Subpanel Grounding: Why Neutrals and Grounds Must Be Separated

This is the single most common defect in owner-installed subpanels, and it's worth understanding rather than just following as a rule.

Neutral and ground are bonded together at exactly one point in the electrical system: the service disconnect. Everywhere downstream of that point, they stay separate. A subpanel is downstream. Therefore a subpanel's neutral bar must be isolated from the enclosure, and grounds land on a separate bar bonded to the can.

Why is ground bonded to neutral only once?

The two conductors do different jobs, and the distinction only holds if they meet in one place.

The neutral (grounded conductor) carries normal return current. In a 120V circuit, current goes out on the ungrounded conductor and comes back on the neutral, every second the load runs. It is a current-carrying conductor under normal conditions.

The equipment grounding conductor carries current only during a fault. Its job is to provide a low-impedance path back to the source so that a fault to metal draws enough current to open the overcurrent device fast. The rest of the time it carries nothing.

Bonding them at the service establishes the reference to earth and gives fault current its return path. Bonding them again downstream creates a second parallel route for normal return current — and current, as always, divides between all available paths in inverse proportion to their impedance.

What happens if a subpanel neutral is bonded?

With a subpanel bonded, normal neutral current no longer stays on the neutral. It splits between:

• The feeder neutral conductor, and
• The equipment grounding conductor, the metal conduit, the enclosures, the boxes, and anything else metallic tied into the grounding system

The consequences:

• Normal current on metal you can touch. Appliance frames, conduit, boxes and enclosures carry current during ordinary operation. They're not supposed to.
• The fault path is compromised. The grounding system's reliability depends on it being a dedicated, unloaded path. Sharing it with return current undermines that.
• Nuisance tripping. GFCI and AFCI devices detect current imbalance between the ungrounded conductor and the neutral. When part of the return travels on the grounding conductor instead, the device sees an imbalance and trips — correctly, as far as it can tell.
• Stray voltage and noise, which shows up in sensitive equipment and in odd tingles on metal.

The last point is worth connecting: electrical shocks and tingles on appliances or plumbing sometimes trace back to exactly this.

How should a subpanel be wired?

Feeder: four wires.

Neutral and ground run separately the whole way. Three-wire feeders to subpanels were permitted under older rules in limited circumstances and are not permitted for new work — if you find one on an existing installation, it's a legacy condition worth discussing with an electrician.

At the subpanel:

• Neutral bar isolated from the enclosure — sitting on insulating standoffs, with the bonding screw or strap removed (or never installed)
• Ground bar separate, screwed directly to the enclosure so it's bonded to it
• Neutrals to the neutral bar, grounds to the ground bar, and never mixed
• One conductor per terminal unless the bar is listed for more — doubled-up neutrals are a common defect on their own

At the main panel / service disconnect:

• The main bonding jumper — usually a green screw or a strap supplied with the panel — bonds the neutral bar to the enclosure. This is the one bond, and it stays.

Panels ship bonded — that's the trap

Load centers are sold with the bonding screw or strap included, often installed or bagged in the enclosure, because the same panel can be used as either a service panel or a subpanel.

Used as a main panel, the bond goes in. Used as a subpanel, it must come out (or stay out). A subpanel installed straight from the box without removing the bonding screw is bonded — which is exactly how this defect happens so often.

How do you check a subpanel for a bonded neutral?

With the panel de-energized and the cover off — and if you're not confident doing this safely, have an electrician look instead:

• Is there a green screw or metal strap connecting the neutral bar to the back of the enclosure? In a subpanel, there shouldn't be.
• Are neutrals and grounds landing on the same bar? In a subpanel, they shouldn't be.
• Does the feeder have four conductors, or three?
• Is the neutral bar isolated on standoffs, or bolted directly to the can?

If any of those look wrong, get an electrician to assess and correct it. It's not a complicated fix, but it's inside an enclosure where the feeder lugs may remain energized even with the subpanel's own breaker off.

How do you ground a subpanel in a detached building?

A subpanel in a separate structure has additional requirements beyond the four-wire feeder:

• A disconnecting means at the separate building
• A grounding electrode system at that building, connected to the equipment grounding conductor
• The neutral and ground still stay separate at the detached building's panel

Older rules permitted a three-wire feeder with the neutral bonded at a detached building under limited conditions. That allowance was removed, and the current requirement is a separate equipment grounding conductor. Detail in feeding a detached building.

Related bonding topics

• Grounding vs bonding as concepts — see grounding vs bonding
• A bootleg ground is a different but related defect — a ground jumpered to neutral at a receptacle, which produces similar problems at device level. See what is a bootleg ground
• Generator neutral bonding is the same principle applied to a portable or standby source, and getting it wrong causes the same issues — see generator neutral bonding
• Multi-wire branch circuits interact with neutral loading in their own way — see what is a multi-wire branch circuit

Code and permits

The separation requirement comes from the NEC's grounding and bonding articles, and the specific section numbering shifts between code cycles. Your jurisdiction's adopted edition and local amendments govern — check with your building department rather than relying on a section number quoted anywhere, including here.

Subpanel work is permitted and inspected work in essentially every jurisdiction, and it involves connecting a feeder breaker in an energized main panel where the main lugs stay live behind the deadfront. See electrical work homeowners can legally do and panel upgrade permits and inspection.

The bottom line

One bond, at the service, and nowhere else. A subpanel gets a four-wire feeder, an isolated neutral bar with the bonding screw removed, and a separate ground bar bonded to the enclosure. Bond it a second time and normal return current starts flowing on grounding conductors, conduit and appliance frames — which is a real hazard, not a technicality, and it causes GFCI and AFCI nuisance tripping as a side effect. Panels ship with the bonding screw included, which is why this gets missed.

Next: how to size a subpanel.

Code reference

The requirements behind this guide, for looking up in your adopted edition:

• NEC 250.24(A)(5) — the neutral-to-ground bond is made at the service and nowhere downstream
• NEC 408.40 — panelboard equipment grounding, requiring the isolated neutral bar in a subpanel

Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ before relying on any specific article.

Where to go next

• Wire & breaker size chart