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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

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  • Isn't sch 80 required above ground?
    – Armand
    Commented Dec 30, 2023 at 5:44
  • diy.stackexchange.com/questions/38869/…
    – Armand
    Commented Dec 30, 2023 at 5:44
  • Is returning the meter socket and getting a different one an option? Commented Dec 30, 2023 at 6:01
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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. Commented Dec 31, 2023 at 1:29
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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
    Commented Dec 31, 2023 at 16:56

2 Answers 2

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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.)

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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.

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  • Side notes that I haven't addressed as I'm not sure about US code in this regard: Grounding. Not sure on if the earth-neutral link would go in the meter cabinet, just one, or each of the two panels grounded separately. Commented Dec 30, 2023 at 10:52
  • If OP used just a breaker in a box to feed the house, without any additional circuits on that panel, could they omit the separate ground wire to the house? Commented Dec 30, 2023 at 10:55
  • It's not clear how OP reached the volt-drop calculations. Could they tweak the numbers to reasonably allow 600kcm cable (lower cost and no extra lugs). Would reducing total supply to 350A (150+200) help, or arguing diversity between the two load groups? Commented Dec 30, 2023 at 10:57

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