A subpanel is fed from the main panel and provides additional circuit positions. It is a straightforward installation with one rule that is broken constantly.
The rule
The neutral and the equipment grounding system are bonded together at exactly one point in the entire installation: the service disconnect, which for most houses is the main panel.
Everywhere downstream of that, they are kept separate.
What that means in a subpanel
The neutral bar is isolated from the enclosure. Panels ship with a bonding screw or strap that connects the neutral bar to the metal can; in a subpanel that screw is removed or left out.
A separate equipment ground bar is added and bonded to the enclosure.
Neutrals land on the neutral bar, grounds on the ground bar, one conductor per terminal.
The feeder has four conductors: two hots, a neutral and an equipment grounding conductor.
Why it matters
If the neutral and ground are bonded at the subpanel, the neutral has two return paths back to the service: the feeder neutral, and the feeder's grounding conductor plus every other metal path between the two panels — conduit, water pipes, structural steel.
Normal load current then flows continuously on the grounding system. That means:
Metal enclosures, conduit and piping carry current in normal operation, where they should carry none.
If the feeder neutral is ever lost, all of the return current goes onto the grounding conductors, and the voltage rise on them becomes a shock hazard.
GFCI devices downstream may trip or fail to work correctly.
The exception that used to exist
Older Code editions permitted a three-wire feeder to a detached building, with the neutral bonded there, under specific conditions.
That allowance was removed. A detached garage or workshop fed today requires a four-wire feeder and separated neutral and ground, plus its own grounding electrode — usually ground rods at the detached building — and a disconnecting means.
Existing three-wire installations are frequently found and are the most common legitimate reason someone read an older instruction and applied it.
How to check an existing subpanel
Open the cover — with the appropriate care, and this is a job for someone who knows what they are looking at.
Are the neutrals and grounds on separate bars? If white conductors and bare conductors share a bar, they are bonded.
Is there a green bonding screw through the neutral bar into the can?
Does the feeder have four conductors?
Is there more than one conductor under a single neutral terminal? Terminals are generally listed for one, and doubled neutrals are a common finding.
In the main panel
The opposite applies. The bonding screw or strap is installed, the grounding electrode conductor lands there, and neutrals and grounds may share a bar.
The main panel is the reference point for the whole installation, which is why the rule is expressed as "one point" rather than as a property of subpanels.
Multi-wire branch circuits
A related detail: where two circuits share a neutral, they must be on opposite legs and must have a common disconnect — a two-pole breaker or handle ties.
If they are not on opposite legs, the shared neutral carries the sum of both currents rather than the difference, and it can be overloaded without any breaker tripping.
This is another finding that is invisible from outside the panel and dangerous inside it.
Sizing the feeder
A subpanel's feeder is sized for the calculated load it will serve, not for the subpanel's own bus rating.
A 100 amp rated subpanel fed by a 60 amp breaker is entirely normal, and the breaker in the main panel is what protects the feeder. The subpanel's rating only has to be at least as large as its overcurrent protection.
Voltage drop matters on a long run to a detached building, and it frequently sets the conductor size rather than the ampacity does.