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I want to upgrade my 200 amp service to 400 amp, to service multiple buildings (I'm told this is cheaper than two 200 Amp services). This is in Washington State.

I know I can do 200 and 200, but that may be underkill for the main house and overkill for the rest.

For the main house, I'm concerned 200 Amps may not be enough, as I'd like to add an electric tankless water heater.

For the other buildings (a bunkhouse, garage, and boathouse), I think 150 amps is enough. I'd start with a 150 amp box in the adjacent building (the bunkhouse) which will provide power to a 100 amp subpanel in the garage across the street, and 50 amps for the circuits in the bunkhouse and boathouse. I want 100 amps in the garage so I can have a 50 amp circuit for an electric car charger, 20 amp circuit for a well pump, and a couple other circuits for lights and power in the garage.

If I have the 400 amp service come into something like the Milbank R3548-X (320-Amp 1 Gang Ring Overhead Only Meter Socket) mounted outside the main house wall, can I then feed through the wall into something like the Square D HOM3060L225PGCVP (225 Amp, 30 space, 60 circuit load center) with a 225 amp breaker in that box, and also feed from the meter box to something like the Siemens W0202MB1150CU 150 Amp Outdoor Circuit Breaker Enclosure mounted next to the meter box, which then enters a conduit to go the 20 feet or so to the bunkhouse, where it would enter a 150 amp main lug center, containing all the circuits for the remaining buildings? I don't know if I can go straight from the meter box to the bunkhouse without first having a breaker. If I did, I'm worried I'd have to have wire sized for 400 amp. Or if I have a 150 amp breaker in the bunkhouse, can I go straight there from the 400 Amp meter box using wire appropriately sized for 150 amps?

Assuming this all make sense, I have questions about wire size/type, conduit size, and how to make the connections from the meter box to the two panels, but I'll hold off on them until I first know what I've described is reasonable.

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  • Have you discussed this with the local inspection people? – JACK Aug 5 '20 at 18:01
  • I have not had any luck talking with local inspection people. They seem to only want to give feedback during an actual inspection, after I've spent hundreds of $ on equipment and many hours installing it. – Tom Getzinger Aug 5 '20 at 18:21
  • Current 2020 code requires the disconnects to be at the service I know Oregon is adopting that section and our states are fairly close having reciprocal agreements for electrical , neither state will adopt the 2020 code until November and the last I saw it might be may of next year but some local counties already require the disconnects to be at the service so I would plan for all the mains to be at the service (not having the only main in the bunk house) I like mains at each panel unless they are in the same room. – Ed Beal Aug 5 '20 at 18:59
  • I've heard WA adopts NEC 2020 on Oct 29. So, the Siemens 150 amp box next to the meter box will be needed for the bunkhouse. I'm hoping having the 225 amp breaker in the box inside the house right on the other side from the meter box is sufficient. I haven't found any 225 amp boxes like the siemens 150 amp one - highest I've found is 200. I suppose I could get a 225 amp outdoor panel, but would prefer the main house's panel be indoors. – Tom Getzinger Aug 5 '20 at 19:32
  • Actually, I found this: Siemens WB3225 225-Amp Outdoor Circuit Breaker Enclosure. So I could have three boxes outside: the 150 amp breaker enclosure, the meter socket, and this 225 amp breaker enclosure. Then, I'd have a 150 amp main lug box in the bunkhouse and a 225 amp main lug box in the main house. That just feels like a lot of boxes that will be mounted on the front of my house. I'd like to minimize that as much as possible. – Tom Getzinger Aug 5 '20 at 19:41
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I'm concerned you're overthinking this. You're not required to section out your 400A allotment to X for one building, Y for another etc. that totals up to 400. You are allowed to "oversubscribe" the buildings, so long as they're behind breakers.

Also keep in mind, 400A is a lot of power. All-electric houses have long gotten by on 100A.

Build your system to have the versatility for any site to use full power

For simplicity and economy, I'd simply do the bog-standard "two 200A panels" that everybody does with 400A service. House panel A and B.

However, I'd make house panel B "special". I'd use a panel with thru lugs. The thru-lugs are after the panel's main breaker, but otherwise just hotshot straight off the bus.

