400 Amp service to 225 amp and 150 amp panels

Question

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.

Answer 1

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.