Grounding at detached garages, ADUs and outbuildings

What the code requires

When a detached garage, ADU, shop or other separate building is supplied by a feeder or branch circuit from another building, the code requires it to have its own grounding electrode system and grounding electrode conductor, installed under Part III of Article 250.

The supply must include an equipment grounding conductor of a type allowed by 250.118(A), run with the circuit conductors and sized from 250.122. It connects to the building's disconnect and to its electrodes. Any neutral that is run may not be connected to that ground or to the electrodes at the second building. On a typical 120/240 V feeder, that means four wires (two hots, a neutral and a ground), with the neutral isolated.

The grounding electrode conductor at the second building is sized from Table 250.66, based on the largest ungrounded feeder conductor. For example, a feeder of 4/0 copper calls for a 2 AWG copper grounding electrode conductor. The caps in 250.66 still apply, so a conductor that runs only to ground rods never needs to be larger than 6 AWG copper.

On the job

• A common correction at a detached garage or ADU subpanel is the bonding screw or strap left in place, or neutrals and grounds sharing one bar. At a second building they must stay separate.
• A 3-wire feeder with no separate ground to a new outbuilding does not pass. Only existing installations that meet the exception below can keep using the neutral as the ground path.
• Missing ground rods or other electrodes at a building fed by a feeder, or by more than one branch circuit, is another frequent correction.
• Feeding a separate building is work for a licensed electrician. Your building department can tell you what permit and inspections it needs.

Exceptions that matter

• No grounding electrode system is needed if the building is supplied by a single branch circuit, with a multiwire circuit counting as one, and that circuit includes an equipment grounding conductor.
• Existing installations only: the neutral may still carry fault current back if no equipment ground was run, no continuous metal path joins the buildings, and there is no ground-fault protection of equipment on the supply side.
• Under that exception, the neutral must be at least the larger of the size needed for the 220.61 calculated load or the Table 250.122 size.

Related sections

• NEC 225.31(B): Disconnect location for detached buildings
• NEC 250.24: Grounding and bonding at the service
• NEC 250.50: Bonding all grounding electrodes together
• NEC 250.66: Sizing the grounding electrode conductor
• NEC 250.118: What can serve as the equipment ground
• NEC 250.122: Sizing the equipment ground wire
• NEC 250.142(B): No neutral-to-ground connection past the main
• NEC 220.61

Where this comes from

• NFPA: 2026-cycle CMP-5 First Draft ballot (2023 baseline text)
• EEPower, National Electrical Code 2023 Basics: Grounding and Bonding Part 8
• Read the official NEC free at NFPA