I want to know what size wire to use running underground aluminum triplex in 3" electrical conduit from PUD transformer to my above ground free standing (mounted on posts/plywood back) 100 amp service panel which will feed a 100 amp RV panel on same posted/plywood. But, I want the wire to be uprated for future 200 amp home service. So, what size should I use to ensure I have adequate wire size for a 200 amp service for a home and not have to worry about pulling new wire at a later date. From what I have read from charts in code, it says 4/0. But what I am not sure about is what the other two wire will be. Will it be 4/0 4/0 2/0 or 4/0 4/0 4/0 or something different all together. Thanks Dean

  • You should consult a local professional. You have to consider things like distance, temperature, and material (copper vs aluminum). You will also need to consider all of your existing 100 amp connectors; are they rated for a 200 amp wire?
    – MonkeyZeus
    May 5, 2021 at 12:09
  • Are you considering how much headache you're trying to repurpose your 100A feed for a relocated service? Direct burial 2-2-4 AL URD for a 100A service is going to cost you about a buck a foot. May 5, 2021 at 12:51
  • I have not seen a 100 amp RV panel I have seen many panels feeding 50 amp RV’s. We do not know the distance to calculate the possible voltage drop. 4/0 al will be limited to 180 amp and if that is the feeder to the home it would be allowed to feed a 200 amp service but the distance could mean the wire size would need to be increased we don’t know. The allowance for a smaller feeder to the home only counts to the house feeder.
    – Ed Beal
    May 5, 2021 at 13:43
  • There are 100 amp RV panels. They have 50amp/30amp/20amp breakers and plugs. Just pull up on internet. Going 160ft underground in 3" conduit. This is a new service line coming from a new transformer on my property. I know I can run 100amp wire, but just in case I do build, I want the wire pulled for 200amp service just in case. I know it cost more. I am not worried about that. Since I have to run conduit and wire anyway, just makes more since to run large enough for 200 amp now. Make sense NOSPARK
    – Dean
    May 6, 2021 at 16:47

1 Answer 1


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.


Now, often when you go shopping for 100A wire, some fool will tell you "oh use #2 aluminum or #4 copper". That's wrong. The person is incorrectly applying 310.15(B)(7) on the assumption that your entire service is 100A, and they don't even realize their blunder because they're just giving 4th generation hand-me-down advice from someone else who misapplied 310.15(B)(7).

Actually, you need #1 Al / #3 Cu for 100A, and 310.15(B)(7) is saying that you never need larger than 4/0 (which isn't very helpful lol).

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

  • Sometimes, a meter
  • A 200A main breaker
  • 8 breaker spaces (they also make 4-space versions, avoid - too few).
  • "Thru Lugs" carrying the full 200A onward.

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.

  • It sounds like there may be some confusion about what I am doing. I am only going to have a 100amp service panel, with meter feeding a 100 amp RV panel which has a 50amp/30amp/20amp breakers and plugs. The wire size increase is only to uprate for a later date.
    – Dean
    May 6, 2021 at 17:02
  • @Dean Oh, sorry. you know I thought I'd seen something about that.... I didn't worry about it too much because it doesn't change my answer, except, change the 200A main breaker in the farm panel for a 100A and save it for that happy future day. May 6, 2021 at 18:28
  • The reason I don't go with 200 amp main breaker panel is because I am only buying 100 amp service from PUD. If I use a 200 amp service panel/meter I have to have 200 amp service. Since the panel and meter really have no future use application if I was to upgrade, it does not matter if I have a 200 amp panel, it would to be usable on a house.
    – Dean
    May 6, 2021 at 19:44
  • Regarding the wire size from the 100amp main breaker to the RV subpanel. What wire are you saying I should be using? (oh use #2 aluminum or #4 copper". That's wrong. Actually, you need #1 Al / #3 Cu for 100A) So you are saying that #1 al & # 3 copper should be used for a 100 amp service to sub panel?
    – Dean
    May 6, 2021 at 19:50
  • @Dean yes, #2 would be good enough so long as your entire service was 100A, but since you are planing an upgrade to larger service, #1 is needed for 100A. However, to complicate this, you only need 50A at an RV stand . If you have two RVs, then other rules apply, and you may not need 100A. May 6, 2021 at 22:01

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