Subpanels and Outbuildings

The four-wire feeder

A detached garage, shop, or barn with its own panel is a separate building supplied by a feeder. The modern way to feed it runs four conductors ¹:

• two hots (for 120/240 volts),
• an insulated neutral,
• an equipment grounding conductor, the low-impedance path that lets a fault trip the breaker back at the house.

Older installations used the neutral itself as the bonding path to the outbuilding. Mike Holt calls that “a dangerous practice”: any problem with the neutral puts current on every grounded metal part in the building ¹.

The feeder’s breaker sits in the main panel; the conductors are sized for the load and, on a long run, for voltage drop.

Inside the subpanel

At the outbuilding ¹:

• A disconnect for the building, often the subpanel’s main breaker.
• The neutral bar is isolated from the enclosure, with its bonding screw left out. The neutral at the separate building is never bonded to the equipment ground or grounding electrode.
• The ground bar is bonded to the enclosure, and all the building’s equipment grounds land there.
• A grounding electrode at the building, such as a ground rod, bonded to the disconnect, to limit lightning and surge voltage.

Neutral and ground meet in exactly one place: the service ². Join them again at a subpanel and normal neutral current divides onto the grounding wires, conduit, and metal parts.

Exception: a building fed by just one branch circuit that includes an equipment grounding conductor, a shed with one 20-amp circuit for a light and an outlet, doesn’t need its own grounding electrode ¹.

Sizing the feeder

Size the feeder for the load first, then check voltage drop over the distance: about 3 percent on the branch circuits and 5 percent total is the NEC’s recommendation, which leaves roughly 2 percent for the feeder ³. Long runs to a barn often need wire a size or two bigger than ampacity alone would demand, or a higher-voltage, lower-current feed.

In Wisconsin, a new or added feeder or branch circuit serving a farm needs a permit, and so, since September 1, 2026, does work on a structure without a prior permit ⁴.

Long feeders and farms

A feeder’s neutral carries the unbalanced current, and over a long run its resistance makes a voltage drop. When the outbuilding’s neutral is also grounded locally, that drop shows up as neutral-to-earth voltage at the building. MSU’s example shows a 3.0-volt drop on a feeder neutral producing 2.25 volts of NEV at the second building, enough to affect livestock, and it can add to NEV coming from the utility side ⁵.

Cows standing on wet concrete, touching a waterer or stanchion, feel voltages people never notice; reluctance to drink or enter stalls can follow. Equipotential grids in barn floors and properly sized, four-wire feeders help, and finding an actual problem takes trained testing because the source may be on the farm, the utility line, or a neighbor’s system ^{5 2}.

What to take from this

Feed a separate building with four wires: two hots, an insulated neutral, and an equipment ground. At the building, install a disconnect, keep the neutral bar isolated, bond the ground bar to the enclosure, and drive a grounding electrode, unless it’s served by a single branch circuit with a ground wire. Size feeders for voltage drop as well as ampacity; on farms, a drooping feeder neutral can show up as stray voltage that livestock feel.