One supply per outbuilding: garages, ADUs and shops
A detached garage, ADU, shop or other building fed from another building's service is generally limited to one feeder or one branch circuit. Additional supplies are allowed only under specific conditions the code lists.
What the code requires
When a separate building or structure gets its power from another building's service, the code generally limits it to one feeder or one branch circuit, and a multiwire branch circuit counts as one circuit. Common examples are a detached garage, an accessory dwelling unit (ADU) or a workshop fed from the main house. The same one-supply limit applies to a circuit run from the outbuilding back to the original building.
Additional supplies are allowed only under the conditions listed in 225.30(A) through (F). Examples include fire pumps, emergency and standby systems, listed EV power transfer systems, docking facilities and piers, different voltages, frequencies, phases or uses, and capacity needs over 2,000 A at 1,000 V or less (225.30(D)).
Under 225.30(B), up to six feeders are allowed when they all start in the same panelboard, switchboard or other distribution equipment and each ends in a single disconnect. Those disconnects must be grouped in one location and each marked with the load it serves.
Where a building does have more than one supply, a permanent plaque or directory must be installed at each disconnect identifying all the other supplies (225.37). It is not required for branch circuits run from a dwelling to a second building.
On the job
- Running a second feeder to a detached garage or ADU for convenience is not permitted unless a 225.30 condition applies. The common lawful route is 225.30(B): feeders from the same panel, each ending in a single disconnect, with the disconnects grouped and marked.
- Where more than one supply is allowed, a missing directory at each disconnect (225.37) is a common correction.
- A feeder-supplied ADU also brings in 225.41, the outdoor emergency disconnect, and 225.42, surge protection.
Related sections
- NEC 225.31(B): Disconnect location for detached buildings
- NEC 225.33
- NEC 225.37
- NEC 225.41
- NEC 225.42
- NEC 230.2
- NEC 250.32: Grounding at detached garages, ADUs and outbuildings
Where this comes from
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Kyle Robert Anderson KBC Electric · CSLB #1073066 · C-10 Electrical · Licensed since 2021