Field guide: installing a subpanel, final inspection (edition 3)

Before the inspector pulls up

Final inspection on a subpanel is won or lost before anyone shows up. Walk the install with the same checklist the AHJ uses. If you catch it, they do not.

Energize nothing until the grounds, neutrals, and bonding are verified. A subpanel that looks clean but has a bonded neutral in a detached structure will fail every time, and the fix after drywall is painful.

• Panel schedule filled out, legible, in pen or printed label
• Dead front installed, all unused openings closed with listed KO seals
• Working clearance per NEC 110.26: 36 in depth, 30 in width, 6.5 ft headroom
• No storage, shelving, or clothes rod in the working space

Grounds and neutrals, separated

This is the number one red tag on subpanels. Per NEC 250.24(A)(5) and 408.40, the grounded (neutral) conductor is bonded only at the service disconnect. At the subpanel, neutrals land on an isolated neutral bar, grounds land on a bonding bar screwed to the enclosure, and the main bonding jumper or green bonding screw is removed.

If the subpanel came from the factory with the bond screw installed, pull it and bag it to the panel for the inspector to see. Some inspectors want to physically confirm the screw is out, so make it obvious.

Tag the bonding screw to the inside of the dead front with a zip tie. Inspector sees it in two seconds, you do not get asked to pull the cover twice.

The feeder and the EGC

Four wire feeder for a subpanel in a separate structure or on the same premises, no exceptions under the 2017 and later cycles. Two hots, an insulated neutral, and an equipment grounding conductor sized per NEC 250.122 based on the feeder OCPD.

Confirm the feeder breaker at the service matches the subpanel rating or less. A 100 A subpanel fed from a 125 A breaker is a tag unless the panel is rated 125 A. Check the label inside the door, not the marketing sticker on the front.

• Feeder conductors sized per NEC 310.16 with the correct temperature column, 75C terminations on most residential gear
• EGC sized per Table 250.122, upsized if the ungrounded conductors were upsized for voltage drop (250.122(B))
• Neutral insulated through the entire run, no nicks at the clamp
• Listed connector at every KO, no double lugging unless the terminal is listed for it

Grounding electrode at a separate structure

If the subpanel feeds a detached building or structure, NEC 250.32 requires a grounding electrode system at that structure. Ground rods, Ufer, or a metal water pipe electrode, bonded to the subpanel ground bar with a conductor sized per 250.66.

Two rods 6 feet apart unless you can document 25 ohms or less with a single rod, which nobody actually does. Drive them full depth, use a listed acorn or clamp rated for direct burial, and keep the GEC continuous or use irreversible compression.

Inspectors love to pull on the GEC at the rod. If your acorn spins or the wire slides, you are re-driving. Torque it, then paint a witness mark across the clamp and conductor.

Breakers, torque, and labeling

Only breakers listed for the panel. A UL classified breaker is acceptable if the panel manufacturer has not voided it, but many inspectors prefer factory matched. Check the label on the interior for the approved breaker list.

Torque every lug to the manufacturer spec per NEC 110.14(D). This became explicit in the 2017 cycle and inspectors are enforcing it. A calibrated torque screwdriver with a witness mark on each lug head is cheap insurance.

1. Verify breaker type against the panel door label
2. Torque feeder lugs, neutral lug, ground bar screws, branch breaker lugs
3. Mark each lug with a paint pen once torqued
4. Fill out the circuit directory with room and load, not just "lights"
5. Verify AFCI and GFCI requirements per NEC 210.8 and 210.12 for the served circuits

Final walk and documentation

Open the dead front one more time. Look for loose strands at the neutral bar, a grounding conductor that missed the lug, a KO without a connector. Close it up, verify the panel cover screws are all in, and confirm the disconnect means at the service is labeled per NEC 408.4(B) identifying the source of the feeder.

Have the permit, the approved plan, and the load calculation at the panel. If the jurisdiction requires a sticker or QR tag, apply it before the inspector arrives. Clean the area, sweep the debris, and leave the dead front off so the inspector is not waiting on you to pull six screws.

• Permit posted and visible
• Load calc on site, matches the panel schedule
• Dead front off, breakers in the off position until approval
• GFCI test buttons verified on any receptacle circuits fed from the subpanel
• Arc fault and surge protection per the adopted code cycle in your jurisdiction