Field guide: installing a subpanel, after the job (edition 6)

Final inspection walkthrough

The subpanel is energized, loads are balanced, and the cover is on. Now comes the part that separates a clean install from a callback. Before you pack out, walk the job with the same critical eye the AHJ will bring. Most red tags on subpanels come from the same handful of issues, and catching them now costs nothing.

Start at the feeder and work toward the branch circuits. Verify the feeder OCPD at the source matches the subpanel bus rating or the feeder ampacity per NEC 215.3. Confirm the equipment grounding conductor is sized per NEC 250.122 based on that upstream breaker, not the subpanel rating. If you pulled SER or SEU, check that the cable is protected where it transitions through framing and that the sheath terminates properly inside the enclosure.

• Feeder OCPD sized correctly at the source panel
• EGC sized per Table 250.122 to the upstream breaker
• Neutral and ground isolated at the subpanel (NEC 250.24(A)(5))
• Bonding screw or strap removed from the neutral bar
• Knockouts filled or plugged, no open holes

Neutral and ground separation

This is the single most common failure on subpanel inspections. At a subpanel, the grounded (neutral) conductor and equipment grounding conductors must land on separate bars. The bonding screw that ships green in the neutral bar gets removed or left out entirely. NEC 250.142(B) prohibits using the grounded conductor for grounding on the load side of the service disconnect.

Check every neutral landing. One loose strand from a neutral touching a ground bar or the enclosure creates a parallel path and nuisance GFCI trips downstream. Double check that multiwire branch circuits have their neutrals landed on the correct bar and that shared neutrals are handle-tied at the breakers per NEC 210.4(B).

If the can came with a factory bonding strap between the neutral bar and the enclosure, pull it. I have seen apprentices loosen the green screw and call it done, missing the strap entirely. Ohmmeter between neutral bar and ground bar should read open.

Torque, terminations, and labeling

Torque specs are not optional anymore. NEC 110.14(D) requires terminations to be tightened to the manufacturer's listed torque value, and inspectors are increasingly asking to see a calibrated torque tool on site. Hit every lug on the feeder, every breaker terminal, and every bar screw. Mark each one with a torque seal paint pen so you and the inspector can see what has been verified.

Panel labeling per NEC 408.4(A) means every breaker gets a legible, specific description. "Lights" is not a description. "Kitchen south outlets" or "Garage door opener" is. If the subpanel is fed from a panel in another building or another room, NEC 408.4(B) requires a label identifying the source panel and its location. Write it on the inside of the dead front with a permanent marker if you do not have a printed label handy.

1. Torque all feeder lugs to spec, then paint mark
2. Torque branch breaker terminals, then paint mark
3. Label each circuit with room and load type
4. Label the source panel location on the dead front
5. Apply the available fault current and date label per NEC 110.24

Arc fault and ground fault coverage

Before you close up, verify every circuit that requires AFCI or GFCI protection actually has it. NEC 210.12 covers AFCI in dwelling unit kitchens, family rooms, bedrooms, laundry, and more. NEC 210.8 covers GFCI in bathrooms, garages, outdoors, basements, kitchens, laundry, and within six feet of sinks. The 2023 code expanded GFCI requirements to include more 240 volt loads, so check your adopted code cycle.

Test each dual function breaker with its test button and verify the indicator. If you installed any GFCI receptacles downstream fed from this subpanel, trip test them too. A breaker that test trips but will not reset usually means a shared neutral crossing between circuits, which you want to find now.

Documentation and closeout

A clean closeout package saves you on the next service call and protects you if something goes wrong. Take photos of the finished panel interior with the dead front off, the schedule filled in, and the feeder routing. Save the torque tool calibration cert for the year. Note the panel manufacturer, model, and main breaker size in your job file.

Leave the homeowner or GC a one page summary with the subpanel location, what circuits it feeds, the source panel, and any AFCI or GFCI breakers they should know how to reset. If the job required a permit, confirm the inspection is scheduled and the correction list, if any, is closed out in writing.

On residential work I leave a laminated card in the panel with my company name, the install date, and the feeder size. Six years later when someone adds a hot tub, they know who to call and what they are working with.

Common callback triggers

Most warranty calls on subpanels come back to three things: loose neutrals causing intermittent dimming, shared neutrals on AFCI circuits causing nuisance trips, and undersized EGCs flagged on a later inspection when someone adds load. Address all three at closeout and your callback rate drops.

Walk the load side one more time with a clamp meter under normal load. Check the feeder neutral current, then each branch. Anything running hot gets investigated now, not at 10 PM when the customer calls. A five minute sweep with the meter has saved more reputations than any single tool in the bag.