1

Existing 100A service and main panel from PG&E here in California.

Planning for a future detatched ADU to be built potentially years down the road. What I'm going to do now is relocate the service to a new pole in the backyard, and replace the single meter with a dual-meter panel. What I'm trying to figure out is whether to go 200A and have only 100A for the existing house and 100A for the future house, or go >200A and have whatever capacity I'll realistically need for a 10kW solar system AND the two houses. If I do 200A, the single 100A side will only be able to support a small solar system, so I need to go bigger than 200A I believe.

This means PG&E will have to increase the size of their service drop, although they say that with no additional load the wire won't have to be any bigger...for now.

So, my question for you folks is, what are my options for a dual meter panel greater than 200A, which I assume I'll have to do in order to accomodate a 100A house, a 10kW solar system, and a future small house.

Thank you.

15
  • Are you sure you really need more than 200 A? Have you done a load calculation? Commented Jan 21, 2023 at 5:57
  • 1
    Why do you think you need more amps for a solar system? Solar subtracts from your service demand. If you have a 200A house and 160A solar, you don't need 360 amps of service! You need (best case) 40A of service and (worst case) 200A of service. So 200A will suffice. The reason to think about 400A for house+ADU is electrification of appliances. e.g. a house that doesn't even have gas plumbed into it, which some cities are starting to codify. (Carmel Indiana did it in the 60s). Commented Jan 21, 2023 at 8:15
  • What is the square footage of the existing house, and what square footage are you planning the ADU to be? Commented Jan 21, 2023 at 15:26
  • Existing house is small, like 900 sq ft. A new ADU would be a much bigger house but still I don’t know, 2500 sq ft. Gas is nearby but I’d rather use electric everything. Imagine 100A to the new ADU. A dryer and AC and car charger would eat that up pretty quickly. Let alone hot water heater, oven, etc. Commented Jan 22, 2023 at 5:57
  • @blackmountain -- I'm going to assume the new building will become the primary dwelling unit with the existing unit flipped into the ADU for purposes of this discussion (this'll make it a bit easier/more standard to talk about), first off. Second, is it OK if the solar is net metered with the "house" loads, or would you want to virtually net meter the property instead? (makes a difference in complexity here) Commented Jan 27, 2023 at 3:50

1 Answer 1

2

Fortunately, you only need two

Sometimes, a two-dwelling-unit property will require three meters, as NEC 210.25(B) prohibits having loads that are common to both (all) tenants (such as an apartment building's fire alarm system, a shared laundry area, or exit lighting in a corridor or shared stair) run from a tenant's electrical service equipment, and many places prohibit this as well as a function of landlord-tenant law. Fortunately, in your situation where you have two detached dwelling units on the same property, the only plausible shared load would be freestanding (i.e. not attached to one house or the other) outdoor lighting, and since you don't have any of that, a two-meter pack will work for your application.

As far as specific hardware goes, the most cost-effective thing I've been able to find for this job is the Siemens WEP4212 -- at the time of this posting, you should be able to get one for somewhere around $1000 through your local electrical supply house (perhaps less, perhaps somewhat more).

Mounting this thing

Unfortunately, you'll have to work with PG&E to rearrange your electrical service into what's called an "underground service dip". This is because PG&E document 025055 prohibits service equipment larger than 225A, or any multi-meter equipment, from being pole-mounted; instead, you'll have to "dip" the service underground to an adjacent location where a suitable mounting arrangement (backerboard or horizontal struts, with pipe or timber supports) can be installed.

The aboveground (mast) section of conduit on the existing pole will need to be 2½" galvanized rigid metal conduit with a mating weatherhead, as PG&E does not accept PVC or EMT for service mast duty; it is permissible to run Schedule 40 PVC once past the sweep from the pole into the trench though. Once this is done, PG&E will run its conductors from the existing service point, through the conduit, to the pull section in the new service equipment.

Getting grounded

You'll also need to run a grounding electrode system at the meter rack. This consists of two 8' ground rods driven 6-8' apart and connected to each other and to the meter-pack hardware by a 2AWG grounding wire. You may need to use a length of ¾" rigid steel conduit with Bridgeport MCH-075 grounding hubs at each end as a damage shield for the aboveground section of the grounding wire, as well. If not, you can use a Bridgeport MC-075 to bring the grounding conductor into the box; either way, you'll want to punch or otherwise cut a ¾" trade size knockout (1⅛" actual hole size) in the bottom of the meter-main, below the center compartment that contains the grounding wire lugs, as service grounding fittings and concentric knockouts don't mix very well.

Homeward bound

The wiring from this meter-pack to the homes in question will be a 4-wire feeder -- fortunately, mobile home feeder cable is a readily available and inexpensive solution to this problem, as NEC 310.12 applies to individual dwelling unit feeders in a multifamily property. Protecting this wire will require a 200A QS and a 100A QP breaker of the appropriate interrupting rating for your location, something you'll have to ask PG&E about since they don't publish any guarantees on that for non-single-family properties.

Once you reach each individual unit, you'll then be able to land the wire into a main breaker subpanel of the appropriate rating and size -- I'd recommend a 30 space, 125A panel as a replacement panel for the existing home if the main panel there cannot be converted to a subpanel by pulling the bonding screw and moving the existing grounding wires to a new, separate grounding bar, while the new home will get a 40 or 42 space, 200 or 225A panel fitted.

1
  • Wow man, that's a great response, thank you. I tried upvoting it but evidently I'm too new of a user. I really appreciate the info. That part about dipping the service underground is interesting, I hadn't thought of that. I wasn't actually considering mounting anything on the pole but I hadn't really throught through how I was going to wire from the pole to the free-standing meter support setup. Commented Jan 29, 2023 at 20:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.