conduit to connect panels outside

Question

I am connecting a 200 amp meter/distribution panel to a 100 amp RV panel (both mounted on the same post outside). Can I connect the two with metal, pvc, or the flexible non metal or metal conduit to run my wires? They will only be about 6 inches apart, back to back. So, it would work best to come out side knockouts, because the RV panel is only 9inches wide and i would not be able to access the back knockout. Please advise. Thanks Dean

Answer 1

Honestly, I would affix a chunk of marine plywood to the pole, prime it, paint it with exterior paint, and attach the RV panel to that. That will give you more freedom to position it for a "back to back", "out the bottom of the RV panel and then in the back of the main" with a single bevel 90, or down, over and up with two 90s.

Given the very short distance, the cost of wire is negligible, so I would use #3 copper. I'm not a huge fan of copper for feeder, but it's the most compact, so it'll fit in the smallest conduit.

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Here's the thing about those "50/30/20" RV stands. Generally, you're expected to install one of those per RV site. And then, the RV is expected to use the one socket that is compatible with its power.

You keep talking like using all three of them at the same time, you're not required to provision for that. You only need to provision for the largest socket available to any given site, and you ignore all the smaller sockets.

If you follow the "One 50/30/20 panel per site", then it's pretty simple. You calculate based on 50A/12,000 watts per panel/site, and then apply the favorable derate that Ed Beal mentioned from 551.73.

• 90% if 2 sites (so 90A for two 50/30/20 RV stands)
• 80% if 3 sites (so 120A for three 50/30/20 RV stands)
• 75% for 4 sites
• 65% for 5 sites
• 60% for 6 sites
• 55% for 7-9 sites (thus you can support seven 50A sites from 200A service)

So again, you don't have to provision more sockets than you have RV sites. You just count your RV sites and provision that many sockets, largest first.

And follow what Ed Beal says about how 50A RVs take a lot more power than 30A RVs. That is because 50A RVs take 240V power, not 120V. They take over triple the power of a 30A.

• 50A/240V sockets are 12,000 watts
• 30A/120V sockets are 3,600 watts
• 20A/120V sockets are 2,400 watts
• By the way, tent sites follow a special rule and you must only figure them for 600 watts.

And then apply the derate as Ed says.

Like I say, given your 100A provisioning, I would just go ahead and install two 50/30/20 RV subpanels at wherever you expect will be most useful. Because the subpanels are good for 100A busing, you don't actually need to give them separate breakers - you can simply daisy chain from one subpanel to the next. But the wire needs to be 100A.

Does it matter how many sites you support with two 50/30/20 RV stands? Actually it doesn't -- no combination will exceed 100A.

• 2 sites: 12,000 x 2 = 24,000 W x 90% derate = 90A
• 3 sites: 12,000 x 2 + 3600 = 27,600 W x 80% derate = 92A
• 4 sites: 12,000 x 2 + 3600 x 2 = 31,200 W x 75% derate = 97.5A
• 5 sites: 12,000 x 2 + 3600 x 2 + 2400 = 33,600W x 65% derate = 91A
• 6 sites: 12,000 x 2 + 3600 x 2 + 2400 x 2 = 36,000 W x 60% derate = 90A

Answer 2

Dean the 50/30/20 panel can only draw 80 amps there is 1 240v load the other 2 are 120v and are each put on 1 leg so the maximum load would be 80 amps with all 3 maxed out, code requires the load to be balanced one leg would have a max of 80 the other 70 and that is IF the 3 were drawing maximum (they never do).

Now if we look at code NEC 551.73 the feeder shall be based on 12000 per site with 50 amps, 3600 va per site with 30 amp and 2400 per site with 20 amp.

That’s all well and good 50/30/20 but now if you start with multiple units table 551.73.A allows the feeder to be have a derate 3 sites is a .80 18000x .80=14400 /240= 60 amp

So by code if you were planning for 3 sites on this feeder you could legally feed this with a 60 amp breaker and wiring and be fine as I said above you never draw full capability and yes I have wired several MH parks.

As I suggested if you want some over kill go 80 amps on your feeder breaker, If you want a little buffer above the absolute minimum go 70 amps And the minimum is 60 amps for these 3 sites.

if in Arizona where you will need lots of cooling go with 80 again that is the max the 3 sites can draw as 2 are 120v and they will be on opposite legs.

70 amps provides a nice point between the max draw and minimum if just the 2 main sites are in use a 60 amp feeder would be code compliant.

2 sites is a derate of .90 and the calculated load of 58.5 rounds up to 60 amps. Again 70 would provide a nice cushion between the minimum allowed by code and the maximum that could be drawn 80.

Putting in a 100 amp breaker and feeder is a waste unless you already have the breaker. If you like to go big go 80 that is more than needed and totally future proof. If you want to save a little go 70 still going to work for most areas most of the year. High heat areas when multiple AC’s are the worst case I have seen.