I'm running a service to an RV that’s about 300 feet from the sub panel that I’m going to install off of the main panel in the barn.
I plan on having two RV spots about twenty feet from each other with each having its own electrical (pedestal) for them to get their power from.
Would one 200 amp service be enough to carry both RVs? And would #8 AWG be enough or would it be better to run #1/0?
What would be best? Remember it’s 300’ from the existing supply panel.
The gold standard way to provide power to an RV site is to provide each of these:
Because an RV site can only hold one RV, you only have to assume one is in use at a time - the 30A or 50A. Not both. So you only need to provision 50A@240V to the site.
Most novices either know nothing about wire metal, or have heard "scary stories" about aluminum. Those stories do not apply to feeder in the 50A and larger range. For a 300' run, aluminum is the only correct choice.
There's no money savings to supporting only the smaller capacity, because voltage drop is 4 times worse at half the voltage, so it actually has the need for the largest wire size.
The advantage to separate feeders is you can wire it straight to commercially available RV "pedestals". However you pay for it dearly in higher wire cost. And worse, each RV experiences the worst voltage drop; if only one RV is present, the wire capacity of the other feeder is completely wasted.
The advantage to a single feeder is you only need one cable run. You either have one subpanel and build your own sockets at the stands; or you simply install two RV pedestals and daisy-chain your feeder from one to the other. See notes at the end. But more importantly, single feeder will perform extremely well with one RV.
Let's follow the Canadian rules: 3% (ish) at 80% of load. (so 40A @ 240V or 24A @ 120V).
2 separate feeders:
Worse, the #1 size is difficult to find; it's not made in MH or URD cable, forcing you into individual XHHW wires in conduit. Now I happen to recommend conduit, but still.
Single feeder:
And here's where a trick comes in. NEC allows for the fact that both RVs are unlikely to be maxed out at once. That's covered in Table 551.73, which gives a 90% derate for two RV stands. So we don't double it to 80A, quite. We only compute on 90% of that, or 72A. With the 120V/30A circuits, we can use a "trick" where we "balance" the two 120V RVs on opposite "legs" of power. It's legal in principle but doesn't work that well in practice, because their A/C units don't cycle at the same time.
For the 120V RV, the worst case scenario is one RV at max draw, so 24A @ 120V gives us 1.9% voltage drop on 2/0 aluminum. That's plenty fine.
But again we're following the rules in the People's Republic of Canada. In the USA we have freedom... Yes, we have to provision 90 amps to these dual 50A RV stands, but that only requires #2 aluminum wire ($400) if we don't mind getting murdered by voltage drop when two RVs are going hot and heavy at the same time. Freedom isn't free :)
With only one RV, #2 aluminum won't be an issue. When two 50A RVs are running all A/C packs on full and somebody's doing a dryer load, that's when voltage drop will be apparent.
I would definitely run a single feeder to a subpanel out at the RV stands, which then feeds 50A, 30A and 20A receps at each RV stand. As far as wire size:
In Canada you must use #2/0 cable and you must breaker it at 100A - that's Code there.
In the US I would back it down to exactly what you suggested - #1/0 aluminum ($525). That more reflects real world usage. Even with both RVs heavily drawing (80A) that will only have 4.16% voltage drop. 5.20% at 100A, but that isn't likely to happen and it's still not terrible.
For 120V RVs, the worst-case of one RV pulling 24A gives 2.5% voltage drop, or at full 30%, 3.1% drop. That is fine.
For the short feeders from the subpanel to the RV stands, I would run Rigid Metal Conduit (RMC) - hella expensive, but it only needs trenching to 6" cover. THHN wire can handle 50A with #8 wire (most cables need #6). #10 for the 30A and #12 for the 20A. One ground wire for all of them.
ThreePhaseEel's comments on feeding to multiple RVs:
The nifty trick with a single feeder is you can either get a RV pedestal with loop feed lugs, or a back-to-back unit that has two pedestal "heads" fed from the same feed lugs, and not need a separate subpanel to feed both RVs from one feeder as a result...
...(For instance, a Milbank U5220-XL-75 or a Midwest Electric U075CB6010 would take care of all the OP's RV power outlet needs)
Given the "standard" "big" RV outlet is 50A/240V, 200A would carry 4 of them - if there were no other loads. Per NEC guidelines, 12000 VA per 50A RV socket, meaning you provision them as if they were fully loaded.
Unless you mean the smaller 30A 120V service? That is also provisioned as if fully loaded, but fully loaded is a lot smaller (3600 VA), particularly given that you can run two of them off one 30A MWBC.
Whether you can add two of them to a 200A panel with other loads depends what the other loads are, and how their use and the RV use overlap in time.
8AWG copper is not a good choice for either configuration at 300 feet. 1/0 AWG aluminum would be fine (1/0 copper would be outrageously expensive and foolish) though it would not meet the "guideline" (and that's all it is unless you are in Canada) of 3% drop for the 100A @ 240V case - but it's less than 4% drop and if you want to reduce the drop, larger aluminum will cost a good deal less than copper that size.
