Field guide: installing a subpanel, final inspection (edition 1)

Before the inspector pulls up

Final inspection on a subpanel is pass or fail on details that took thirty seconds to get right during rough. Walk the job yourself first, with the cover off and the main off. Treat the AHJ's visit as a formality, not a discovery phase.

Most red tags on subpanels come from the same short list: bonding, grounding separation, breaker fill, and terminations. Run through each before you call it in. If you find something, fix it and re-walk the whole panel, not just the spot you touched.

Grounds and neutrals, separated

At a subpanel fed from the same structure, neutrals and equipment grounds must land on separate bars. The bonding screw or strap that ties neutral to the enclosure gets removed or left out. This is NEC 250.24(A)(5) on the service side and 408.40 at the subpanel. Miss this and you have objectionable current on the EGC and raceways.

If the feeder is a 4-wire to a separate structure, same rule applies under NEC 250.32(B)(1). The old 3-wire allowance for detached buildings is gone. Inspector will look for four conductors and a floating neutral bar every time.

• Neutral bar isolated from the can (bonding screw out, plastic washers intact)
• Ground bar bonded to the can, sized per NEC 250.122
• One conductor per terminal on the neutral bar, NEC 408.41
• GECs and bonding jumpers landed only where listed

If you pulled the bonding screw and tossed it, tape a spare inside the cover. Next guy working a service change will thank you, and so will the inspector who wants to see it was intentional.

Feeder, OCPD, and conductor sizing

Confirm the feeder ampacity matches the subpanel's main or the upstream breaker protecting it. NEC 215.2 and 215.3 govern feeder conductor sizing and overcurrent protection. If the subpanel is a main-lug-only, the upstream breaker is the OCPD, full stop. Label it.

Check the conductor insulation rating against the terminal rating. Most residential gear is 75 C terminals, so size from the 75 C column of NEC Table 310.16 even if you pulled THHN. A 100 A feeder on 75 C needs #3 Cu or #1 Al, not #4. Aluminum feeders get antioxidant and a torque check.

1. Verify breaker size at source matches feeder ampacity
2. Torque all lugs to the label value, mark each with a witness stripe
3. Confirm conductor is rated for the wet or dry location of the raceway
4. Check working space per NEC 110.26: 36 in depth, 30 in width, 6 ft 6 in height

Breakers, fill, and AFCI/GFCI

Only breakers listed for the panel. No UL classified cross-brand unless the panel's label explicitly allows it. Tandem breakers only in slots marked for them. NEC 408.36 and 110.3(B) cover this, and inspectors check panel labels now more than they used to.

AFCI and GFCI coverage shifts with every code cycle. Under the 2023 NEC, dwelling unit circuits for kitchens, laundries, bathrooms, garages, outdoors, basements, and similar require GFCI per 210.8(A) and (F). AFCI covers most habitable rooms per 210.12. If the subpanel feeds outdoor loads or a detached structure, check 210.8(F) for the outdoor outlet GFCI requirement that catches a lot of people.

Labeling, directory, and workmanship

The circuit directory has to be specific. "Lights" fails. "Kitchen counter small appliance, south wall" passes. NEC 408.4(A) requires legible, specific identification, and the inspector will pick one at random and ask you to point to it.

Arc flash and available fault current labeling apply where required by 110.16 and 110.24. On residential subpanels this is usually minimal, but commercial jobs need the available fault current, the date, and the source legibly marked. Check the local amendment list, some jurisdictions want this on every new panel.

• Panel directory filled out in ink, not pencil
• Feeder source identified on the panel (from where, what breaker)
• Dead front flush, all screws in, no knockouts open
• Cables secured within 12 in of the can and every 4.5 ft, NEC 334.30
• Connectors listed for the cable type, no NM in an EMT connector

Take a phone photo of the finished directory before you button it up. If the homeowner scribbles on it later and calls you back on a miswired circuit, you have your baseline.

Walk-through with the inspector

Have the permit, the plan, and the load calc on site. If the inspector asks for the feeder size and you have to go look, you have already lost ground. Know your numbers before the truck pulls in.

Open the cover before they ask. Hand them the torque screwdriver if they want to spot check. Inspectors appreciate a clean install and a contractor who knows their own work, and that reputation carries into the next job. A subpanel that passes the first time is worth more than one that passes the second.