House and ADU on a two-meter 400A service single line diagram

A two-position meter socket with a 200A main for the house and a 100A main for a detached ADU. Use this when a detached accessory dwelling unit (ADU) gets its own meter. The service to the lot is replaced with a 400A two-position meter socket, with a main for the house and a main for the ADU.

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What the reviewer checks on this drawing

The reviewer checks the two service disconnects grouped and labelled, the load calculation for each dwelling, the ADU feeder with its building disconnect and grounding electrode, surge protection for each dwelling, and the utility’s approval of the meter equipment.

What the drawing shows

Every device on the sheet, in the order power reaches it.

Notes on the sheet

• The service to the house is replaced with a 400A two-position meter socket. The house panel stays and becomes a subpanel: four-wire feeder, neutrals isolated, bonding screw removed.
• Each main is a service disconnect. The two are grouped at the meter socket, outdoors (NEC 230.72). Each is marked with the dwelling it serves and EMERGENCY DISCONNECT, SERVICE DISCONNECT (NEC 230.85, 2020 and later).
• ADU feeder: four-wire, neutral isolated at the ADU, with a grounding electrode at the ADU connected to its equipment grounding bar (NEC 250.32). The main breaker in the ADU panel is the building disconnect (NEC 225.31). The 2023 NEC also asks for an emergency disconnect outdoors at a dwelling supplied by a feeder (225.41): where the ADU panel is indoors, add one on the outside wall, marked EMERGENCY DISCONNECT, NOT SERVICE EQUIPMENT.
• Grounding electrode conductor and main bonding jumper at the meter socket, sized from the service entrance conductors. Surge protective device, Type 1 or Type 2, for each dwelling, here at its panel (NEC 230.67).
• Ground-fault protection on the ADU’s range (NEC 210.8(D), 2023) and on its outdoor heat pump (NEC 210.8(F), 2020 and later; the 2023 edition exempted listed HVAC equipment until September 2026 and local amendments often still do). The CEC asks for neither. Disconnect within sight of the heat pump (NEC 440.14).
• Load calculation for each dwelling (NEC 220.82; CEC Rule 8-200). The service conductors carry the two together.
• Meter socket listed as metering equipment with each main in its own compartment (NEC 230.71(B), 2020 and later). Utility to approve the meter socket and assign the second account. The ADU usually needs its own civic address first.

Diagram is schematic and not to scale. Field-verify existing equipment ratings. Ratings and conductor sizes shown are typical. Confirm against the adopted code, the equipment listing and a load calculation before installation. Code references are to the 2023 NEC and the 2024 Canadian Electrical Code: see references and editions.

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