In a subpanel, the neutral bar must be isolated from the enclosure and from the equipment grounding conductors. The neutral-to-ground connection belongs at one point only: the service disconnect. Repeating that bond downstream is a common and consequential wiring error, and it produces a panel that looks entirely normal while behaving incorrectly.
The system is designed to have exactly one neutral-to-ground bond
Grounded circuit conductors and equipment grounding conductors serve different purposes. The neutral carries normal return current. The equipment grounding conductor carries current only during a fault, and otherwise sits at approximately the same potential as the enclosures it connects. Those two functions are deliberately joined at a single location, at the service disconnecting means, so that fault current has a defined low-impedance route back to the source. Every panel downstream of that point is expected to keep the two separate. A subpanel therefore needs an isolated neutral bar plus a separate ground bar bonded to the can.
A second bond creates parallel return paths you cannot see
When a subpanel neutral bar is bonded to the enclosure, ordinary return current no longer has one route home. It divides between the feeder neutral and every other conductive path back to the main panel, which can include the equipment grounding conductor, metal raceway, metal water piping, and duct work. The split follows relative impedance, so a portion of everyday load current flows continuously on parts that are supposed to be current-free. Nothing trips, nothing smells hot, and the lights work. The defect is invisible from the front of the panel and usually found only by measurement or by removing the cover.
The consequences show up in shock risk and in protective devices
Current on grounding conductors means a voltage drop along them, so enclosures and exposed metal no longer sit at a common potential. That is the mechanism behind stray voltage complaints in outbuildings, workshops, and around pools and barns. It also interferes with protection: a ground-fault circuit interrupter compares the current leaving on the ungrounded conductor with the current returning on the neutral, and a parallel path that diverts some of that return looks exactly like a ground fault. A downstream bond can cause persistent nuisance tripping, or can mask the imbalance the device is meant to detect. If the feeder neutral later opens, bonded grounding conductors may carry the full return current.
The feeder needs four wires, and the bonding screw needs to come out
A 120/240-volt subpanel fed from a main panel requires two ungrounded conductors, a neutral, and an equipment grounding conductor run with the circuit. Panelboards ship with a main bonding jumper, usually a green screw or a strap, that ties the neutral bar to the enclosure. That jumper is correct in a service panel and must be removed when the same panelboard is used as a subpanel. Add a separate equipment ground bar, bond it to the enclosure with the listed hardware, and land neutrals and grounds on their own bars with one conductor per terminal unless the terminal is listed for more.
Detached buildings and generators follow their own rules
Older installations sometimes fed a detached garage or barn with three wires and re-bonded neutral to ground at the remote building. Current requirements call for an equipment grounding conductor run with the feeder and no re-bonding at the second structure, though grounding electrodes are still required there. Separately derived systems behave differently again: a transformer secondary or a generator configured as a separately derived system is bonded at its own source, which is why a transfer switch must be matched to whether the generator neutral is bonded or floating. Treat these as distinct cases rather than extending a rule of thumb across them.
What to check before you act
- Confirm the main bonding jumper or green bonding screw is removed in any subpanel.
- Verify neutrals and equipment grounds land on separate bars, one conductor per terminal.
- Confirm the feeder includes an equipment grounding conductor, not three wires only.
- Treat detached structures and generator systems as separate cases with their own requirements.
Common questions
Will a wrongly bonded subpanel fail an ordinary outlet test?
Usually not. A plug-in receptacle tester checks conductor arrangement, not whether return current is sharing grounding paths.
Is it acceptable if the subpanel is in the same building and only a few feet away?
No. Distance and location do not change the requirement; the bond belongs at the service disconnect regardless.
The practical takeaway
One bond, at the service. If a downstream panel has its neutral tied to the can, normal load current is travelling on grounding conductors right now, and the panel will keep looking fine until something else goes wrong.
References and further reading
- OSHA 1910.304: wiring design, grounding and protection
- NFPA: understanding the National Electrical Code
- NIOSH: Electrical Safety, Safety and Health for Electrical Trades
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
