Field guide: installing a subpanel, inspector tips (edition 3)

Plan the feeder before you open the main

Subpanel installs fail inspection on paperwork more than on craft. Size the feeder against the calculated load per NEC 220, not the panel's bus rating. A 100A subpanel fed from a 60A breaker is legal if the calculated load fits; inspectors want to see the math on the permit, not a guess.

Confirm the conductor ampacity against NEC 310.16 and the terminal temperature rating at both ends. Most residential breakers and lugs are listed at 75°C, so size THHN/THWN-2 accordingly even though the insulation is rated 90°C. Aluminum SER is common for 100A and 125A feeders; derate for the 60°C column if you land on older equipment.

Check voltage drop on long runs. A detached garage 180 feet out on #2 aluminum at 100A draws roughly 4.5% drop at full load. NEC 210.19(A) Informational Note recommends 3% on branch circuits and 5% total; inspectors in some jurisdictions enforce it as policy even though the Informational Note is not mandatory.

Four wires, always, between structures and in the same building

Since the 2008 NEC cycle, a subpanel in a separate structure requires a four-wire feeder with an isolated neutral and a separate equipment grounding conductor. NEC 250.32(B) killed the old three-wire exception for existing premises wiring in the 2008 code. If you are replacing a feeder to a detached garage or shop, it gets four wires and grounding electrodes at the second structure.

Inside the same building, four wires have always been the rule under NEC 215.6 and 250.142. The neutral bar floats. The ground bar bonds to the enclosure. The green bonding screw or strap that ties neutral to the can in the main panel comes out of the subpanel and goes in the parts bag taped inside the door.

Inspector tip: "I check the bonding screw first. If I can see a green screw threaded into the neutral bar on a subpanel, I do not even open my clipboard. Red tag, come back when it is out."

Grounding electrodes at a separate structure

A detached building with a subpanel needs its own grounding electrode system per NEC 250.32(A). Two ground rods eight feet apart, or one rod if you can document 25 ohms or less, which nobody measures, so drive two. Bond them to the ground bar in the subpanel with a #6 copper minimum, continuous, no splices unless irreversibly compressed or exothermically welded.

If the structure has a metal underground water line, a concrete-encased electrode, or a metal building frame in contact with earth, those become part of the grounding electrode system too. NEC 250.50 is a list, not a menu. Miss one and the inspector will ask where the rebar stub-up is.

• Two 8-foot ground rods, 6 feet minimum apart, 8 feet preferred
• #6 bare copper GEC, continuous run to the subpanel ground bar
• Acorn clamps listed for direct burial, not the indoor-only pot metal variety
• Label the disconnect at the structure with the source location per 225.37

Breaker and conductor fill, the boring stuff that fails

Count your circuits before you pick a panel. NEC 408.54 limits the number of overcurrent devices to what the panel is listed for, which is printed on the label inside the door. A 20-space panel does not always mean 20 breakers; tandem-allowed positions are marked with a notch or a CTL designation. Using tandems in non-CTL spaces is a violation the inspector will catch by reading the label you did not.

Conductor fill at the top of the can matters. NEC 312.6 sets bending space and wire-bending distance at terminals. A 100A subpanel with a 1-1/4" feeder coming straight at the lugs from a short nipple is tight; inspectors look for sharp radius bends and conductors crossing over the neutral bar. Pull in from the back or the side when you can.

Working space, labels, and what inspectors actually read

NEC 110.26 working space is 36 inches deep, 30 inches wide or the width of the equipment, and 6.5 feet high. A subpanel behind a water heater, next to a furnace plenum, or in a closet with clothing storage fails on the first walk-through. Closets are specifically prohibited in dwelling units under 240.24(D) and (E).

Label every circuit with a specific description. "Lights" is not a description. "Kitchen counter GFCI, north wall" is. NEC 408.4(A) requires circuit directories to be legible and specific enough that the circuit's purpose is evident. Handwriting is fine if it is readable; inspectors do not require printed labels, but they do require accuracy.

Inspector tip: "If the directory says 'spare' and the breaker is hot when I flip it, that is a failed inspection. I do not care that you ran out of time. Label it or kill it."

Pre-inspection walk, five minutes saves a trip

Before you call for rough or final, walk the panel like the inspector will. Bonding screw out on the subpanel, in on the main. Neutrals and grounds separated, one conductor per terminal on the neutral bar per NEC 408.41. Breakers torqued to the manufacturer spec on the label; NEC 110.14(D) made torque marking or documentation mandatory in the 2017 cycle.

Check AFCI and GFCI coverage against 210.8 and 210.12 for the occupancy type. Dwelling unit kitchens, laundry, bathrooms, garages, outdoors, basements, crawl spaces, and within 6 feet of a sink all need GFCI in the 2023 cycle. AFCI covers most dwelling unit 120V 15 and 20A circuits except a short list of exceptions.

1. Bonding screw status correct for main vs sub
2. Neutrals and grounds separated, one wire per hole
3. Torque marks on every lug and breaker
4. GEC continuous, acorn clamps tight, rods driven flush or below grade
5. Directory complete, specific, and accurate
6. Working space clear, no storage, door opens 90 degrees