New electrical service to detached ADU/garage

Question

I'm planning to bring dedicated 240V 1Ø power to a meter box attached to an outdoor face of a detached ADU/garage building, then on inside to a 225A main breaker in the ADU (apartment) portion of the structure, and then continue the main service wiring on to feed another 125A breaker sub-panel located in the attached RV bay.

I'd rather use a main breaker box as my sub-panel, as opposed to using a main lug box.

Does anyone foresee a problem with the AHJ (WA State) for this arrangement?

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Thanks to all for your questions and comments. New 240V 1Ø service will be installed underground from the street transformer by IP&L to a new meter box w/ disconnect on side of the structure, then directly through the wall to a 200A main breaker box in the RV bay (serving an anticipated instantaneous max load of ~50A, primarily 120/240V lights and outlets in the unheated RV Bay), and then approx. 25' feeder through 3"Ø PVC conduit buried under the RV Bay concrete floor to a 200A subpanel breaker box in the garage, serving the 800sf ADU (gas heat) and unheated 775sf garage.

I have yet to finalize the wiring map, but am confident that the 200A service and 60 combined breaker spaces between the two boxes will be entirely adequate to handle all my initially desired circuits, as well as those possibly needed in the foreseeable future -- at least mine!

If anyone cares to comment on wire size/type needed for the 25' buried feeder run, I'd be pleased to hear them. Otherwise, the branch circuit wire sizing will be IAW standard practice, with no individual branch runs anticipated to be longer than 75-85'.

Thanks again to all who responded -- you've been a huge help!

Best regards, Russ

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Thank you TPE, your responses are much appreciated. However, your 2nd assumption regarding the ADU/garage ". . . that it's the only garage on the property", is incorrect:

1. This detached ADU/Garage addition is built on a single residential lot, and co-located behind my home/garage (currently served by 200A metered service, with the service panel located inside the attached garage). As such, according to Spokane County Building and Planning, land accommodating the ADU structure will never be allowed to be sub-divided into a separate residential plat. I.e., any future sale of my current residence must also include the ADU/Garage unit. Installing the second metered service line to the new ADU/Garage is a matter of convenience only to avoid having to upgrade my home's existing 200A service line/panel to 400A, and subsequently running a 200A feeder from there.

2. My home, as well as numerous others that I'm aware from the 1990 build era, has it's main service panel located inside the attached garage: does the IRC specifically prohibit this practice currently? In my reading of NEC Art. 240.24, “readily accessible” means being located so a person can reach the OCPD quickly without having to climb over or remove obstacles; I don't find specific distance or location limitations, such as having to descend a stairway, or opening man-door(s) to access the panel. If such a limitation exists, is there any requirement preventing the new ADU feeder sub-panel from being located in the wall adjacent to a mid-flight stair landing (not on a step), but still nine steps below the ADU floor level?

Once again TPE, I certainly appreciate all your input, and will definitely get explicit approval from the AHJ for the final component configuration prior to electrical rough-in.

(P.S. I haven't figured out how to get all of my verbose thoughts into a "Comment", so have once again resorted to opening another "Answer".)

Answer 1

Normally, you wouldn't be able to do as you describe

Normally, a carriage-house ADU setup (with the ADU atop a garage space) is set up on the only garage on the property, with use of the (detached from the main house) garage shared between the main unit and the ADU. Were that the case in your situation, your proposal would be no good, even though your utility's metering rules are silent about separate metering vs. master metering, as it would involve serving what effectively can be treated as a common area of a two-family or multifamily dwelling (the garage, shared between both/all units on the property) from equipment that serves an individual dwelling unit (the ADU), which violates the intent of NEC 210.25(B), if not the letter:

(B) Common Area Branch Circuits. Branch circuits installed for the purpose of lighting, central alarm, signal, communications, or other purposes for public or common areas of a two-family dwelling, a multifamily dwelling, or a multi-occupancy building shall not be supplied from equipment that supplies an individual dwelling unit or tenant space.

Since that's not your situation, though...

Thankfully, in your case, the main house has its own attached garage, separate from the ADU's garage, which eases things significantly, as it allows us to treat this in a fashion much closer to what you describe, which is basically a detached duplex configuration, with each dwelling-unit/garage combination being served and metered separately, but with the garages treated as part of their respective dwelling units.

This makes your existing plans for the ADU much more feasible. Load-wise, your ADU comes out to a conservative 102A of load using the Article 220 procedures, given:

• 800 sf of ADU at 3VA/ft2
• 3 20A small appliance branch circuits (2 kitchen, 1 laundry)
• an 8kVA range allowance (good for ranges up to 12kVA)
• a 5kVA allowance for an electric dryer
• a 4.5kVA allowance for an electric hot water heater
• and a 1.5ton, 13 SEER air conditioner outdoor unit (9A @ 230VAC).

