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Goal: I’m installing a 400 Amp service in Washington State through Mason County PUD 3. The AHJ is Washington State L&I. For now, I’m installing what’s necessary to power my RV and well pump but I want the service entrance to be done in anticipation of 200A for my Shop/ADU/RV and 200A for my house, to be built in 1-2 years. Before I go asking stupid questions to L&I, I wanted to run my plan by you wise people.

Electrical experience: Not zero, but pretty close.

Q: Does this plan and shopping list sound reasonable and up to code: One in-ground run 750-750-500 MCM Aluminum XHHW-2 from transformer 170ft to meter, in 4” schedule 40 @ 18.96% fill. Replace 600 MCM meter socket line side lugs to accept 750 MCM. 200A panel w/ disconnect, 100A fuse for 100A sub panel, 30A fuse for well pump and one 110 outlet. Ground panels with 2/0 CU. In-ground run of 1/0-1/0-#2 AWG AL XHHW-2 175ft to 100A sub panel for RV (has 50A, 30A, and 20A fuses). Use 1.5” sched. 40 @ 21.09% fill. Ground with #6 CU.

Shopping list: Eaton B-Line 400A, 4P, 4 Jaw, Ring-Type Meter Socket (already purchased)
750 MCM AL XHHW-2 x 360ft = $1199
500 MCM AL XHHW x 180ft = $407
4” sched. 40 x 140ft = $1200
4” sched. 80 x40 = $32
Lug kit = $128
2/0 awg CU THWN-2 for groundx 10ft = $32
Square D Homeline 200 Amp Outdoor Main Breaker = $235

For 100amp RV subpanel 1/0-1/0-#2:
1/0 AWG AL XHHW-2 x 360ft = $227
#2 AWG AL XHHW-2 x 180 = $74
1.5” sched. 40 x 180ft = $418
Total= = $3952 Electrical plan map

Renn
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  • Isn't sch 80 required above ground? – Armand Dec 30 '23 at 05:44
  • https://diy.stackexchange.com/questions/38869/can-i-use-sch-40-for-buried-and-above-ground-feeder-wire – Armand Dec 30 '23 at 05:44
  • Is returning the meter socket and getting a different one an option? – ThreePhaseEel Dec 30 '23 at 06:01
  • All of my runs will be buried and inside conduit. Returning the meter socket is an option, but the alternatives that I'm aware of seem much more costly, such as prebuilt/wired meter sockets with two 200 amp breakers. – Renn Dec 30 '23 at 08:03
  • Your conduit pricing looks really messed up. Suspect 40' schedule 80 = $320. In fact, if you can get 4" schedule 80 for $8 ($320 for 40') per foot, use that for the whole run instead of schedule 40 at $8.57 ($1200 for 140'). – manassehkatz-Moving 2 Codidact Dec 30 '23 at 23:37
  • Is the Shop/ADU/RV all going to be at the current RV location? Or is Shop/ADU going to be at the current Meter + 200A panel location? Because if it will be at the RV location then you want to plan ahead by using larger conduit - 1.5" might get you to ~ 150A but definitely not to 200A. – manassehkatz-Moving 2 Codidact Dec 30 '23 at 23:45
  • And if your RV location is only for the standard RV setup (50/30/20) you need at most 80A (because the 30A and 20A are both 120V, so your total on each leg is 80A/70A) and can use 2 AWG aluminum with an 80A or 90A breaker with no loss of functionality but cheaper wire. And in any case, make sure to include an appropriate ground wire in the RV run. – manassehkatz-Moving 2 Codidact Dec 30 '23 at 23:48
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    NO FUSES!!! You have several references to fuses that should be breakers. There are still some uses for fuses, but not in a typical 200A panel ("100A fuse, 30A fuse") or an RV panel ("50A, 30A and 20A fuses"). Plus US standard is normally 120V, not 110V. – manassehkatz-Moving 2 Codidact Dec 31 '23 at 01:29
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    @manassehkatz-Moving2Codidact Thank you for all of that information! And yes, saying fuses has become a bad habit, I certainly mean breaker. The 4" schedule 80 cost twice as much as 4" schedule 40. and I did miscalculate those numbers. I will reconsider the conduit size to RV for shop/ADU. – Renn Dec 31 '23 at 16:56

2 Answers2

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Don't Oversize Your Wire

The supply to an entire property gets a very special 83% derate. For 400A that means 332A, though it is commonly referred to as "Class 320" (for 320A). Take a look at standard ampacity tables and you see 500 MCM for copper and 600 MCM for aluminum for 340A. (Checked another table - the one I linked doesn't have an in-between, but it appears 400 MCM copper would be fine for the neutral (not the hots) as well.) That's why the standard meter sockets for 400A service only go up to 600 MCM. That saves Lug kit = $128, as well as a good bit on the main feed wire.

Is it possible to have voltage drop issues resulting in needing larger wire? Absolutely. But at 170 feet that is unlikely to be an issue, and except for industrial situations, I would expect that a typical 400A service split to two 200A panels (as you are doing, and that is the usual setup) will have one 200A panel with a high usage and one with a much lower usage, so the overall usage is unlikely to be close to 332A, let alone 400A. Don't worry about voltage drop. (This assumes you are not going for high volume electric tankless water heating on both panels. And I don't recommend that even on one panel. Just don't. Please.)

manassehkatz-Moving 2 Codidact
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Do all the work in the meter cabinet now.

Your current drawing would require the meter to be removed, reinstalled, and re-sealed to connect the second panel to supply the house.

Installing the second 200A panel (or an outdoor enclosure with a 200A breaker, which may be cheaper) immediately prevents that, as would using a meter-main with the 2x200A breakers included.

Additional considerations:

  • As the supply neutral is bonded upstream, the 100A feed to the RV needs to be four-wire, with a separate ground wire.

  • Your well pump and local outlet ought to be on separate circuits, with both likely being 20A circuits.

SomeoneSomewhereSupportsMonica
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