. The amperage at 240 will be half (typically), which means the voltage drop is half. Add to that the fact that a given voltage drop is at 240 is only half the "percentage" of the line voltage.

As a result Petey is right. It's 1/4 in an ideal circumstance.

That all makes more sense to me, thanks for the education, my friends!
My target amps load is 45 or better, so a 4/4 copper should be okay, considering everything. Will check price differential on UF vs. THWN with 1-1/2" conduit.

I want to run either 30amp or 50 amp 350 foot from my breaker what size wire will I need? The cost will determine what I put in. The most I will run on it is a RV airconditioner, and that wont be very often. Thanks Mike

can i cut off my male fitting from my rv and ad 350 ft to it and plug it back into my 30 amp outlet at my house,just saying lol

i am running a75 foot underground 6/2 in conduit with a single pole breaker to a 30 rv recepticle box is thus set up acceptable / 2 strand wire. in a 18 inch ditch

6/2 would be way more than you need. A 30A circuit at 75' would be fine at 10g (copper) or 8g (aluminum). Make sure you use wet rated conductors.