With this, we add 3600VA for the currently proposed RV receptacle and another ~800VA (2400VA at 35%) for lighting in the garage, which puts us just shy of 125A, well within what your 200A service can handle.

Equipment-wise, I would use a 200A meter-main on the outside of the ADU/garage that is configured with a single main disconnect feeding a small (8 spaces is common) panel. This provides enough room for a separate shutoff for the ADU feeder by itself, as well as shutoffs for the garage and RV-bay feeders, while allowing a 200A feeder to the ADU, if so desired. Since your utility's metering rules are silent on whether to use ring-style vs. ringless metering hardware, and the approved meter socket list specifies both ring-style and ringless sockets, I would speak with the utility for details.

If you insist on separate metering of the RV bay, then we can use a WEP2212EV with the 200A portion feeding a tap (in a 2" RMC nipple) to a 125A panel for the garage as well as providing the main 200A feeder to the ADU, and the 50A portion providing a separate feed to a RV-panel box in the RV bay.

As to the subpanels: GO BIG OR GO HOME

The key number with panels more often than not is not how many amps a panel can handle (a 125A panel is more than adequate for anything you're doing in the garage, for instance), but how many spaces the panel has. Considering that putting a large panel in now is far cheaper than upsizing the panel later, I would use a 200A, 40- or 42-space panel for the ADU and a 125A, 24- or 30-space panel for the garage. Both of these can be main lug or main breaker panels at your leisure (unless the garage is on a feeder tap; in that case, the garage panel must be main breaker in order to protect the feeder tap and busbars properly), since they are being fed with feeders from the meter-main/meter-pack hardware outside, and can be of whatever current make and model you wish.

Your plan to run a fat PVC conduit for the feeder to the ADU is wise; however, you'll need to LB it from the wall into the back of the meter-pack/meter-main box, and perhaps use a reducer to narrow it down to 2.5" or even 2" for the nipple connecting the LB to the meter-pack, depending on the size of the knockouts available on the meter-pack/meter-main you are using. (This isn't an issue from a fill standpoint since nipples <24" long are permitted to use 60% of the available conduit space, vs. 40% for longer runs.)

ADU panel placement

The ADU subpanel needs to be accessible at all times to the ADU occupants, as per NEC 240.24(B)(2):

(2) Branch-Circuit Overcurrent Devices. Where electric service and electrical maintenance are provided by the building management and where these are under continuous building management supervision, the branch-circuit overcurrent devices supplying any guest rooms or guest suites without permanent provisions for cooking shall be permitted to be accessible only to authorized management personnel.

Given that the garage is effectively being treated as part of the ADU here, putting the panel for the ADU in its attached garage is not an issue; however, there can be merit to having it in a space which is less likely to be blocked by storage or other items, as such blockages violate NEC 110.26(B):

(B) Clear Spaces. Working space required by this section shall not be used for storage. When normally enclosed live parts are exposed for inspection or servicing, the working space, if in a passageway or general open space, shall be suitably guarded.

This brings us to your plan to locate the panel at a stairway landing; this is acceptable, as 240.24(F) only prohibits locating a panel over steps of a stairway, but does require the landing to be large enough to fit the entire clear floor space required by 110.26(A).

RV provisions

While it's certainly possible to simply have a TT-30 receptacle in a regular junction box wired to a 30A, 1-pole breaker in the meter-main or the garage panel to serve the RV, I would instead run a 50A feeder from the meter-main to a dedicated, surface-mounted RV box. This provides enough room for upgrading to a 50A RV receptacle later, and just enough room to tuck a 60VA(W) lighting load into the picture, even with the RV receptacle maxed out (we're exploiting the 220.5(B) rounding rule here, but you probably aren't going to be maxing out the RV load in the bay with the lights in the bay on, no?).

That RV box, at the moment, will need to have at minimum a TT-30 receptacle (for your current RV) and a spare space that can accept an appropriately typed 15A AFCI breaker (for the RV bay lights); a Talon TL37US with a Q115AF for the lights will do the trick here, and provide a 20A duplex GFCI for general use within the RV bay as well.

Answer 2

Using a panel with a main breaker is always ok. You just do not use the bonding screw to bond the neutral to the ground , or remove the jumper wire if it is installed. Have done some work up your way but mostly in Oregon. And it is preferred to have a local disconnect. No separate meter needed.