7.2 Outside Feeders & Separate Buildings (Article 225 & 250.32)

Why separate buildings are tested

Minnesota homes have detached garages and shops, and farms have many outbuildings. DLI's knowledge areas include outside branch circuits and feeders and grounding of separate buildings. Most errors come from treating a feeder-supplied building like a service: bonding the neutral at the second panel, or running only three wires.

Number of supplies (225.30)

A building or structure supplied by a feeder or branch circuit may have only one feeder or branch circuit supplying it, unless 225.30 permits additional supplies for:

• special conditions: fire pumps, emergency systems, legally required and optional standby systems, parallel power production, and systems designed for multiple sources for enhanced reliability;
• special occupancies: by special permission, for multiple-occupancy buildings without space for supply equipment accessible to all occupants, or very large buildings;
• capacity requirements: where the load exceeds what one supply can provide; and
• different characteristics: different voltages, frequencies, or phases, or different uses.

Where a building has more than one supply, 225.37 requires a permanent plaque or directory at each disconnect location identifying all other supplies and the area served by each.

Disconnecting means (225.31 to 225.41)

"Within sight" means visible and not more than 50 ft away (110.29).

Outdoor lighting (225.7)

Part I of Article 225 also covers outdoor lighting supplied by outside branch circuits and feeders:

• Common neutral (225.7(B)): where outdoor lighting circuits share a neutral, its ampacity may not be less than the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase.
• 277 V to ground (225.7(C)): circuits over 120 V between conductors and not over 277 V to ground may supply luminaires lighting outdoor areas of industrial establishments, office buildings, schools, stores, and other commercial or public buildings. This is how 480Y/277 V parking-lot lighting is fed.
• Up to 1000 V between conductors (225.7(D)): circuits over 277 V to ground and not over 1000 V between conductors may supply the auxiliary equipment of electric-discharge lamps where 210.6 allows it, such as luminaires on poles at least 22 ft high for parking lots, roads, and athletic fields, or at least 18 ft high on other structures such as tunnels.

Overhead feeder spans

Underground feeders

An underground feeder to a detached building follows the Article 300 burial depths, which the 2026 NEC moved from 300.5 to 300.7. Conductors in underground raceways are in a wet location and need wet-rated insulation such as THWN-2 or XHHW-2. Raceways entering a building from underground must be sealed or plugged where moisture could contact live parts.

Grounding and bonding at the separate building (250.32)

250.32(A) Grounding electrode: each building supplied by a feeder or branch circuit needs a grounding electrode system per Part III of Article 250, connected to the building disconnecting means. A building supplied by a single branch circuit (including a multiwire branch circuit) with an equipment grounding conductor does not need an electrode.

250.32(B)(1) Equipment grounding conductor: the supply must include an equipment grounding conductor sized from Table 250.122. It connects to the building disconnecting means and to the grounding electrode system. The grounded (neutral) conductor may not be connected to the equipment grounding conductor or to the grounding electrodes at the second building. In practice:

• run four wires for a 120/240 V feeder: two ungrounded conductors, a neutral, and an EGC;
• use a panelboard with an isolated neutral bar, and remove or do not install the bonding screw or strap;
• land the EGC and the grounding electrode conductor on the equipment grounding bar.

An exception allows existing installations, made under earlier Code editions that permitted it, to keep using the grounded conductor for grounding where no EGC was run, there are no continuous metallic paths between buildings, and no ground-fault protection of equipment is installed at the supply. New work never uses this method.

250.32(E) Grounding electrode conductor: sized from 250.66 based on the largest ungrounded supply conductor.

Bonding the neutral at the second building creates parallel paths, so normal neutral current flows on the EGC, metal raceways, water pipes, and the earth. This creates shock hazards and stray voltage, a serious problem around livestock.

Worked example: detached garage

A 60 A, 120/240 V feeder supplies a detached garage from the house panel, underground in PVC.

1. Conductors: 6 AWG copper THWN-2 for the two ungrounded conductors and the neutral (65 A at 75°C), protected at 60 A.
2. EGC: Table 250.122 for a 60 A OCPD is 10 AWG copper.
3. Disconnect: a main breaker or switch at the garage, suitable as service equipment, or a snap switch if the garage is on residential property and the load permits.
4. Electrode: install a grounding electrode system, such as a ground rod supplemented by a second rod at least 6 ft away, unless one rod measures 25 Ω or less.
5. GEC: for 6 AWG copper supply conductors, Table 250.66 gives 8 AWG copper; a GEC connected only to rods need not be larger than 6 AWG copper.
6. Neutral: isolated from the enclosure at the garage panel.