This isn't a shopping channel, but unless money is no object, money will rear its ugly head. So I will heavily price the options, based on prices from random USA big-box stores. (and these are "by-the-foot" prices for the wire, a whole 500' spool may be cheaper.)
This is a factor. The expensive rigid conduit only needs 6" deep burial, the cheap PVC conduit needs 18". You can (in some sizes) buy direct-burial wire that must be buried 24". That's a lot of trenching for 350', consider the cost difference there. The cost of conduit also interlocks with the thickness of wire, as smaller wires need smaller conduit. As some examples of cost for conduit alone:
- $300 -- 2" PVC conduit legal to bury 18" deep
- $1400 -- 2" RMC conduit legal to bury 6" deep
- $1200 -- 1-1/2" RMC conduit legal for 6" deep (barely worth the savings)
- $500 -- 3/4" RMC conduit legal for 6" deep
Before you rush to use the minimum size conduit, price the next larger size. This will make wire pulling a lot easier, which means you are less likely to get stuck and need to call an electrician for the pull.
Do you really need copper? How big a deal is voltage drop?
People often go to copper by default, or because of "problems they hear about". That was never an issue for large cables like this. The fact is, large-wire lugs are made of aluminum. The cost of copper is insane, and nobody buys direct burial cable in it, anyone splurging for Cu also splurges for conduit. I'll price both ways and add conduit to the Cu pricing.
Voltage drop: Realistically your RV will rarely use 30A and almost never use 50A. The "conventional wisdom" is to size for 3% voltage drop of your realistic usage (30-50A). However some people nonetheless spec wire for 3% voltage at absolute max current capacity (100A). Your call. Assuming the latter:
- $3600 -- copper 2/0 conductors (#1 ground) in 2" PVC conduit (buried 18")
- $1340 -- aluminum 4/0-4/0-4/0-2/0 MH feeder cable (direct buried 24")
- $1700 -- aluminum 4/0 conductors (2/0 ground) in 2" PVC conduit (buried 18")
- add $1100 if you'd rather bury rigid conduit 6".
Size voltage drop for actual use
Now if you want to size for 3% at 50 amps, and 6% at absolute max 100A - so a 7V drop on a 120V circuit - you can save some coin. Unfortunately I could not find direct burial cable in these sizes, they would expect you to bump up to the above sizes, or use conduit. So we shall.
- $2750 -- copper #1 (#3 ground) in 2" PVC conduit (18" burial)
- $1400 -- aluminum #1/0 (#1 ground) in 2" PVC conduit (18" burial)
- add $900 if you want to use Rigid conduit buried 6" deep.
Transformers
When you increase voltage, transmission losses drop dramatically. Just for chuckles, let's look at using transformers to kick up the long-haul transmission voltage to 480V. Now you want 240V at 100A, which is 24,000 watts, or 24 KVA. 24 KVA is 50 amps at 480 volts. To contain voltage drop within 3.3 percent, you'd use
- $560 -- #6 copper direct-burial UF-B cable (buried 24")
- $500 -- two copper #6 wires in 3/4" PVC conduit (buried 18")
- add $500 for rigid conduit buried 6".
25 KVA is a common transformer size. With aggressive shopping/deal-hunting, you can find 25 KVA 480V transformers for $300-400 each. With two of those, the total bumps to $1100-1800. Much as I love to throw this option in, this one is a wobbler - is the slightly lower cost worth the added complexity? On the other hand, it sure makes the wires and conduit small!
Keep in mind you are not running neutral or ground, in fact, your two 480V wires are entirely isolated from the electrical system at both ends. This isolation causes your remote site to be separately derived service, thus a *main service" and the panel there will be a main panel.
Best Answer
Apply 310.15(B)(7) to your entire service
NEC 310.15(B)(7)says that you take your entire (planned) service size - 200A - and multiply it by .83 (83%) giving 166A for you. You never need to use wires larger than that anywhere that's served by that service. So any feeder - even if it's 200A feeder - only needs to be 166A not 200A.
So that calculates out to 4/0 aluminum (180A). 3/0 aluminum is too small to carry 166A.
Footnote:
It's totally OK to oversubscribe
So for instance if you have 200A service, it is OK to have 200A going to the house, 100A going to the shed, 50A to an RV stand, 30A to a well, etc. even though these add up to more than 200A. That's fine.
You are restricted on how much total load you can have on the service, and this is based on a Load Calculation. But that allows for intermittent and "non-simultaneous use". You still want to make each feeder big enough it won't trip on its local loads e.g. welding in the shed with the compressor running.
Consider a "farm panel"
This is a type of service panel that has
The idea is you come off the "thru lugs" to the biggest load - e.g. the house. It has access to the full 200A. Then you use the 8 breaker spaces for four 2-pole "feeder breakers" -- e.g. 50A to the RV stand (they don't need more unless there are two RV stands). 100A to the shop. Etc.
If your existing feeder to the house is only 100A, then go ahead and feed it from one of those four 2-pole brakers, just slap a 100A breaker in there (90A if the wire is #2 aluminum or #4 copper).
...Except my service is only 100A
Since you expect to go to 200A, I advise still doing the above.
Only difference is you fit a 100A main breaker in the farm panel instead of 200A. The 200A version of the panel is so popular that cost wise, you are probably better off buying the 200A version of that panel, then buying a retrofit 100A main breaker for it. Then bag the 200A breaker and leave it in the bottom of the panel for that happy day in the future.