Off the thru-lugs, I would continue 200A wires the 20 feet to the bunkhouse. Hey, it's only 20 feet, the cost of 4 wires isn't going to beat you up too bad. There, I'd fit a 225A (for the safety margin) main breaker panel. The breaker's value doesn't matter; it's only there to be a disconnect, which is required for an outbuilding.

Is it an outbuilding? The minimum standard for "not an outbuilding" is "connected by a breezeway", so you can skip the main breaker and go main-lug if there's a breezeway or better.

What this means, is house panel B and the bunkhouse share a 200A breaker.

From this panel in the bunkhouse, go ahead and feed the other buildings. You can feed them with a breaker from bunkhouse's panel... Or if you want to carry full 200A onward to the next building, go ahead and do that - you can just "tee" off the feeder wires coming into the top of the bunkhouse breaker. (that is to say, the 200A line to the garage does not need to be protected by the bunkhouse breaker, since it's already protected by the panel B breaker.)

Personally if I were me, I'd feed the dockside with a <=50A breaker, and the reason is so I can make it GFCI. I feel strongly that all wiring anywhere near water should be under GFCI protection, and that GFCI protection should be placed far from dockside, back at the supply point. That way if something happens inside the dockside subpanel, or to the feeder wires, it too is protected. Electrical drownings are serious, and dockside is where that happens the most.

You won't be maxing out house, bunkhouse, garage and boathouse at once

This is the part where we get to "oversubscribe" a bit. You're vanishingly unlikely to be welding in the garage at the same time you're running the A/C and cooking on a boat at the same time you're using hot water and cooking in a bunkhouse at the same time you're cooking and doing laundry in the main house. There are only so many of you, after all.

So that means it's perfectly OK to potentially have 50A of boathouse load with 100A of garage load (Tesla fast charger) with 100A of all-electric house in the bunkhouse with 120A of stuff in the main house. You simply are not capable of maxing them all at once.

However, back at the house, I would make careful choices about which loads I put in the exclusive panel A vs which in the shared panel B. Have the shared panel contain loads like bedrooms, basement, living rooms, maybe garbage disposal and microwave - loads you are unlikely to "pull hard" while being active in the garage or dockside. Loads that you realistically do pull quite hard, like A/C, heat pump emergency heat, water heater, range etc. go in the exclusive Panel A.

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  • Thank you for the detailed answer. I thought NEC (2020) doesn't allow two panels to feed the same structure. Do you have A and B both on the house? An electric tankless WH can draw up to 150 amps. Add the 50 amp car charger, and I'm pretty loaded. With other random loads, I'm concerned 200A is insufficient. In fact, if I go with a large tankless WH, 200 may be tight for the house. If this gets to be too large of a pain or expense, I will likely go with 400 Amp meter and 2 200 amp panels and make sure I use a smaller tankless water heater. – Tom Getzinger Aug 5 '20 at 21:49
  • Yes, two 200A panels is the standard way of coming off a 400A "320 class" meter. Yes, both at the house right next to each other and very close to the meter (so the unfused service entrance is as short as possible). I know that's surprising when you first see it - I was surprised - but there it is. If you have a very surgey load like 150A of water heater, I'd put that in the exclusive panel. – Harper - Reinstate Monica Aug 5 '20 at 22:39
  • You may be thinking of "two separate services", that's not allowed. But this is one service. – Harper - Reinstate Monica Aug 5 '20 at 22:42
  • If I follow, A (200A) feeds main house. B (200A) feeds main house "special" plus outbuilding. 225.30 says "A building or other structure that is served by a branch circuit or feeder on the load side of a service disconnecting means shall be supplied by only one feeder or branch circuit..." Haven't we violated that? The impression I get is they want a single "disconnecting means" for the structure, and we now have two. Perhaps if I had a single 400 amp breaker for the whole main panel, then feed the two sub-panels? – Tom Getzinger Aug 6 '20 at 1:51
  • @Tom that’s not what that rule is for. It’s saying you can’t have two same voltage feeders to an outbuilding. That doesn’t bear on the dual-200A panel method of breakering a 400A service. You can have up to six main panels as long as they’re next to each other. They must be grouped; they cannot be six all over the place. – Harper - Reinstate Monica Aug 6 '20 at 2:25

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