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  • 3 30-amp breakers for 3 campers
  • 1 50-amp breaker for 1 camper

Total of 140 amps needed to accommodate the 4 campers.

The main house panel has a 200 amp main.

What size breaker do I need for the sub panel for the campers?

What size wire do I need less than 100’ from panel and campers?

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  • Do you know how much load the existing house uses of that 200A? Can you get us photos of the house's panel, as well as the square footage of the house? Feb 21, 2022 at 2:57
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    How many camping spaces are present? You only need to compute for the worst case per space. For instance if you have 3 spaces and one of them has 50A/30A options, you can delete one of the 30A's. Also, read what ThreePhaseEel says as you misunderstand how amps, 120V and 240V work. 50A taps both legs of the panel, 30A is 120V and only taps one leg. So the total is 80A on one leg, 110A on the other (if you think of it that way). Feb 21, 2022 at 5:03

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The calculations are done in VA, not amps

The first issue with your numbers is that 30A campers only use 120V, while 50A campers get 240V. This means you have to do your load calculation in VA, with the 50A camper getting 12000VA and the 30A campers getting 3600VA each, as per NEC 551.73(A), for a baseline total load of 12000 + 3600*3 = 22800VA. However, 551.73(B) also permits a demand factor from its associated table to be applied; since you have 4 sites, this specifies a 75% demand factor to be taken of the calculated baseline total, for a final load number of 17100VA, or 71.25A @ 240VAC.

Fat aluminum is your friend here

For this level of current, I would recommend you use 2-2-2-4 Al mobile home feeder cable instead of copper wire if at all possible; it can handle 90A when landed on the 75°C terminations found in modern distribution equipment, and is far cheaper per-foot than the equivalent copper wire, in addition to being available as a commodity product.

Running it in a 1½" or 2" PVC conduit is preferable since it leaves room for later wiring upgrades, although one can forego that if conduit's not an option for some reason. The subpanel breaker will be 90A for most modern panel types, as well.

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  • 551.73(B) isn't a "requires" so much as an "allows". You can supply 22800VA if you want to, but you're allowed to derate 25% due to 4 camp sites per that table. (since the chances are nil that all 4 sites will be maxed out at once). Feb 21, 2022 at 5:04

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