New Subpanel Neutral Bond Fault At Commissioning Decision Tree

Why this matters

A new subpanel that powers up but shows ground-fault trips, warm neutrals, or buzzing AFCI/GFCI breakers almost always has a neutral that is still bonded to ground at the subpanel. The bonding screw or strap that ties neutral and ground at the service is correct only at the first means of disconnect. Leave that bond in place at a downstream panel and normal neutral return current splits between the neutral and the equipment-grounding conductor, energizing every metal enclosure and tripping any device that watches for ground-path current. Catching this at commissioning prevents a latent shock hazard and a stack of nuisance callbacks.

Symptom presentation

• New GFCI or dual-function breakers in the subpanel trip the moment any load runs.
• AFCI breakers buzz or trip intermittently with no obvious arcing.
• Measurable voltage between the subpanel can and a known earth ground.
• Warm or discolored neutral bar or a warm EGC under normal load.
• Clamp meter shows current flowing on the EGC or the metallic feeder raceway.

These point to parallel neutral current, the signature of a neutral-to-ground bond at the wrong location.

Quick checks

• Confirm the feeder is a true four-wire run: two hots, an insulated neutral, and a separate EGC. A three-wire feeder to a detached structure with no other metallic path was once allowed but is prohibited for new work under current NEC, so a missing fourth wire is itself the fault.
• Look for a green bonding screw, bonding strap, or factory jumper tying the neutral bar to the panel can. In a subpanel it must be removed.
• Verify the ground bar is bonded to the can and the neutral bar is isolated on insulated standoffs.
• Confirm grounding electrode conductors (ground rods at a detached building) land on the ground bar, not the neutral bar.

Isolation tree

Step 1. With the main off, ohm from the subpanel neutral bar to the can.

• Near zero ohms: neutral is bonded to the enclosure. That is the fault for a subpanel. Go to Step 4.
• Open (megohms): neutral is isolated as it should be. Go to Step 2.

Step 2. Clamp the EGC of the feeder under normal load.

• Significant current on the EGC: a neutral-ground connection exists somewhere downstream of this panel, on a branch circuit or at a device. Go to Step 3.
• Near zero on the EGC: the grounding system is clean; the trips are a different fault (look at individual branch circuits for a swapped neutral).

Step 3. Open branch neutrals one at a time at the bar and re-clamp the feeder EGC after each.

• EGC current drops to zero when one branch neutral is lifted: that circuit has a neutral-to-ground contact, often a neutral landed on the ground bar or a neutral touching the can at a junction box. Trace and correct.
• EGC current persists with all branch neutrals lifted: the bond is on the feeder neutral itself or at a second panel. Inspect upstream.

Step 4. Confirm the panel is wired as a subpanel and not a service disconnect.

• It is downstream of the main disconnect: remove the bonding screw/strap, move any neutral-landed grounds to the ground bar, retest neutral-to-can isolation.
• It is the service disconnect: the bond is correct here and the fault lies elsewhere; do not remove it.

Confirming diagnosis

A confirmed misbond reads near zero ohms neutral-to-can before correction and open after. After removing the bond, clamp the feeder EGC under representative load: a clean subpanel shows essentially no EGC current, all return riding the neutral. Voltage from the can to earth ground collapses to near zero. The GFCI/AFCI breakers that were tripping now hold. Record the before/after EGC clamp and the neutral-to-can ohm reading.

A second confirming check distinguishes a panel-level bond from a branch-level neutral-ground contact. With the bonding screw removed, if EGC current is still present, lift branch neutrals one at a time and re-clamp; a branch that carries a neutral-ground contact (a neutral pinched against the can, or a neutral and ground sharing a device screw) will drop the EGC current when isolated. This is why removing the panel bond alone sometimes does not fully clear the symptom: a downstream parallel path can exist independently and must be found and corrected as well.

Remediation

• Remove the neutral bonding screw, strap, or factory jumper from the subpanel.
• Mount the neutral bar on insulated standoffs if it is not already isolated; install a separate ground bar bonded to the can.
• Move every EGC and any grounding-electrode conductor to the ground bar; keep grounded (neutral) conductors on the isolated neutral bar.
• Correct any branch circuit where neutral and ground share a terminal or contact metal.
• For a detached structure, confirm a four-wire feeder and a local grounding electrode system landed on the ground bar.
• Re-energize, re-clamp the feeder EGC, and confirm the protective breakers hold under load.

References

• NFPA 70 (NEC) Article 250.24, Grounding Service-Supplied AC Systems (location of the main bonding jumper).
• NFPA 70 (NEC) Article 250.32, Buildings or Structures Supplied by a Feeder (separate building grounding, four-wire requirement).
• NFPA 70 (NEC) Article 250.142, Use of Grounded Conductor for Equipment Grounding (prohibition on downstream neutral-ground bonds).
• NFPA 70 (NEC) Article 408, Switchboards, Switchgear, and Panelboards (neutral bar isolation in subpanels).
• NFPA 70B, Standard for Electrical Equipment Maintenance (commissioning and inspection practice).