Separate Structure Grounding & Bonding Calculator

The three questions 250.32 answers

Feeding a detached garage, shop, barn, or well house raises the same three questions every time. 250.32 answers each one, and the answers depend on the supply type and whether the wiring is new or existing.

Why the neutral is isolated at a subpanel

The grounded (neutral) conductor and the equipment grounding conductor are bonded together at exactly one place in a grounded system: the service. If the neutral is bonded to the enclosure and ground again at a detached structure, normal neutral current divides between the neutral conductor and every parallel path back to the service, including the equipment grounding conductor, any metal conduit, and metal piping. That puts current on metal that is supposed to be dead and defeats the purpose of the grounding system. That is why 250.32(B)(1) requires a separate equipment grounding conductor with the feeder and the neutral floated at the structure: the neutral bar is isolated from the enclosure, and a separate ground bar is bonded to the enclosure and the grounding electrode.

The existing-wiring exception, in full

Before the 2008 NEC, a three-wire feeder (two hots and a neutral, no separate ground) to a detached structure was common, and the neutral was bonded at the structure with a local ground rod. The 2008 edition ended that for new work. The 250.32(B)(1) Exception keeps it legal for existing premises wiring only, and only when all three of these are true at once:

• No equipment grounding conductor is run with the supply to the structure.
• There are no continuous metallic paths bonded to the grounding system in both structures (no shared metal water line, no continuous metal conduit between them, and so on).
• No ground-fault protection of equipment is installed on the supply side of the feeder.

If any one fails, the grounded-conductor method is off the table and an equipment grounding conductor must be run with the neutral isolated. Where the grounded conductor is used for grounding under this exception, it is sized no smaller than the larger of 220.61 (neutral load) or 250.122 (equipment grounding conductor).

Worked examples

A new feeder runs to a 60 A subpanel in a detached garage. Because it is a feeder, the garage needs its own grounding electrode (250.32(A)). Because it is new work, a wire-type equipment grounding conductor is run with the feeder and sized per 250.122, and the neutral is isolated in the garage panel: no main bonding jumper, neutral bar floated, equipment grounds on a separate bonded ground bar (250.32(B)(1)). That installation complies.

A single 20 A multiwire branch circuit feeds a shed, run in cable with an equipment grounding conductor. Because only a single branch circuit supplies the shed and it carries an EGC, no grounding electrode is required at the shed (250.32(A) Exception), and the equipment grounds land on the circuit's EGC.

An existing barn is fed by an old three-wire feeder with the neutral bonded at the barn and a local ground rod. There is no metallic path between the house and barn and no supply-side ground-fault protection. Under the 250.32(B)(1) Exception for existing premises wiring, that arrangement is permitted to remain. Add a metal water line between the two buildings, though, and the exception no longer applies: the feeder must be upgraded to carry an equipment grounding conductor with the neutral isolated.

Common mistakes

• Bonding the neutral at a new detached subpanel. New work isolates the neutral at the structure; the main bonding jumper belongs at the service only.
• Skipping the ground rod at a feeder-fed structure. A feeder (or multiple branch circuits) always requires a grounding electrode at the structure.
• Reusing the old three-wire method on new work. The grounded-conductor method is an existing-wiring exception, not a choice for new installations.
• Missing a metallic path. A shared metal water line or continuous conduit between buildings voids the existing-wiring exception.
• Confusing a generator with a feeder. A generator at the structure is a separately derived system (250.30), not a 250.32 feeder case.

Frequently asked questions

Do I need a ground rod at a detached garage?

If it is fed by a feeder or more than one branch circuit, yes: 250.32(A) requires a grounding electrode at the structure. The only exception is a single branch circuit (multiwire counts as one) that already includes an equipment grounding conductor.

Do I bond neutral and ground in the detached subpanel?

Not for new work. Run an equipment grounding conductor with the feeder and keep the neutral isolated at the structure (250.32(B)(1)). Neutral and ground are bonded at the service only.

When is the old three-wire feeder still allowed?

Only for existing premises wiring, and only when no equipment grounding conductor is run, there are no metallic paths bonding the two grounding systems, and there is no supply-side ground-fault protection (250.32(B)(1) Exception).

How do I size the conductors?

The equipment grounding conductor is sized per Table 250.122 and the grounding electrode conductor per 250.66. This tool identifies which conductors are required; use the linked sizing calculators for the actual sizes.

Related calculators

• Equipment Grounding Conductor Size Calculator: size the EGC run with the feeder from the overcurrent device per NEC Table 250.122.
• Grounding Electrode Conductor Size Calculator: size the grounding electrode conductor at the structure per NEC Table 250.66.
• Neutral Load Calculator: the 220.61 neutral load used to size the grounded conductor under the existing-wiring exception.
• Ground Rod Resistance & Supplemental Rule Calculator: whether a single made electrode needs a supplemental electrode per NEC 250.53(A)(2).