Field guide: installing a subpanel, inspector tips (edition 2)

Plan the feeder before you touch a knockout

Size the feeder from the calculated load, not from the main breaker. Run a Part III load calc per NEC 220 on the subpanel circuits, then pick the conductor. Oversizing the feeder is cheap insurance; undersizing means ripping it back out when the inspector pulls out a calculator.

Four-wire feeders only. Two hots, a neutral, and an equipment grounding conductor, per NEC 250.32(B) for separate structures and 408.40 for panels in the same building. The days of bonding neutral to ground at the subpanel ended in the 2008 cycle for most installs. Inspectors still find it, and they still red-tag it.

Check the subpanel bus rating against the feeder ampacity and the upstream overcurrent device. A 100A feeder into a 125A panel is fine. A 200A feeder into a 125A panel is a tap, and taps have their own rules under 240.21.

Isolate the neutral, land the ground

Every subpanel needs the neutral bar floating and a separate equipment ground bar bonded to the enclosure. Pull the green bonding screw. Remove it, bag it, tape it to the inside of the dead front so the next person does not reinstall it by reflex.

Ground bar goes on a threaded boss or with the listed kit. Do not sheet-metal-screw a ground bar to the can. The bond path has to be mechanical and listed, per 250.8.

If you can wiggle the ground bar with your fingers, so can a fault current looking for a path. Torque it, mark it, move on.

Conductor fill, torque, and terminations

Count your conductors before you pick the raceway. Four current-carrying conductors in a single pipe triggers the 80% adjustment in 310.15(C)(1). Most residential feeders stay under that, but a multifamily subfeed with shared neutrals can bite you.

Torque every lug with a calibrated screwdriver or click wrench. 110.14(D) made it a code requirement, not a best practice. Inspectors in a growing number of jurisdictions want to see the tool on the truck and the values written on the panel schedule or a sticker inside the dead front.

• Feeder lugs: check the label inside the panel, typically 250 in-lb for 2/0 AL
• Branch breakers: usually 20 to 25 in-lb, verify per manufacturer
• Neutral and ground bars: 35 in-lb is common, but read the sticker
• Anti-oxidant on aluminum, every time, no exceptions

Grounding electrode at a separate structure

If the subpanel feeds a detached garage, shop, or ADU, you need a grounding electrode system at that structure per 250.32(A). One ground rod is almost never enough. Two rods six feet apart, or a single rod with a measured resistance under 25 ohms, per 250.53(A)(2).

Bond the grounding electrode conductor to the ground bar in the subpanel, not the neutral. Size the GEC from 250.66 based on the largest ungrounded feeder conductor. For a 100A feeder with 1 AWG copper, that is a 6 AWG copper GEC.

If the detached structure has only one branch circuit, you can skip the separate electrode under 250.32(A) Exception. In practice, almost nothing is one circuit anymore, so plan on driving rods.

Working space, labeling, and the finish-out

30 inches wide, 36 inches deep, 6 feet 6 inches high, clear, per 110.26. Measure it before you mount the can. A subpanel behind a water heater or under a stair soffit will fail, and rightly so. The inspector is thinking about the next electrician working it hot.

Every circuit gets a legible, specific description. "Lights" is not a description. "Kitchen south wall receptacles" is. 408.4(A) requires it, and it is the single most common callback on a clean install.

• Panel directory filled out in pen or printed, not pencil
• Available fault current marked on the enclosure per 110.24
• Service disconnect and subpanel location cross-labeled if not obvious
• Dead front screws all present, breakers filled or blanked

The inspector spends about four minutes at the panel. Make those four minutes boring. Boring passes.

The five things that fail inspections

After a few hundred subpanels, the same five issues keep showing up. Walk the install once before you call it in and check each one. It takes ten minutes and saves a reinspection fee.

1. Neutral bonded to ground at the subpanel (pull that green screw)
2. Missing equipment grounding conductor, three-wire feeder on a new install
3. No grounding electrode at a separate structure, or a single rod with no resistance test
4. Unlabeled or vaguely labeled breakers, missing available fault current marking
5. Untorqued lugs, no anti-ox on aluminum, or a ground bar on sheet-metal screws

Keep the code book in the truck, keep the torque tool on your belt, and treat the dead front like the inspector is already standing behind you. The work goes faster when you build it right the first time.