Garmaine Staff asked 3 years ago

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?

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

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".)