Do I need a disconnect for a detached building?

Yes. Section 225.31 requires a means to disconnect all ungrounded conductors that supply or pass through a building or structure. That applies to a detached garage, a shop, a barn, a well house, or a pole barn, whether the feed is a full feeder to a subpanel or a single 20 ampere branch circuit for a light and a receptacle.

Where the disconnect has to go

Section 225.32 requires the disconnecting means to be installed either inside or outside of the building or structure served, at a readily accessible location nearest the point of entrance of the conductors. The Code does not define a hard dimension for nearest the point of entrance, but the intent is that a firefighter or an electrician can find and operate it without hunting through the building. Most inspectors read that as within a few feet of where the conductors come through the wall, on the same side, with nothing in between.

The section carries exceptions. For industrial installations with written safety procedures and qualified maintenance supervision, the disconnect is permitted elsewhere on the premises. Where documented safe switching procedures exist and the installation is monitored by qualified persons, the same allowance applies. There is also an allowance for a disconnect on a pole or structure at the point of attachment for a detached structure fed overhead, and for garages and outbuildings on residential property, a suitable disconnecting means is permitted at a readily accessible location remote from the building, which is how a shed fed from a house panel is often handled.

Six switch rule and the rating

Section 225.33 permits the disconnecting means to consist of not more than six switches or six circuit breakers in a single enclosure, in a group of separate enclosures, or in or on a switchboard or switchgear. All of them must be grouped and each must be marked to indicate the load served, per 225.37. If you land a six space panel in the shop and use it as both the disconnect and the distribution, you are compliant only if all breakers are in one enclosure and the panel is at the point of entrance.

Section 225.39 sets minimum ratings. A single branch circuit gets a 15 ampere minimum. Two or more branch circuits get 30 amperes. A one family dwelling served by the feeder gets 100 amperes, 3 wire. All other installations get 60 amperes.

What comes with the disconnect

A detached structure fed by a feeder needs its own grounding electrode system per 250.32(A), which usually means two ground rods spaced at least 6 feet apart unless a single rod tests at 25 ohms or less. The exception in 250.32(A) applies only to a structure supplied by a single branch circuit that includes an equipment grounding conductor.

The neutral and the equipment grounding conductor must stay separated at the detached structure. Section 250.32(B)(1) requires an equipment grounding conductor to be run with the feeder and requires the equipment grounding conductor to be connected to the grounding electrode and to the enclosure, with the grounded conductor kept isolated. The old three wire feeder allowance was removed from the Code in 2008, so pull four wires.

Field takeaway

One disconnect per structure, at the nearest readily accessible point of entrance, sized per 225.39, with a four wire feeder, an isolated neutral bar, and a local grounding electrode system. A single light on a branch circuit still needs a switch or a breaker that kills the ungrounded conductor.