Field guide: installing a subpanel, common mistakes (edition 1)

Subpanels look simple on paper. In the field, they are where code violations stack up fast. Neutral and ground bonded at the wrong point, undersized feeders, missing GECs on detached structures, and breakers packed into a panel that was never rated for them. Here is what shows up again and again on inspection failures, and how to avoid the callback.

Separate the neutrals and grounds

This is the number one subpanel mistake, and it fails inspection every time. In a subpanel, the grounded (neutral) conductors and the equipment grounding conductors must land on separate bars. The bonding screw or strap that ties neutral to the enclosure at the service disconnect does not belong in the subpanel. Remove it, or back it out completely.

Per NEC 250.24(A)(5), the grounded conductor cannot be connected to normally non-current-carrying metal parts on the load side of the service disconnecting means. NEC 408.40 reinforces this: equipment grounding conductors terminate on the equipment grounding terminal bar, which is bonded to the enclosure. The neutral bar in a subpanel must be isolated from the can.

If the panel came with a factory green bonding screw already installed, assume it needs to come out. Nine times out of ten on a subpanel retrofit, that screw is still biting the neutral bar to the enclosure and nobody caught it.

Size the feeder for the load, not the panel label

A 100A subpanel does not automatically mean a 100A feeder. The feeder is sized based on calculated load per Article 220, then protected at its ampacity per 240.4. Running #4 copper to a 100A subpanel because "that is what the panel says" ignores the actual load and the ambient and bundling corrections in 310.15.

Common field sizing for copper feeders at 75 degrees C terminations:

• 60A subpanel: #6 THHN copper, #10 EGC
• 100A subpanel: #3 THHN copper, #8 EGC
• 125A subpanel: #1 THHN copper, #6 EGC
• 200A subpanel: 3/0 THHN copper, #6 EGC

Aluminum sizes up one to two gauges. Always verify the termination temperature rating on both the breaker and the lugs (NEC 110.14(C)). A 90 degree C conductor does not let you skip derating if the equipment is rated 75.

Detached structures need their own rules

Feeding a subpanel in a detached garage, shop, or barn is not the same as a subpanel in the main house. Since the 2008 NEC cycle, 250.32(B) requires a four-wire feeder: two hots, an insulated neutral, and an equipment grounding conductor. The old three-wire feeder with a re-bonded neutral at the detached building is no longer permitted for new work.

You still need a grounding electrode system at the detached structure per 250.32(A), typically two ground rods spaced at least six feet apart, or a compliant single rod with a tested resistance of 25 ohms or less. That GEC bonds to the equipment grounding bar at the detached subpanel, not the neutral.

1. Pull 4-wire feeder from main to detached subpanel.
2. Keep neutrals and grounds separated in the detached panel.
3. Drive grounding electrodes at the detached structure.
4. Bond GEC to the EGC bar, not neutral.
5. Disconnecting means required at the detached structure per 225.31.

Do not exceed the panel busbar rating

A 100A subpanel with a 100A main breaker has a 100A busbar. You cannot feed it with a 125A breaker upstream and call it protected. The overcurrent device protecting the feeder conductors also protects the panel bus, and 408.36 caps the sum of breaker ratings when there is no main, under the six-disconnect rule... and even then, the bus rating is the ceiling.

If the subpanel is fed from a 100A breaker in the main panel, the subpanel bus is good. If it is fed from a 200A breaker because someone "had a spare," the subpanel is now under-protected and the install is illegal. Check the panel label, check the feeder breaker, and match them.

Working space and mounting get skipped

NEC 110.26 is not optional. 36 inches of clear depth in front of the panel, 30 inches of width (or the width of the equipment, whichever is greater), and 6 feet 6 inches of headroom. No shelving, no water heaters, no washing machines occupying that space. Inspectors measure.

Mounting height matters too. The highest breaker handle or switch cannot exceed 6 feet 7 inches above the floor or working platform per 240.24(A). In a crawlspace or attic install, that rule still applies, and "readily accessible" per Article 100 means without a ladder, without removing obstacles, and without a key unless the occupancy allows it.

If the homeowner wants the subpanel behind the water heater or inside the laundry closet behind the dryer, stop and have the conversation before you pull wire. Moving a panel after rough-in costs more than the whole job.

Label everything, torque everything

Every breaker gets a legible, specific circuit directory entry per 408.4(A). "Lights" is not a label. "Kitchen south wall receptacles" is. The directory must reflect what is actually connected, not what was planned on the prints.

Every lug, every breaker terminal, every neutral and ground landing gets torqued to the manufacturer's spec per 110.14(D). Carry a calibrated torque screwdriver. Loose terminations are the leading cause of panel fires and arc faults that show up six months after the final. Torque it, mark it, move on.