Field guide: installing a subpanel, before you start (edition 3)

Scope the job before you open a knockout

A subpanel looks simple on paper. In practice, the job lives or dies by what you figure out before the first lockring goes on. Walk the service, read the existing panel schedule, and confirm the feeder path end to end. If the homeowner hands you a load list, verify it against what's actually energized.

Pull the dead-front on the main and count spare spaces, breaker type, and the bus rating. Note the service size, grounding electrode system, and whether the main panel is a combination service or a main-lug setup fed from a meter-main. These details drive every downstream decision, from feeder ampacity under NEC 215.2 to whether you can even land a double-pole breaker where you want it.

Photograph the panel interior, the meter base, and the grounding electrode connections. You will reference those photos three times before the job is done.

Size the feeder honestly

Load calc first, breaker second. Do the standard calculation per NEC 220.40 and Part III, or the optional method under 220.82 if the structure qualifies. Don't pick 100A because it's what was on the truck. Detached structures often need more than people expect once you add a mini-split, a well pump, or an EV charger.

Match conductor ampacity to the terminal temperature rating, usually 75C for breakers and lugs on modern gear per 110.14(C). Apply any adjustment factors for conduit fill (310.15) and ambient. For long feeder runs, voltage drop becomes the real constraint, not ampacity. Three percent at the subpanel, five percent total to the farthest outlet is the working target.

• 100A subpanel, 75C copper: 3 AWG THWN minimum for the ungrounded conductors.
• 100A subpanel, 75C aluminum: 1 AWG XHHW-2, common for longer runs.
• EGC sized per 250.122, not the same size as the ungrounded conductors unless upsizing for voltage drop (250.122(B)).
• Neutral sized to the maximum unbalanced load per 220.61, full size for most residential feeds.

Grounding and bonding, get this right

This is where most failed inspections happen. A subpanel is not a service. The neutral bar is isolated from the enclosure. The equipment grounding conductor lands on a separate ground bar bonded to the can. No main bonding jumper, no green screw, no neutral-to-case bond. 250.24(A)(5) and 250.142 are explicit.

If the subpanel is in a separate structure, you still run an EGC with the feeder and you still drive ground rods at the separate structure per 250.32(A) and (B). The old three-wire feeder to a detached building is gone under the current code. Four wires, always, and keep the neutral floating at the sub.

Before you energize, ohm from the neutral bar to the enclosure. Any continuity means you missed an isolator or left the bonding screw in. Fix it before the breaker closes, not after.

Plan the physical install

Working space first. 110.26 wants 36 inches of depth, 30 inches of width or the panel width (whichever is greater), and 6.5 feet of headroom. No storage in that zone. Garages and basements are the usual offenders. If the only spot for the sub is behind the water heater, find another spot.

Pick a location that gives you straight conduit runs and serviceable knockouts. Mount the panel so the top breaker is no higher than 6'7" per 240.24(A). Dedicate the space above the panel per 110.26(E). And check local amendments, some jurisdictions prohibit panels in clothes closets or bathrooms outright.

1. Confirm working clearance with the door fully open.
2. Route the feeder to enter the top or sides, not through the dedicated space above.
3. Leave slack for re-termination. Lugs get retorqued, conductors get nicked.
4. Label the sub at the main, and the main at the sub, with permanent markers per 408.4.

Permits, inspections, and the utility

Pull the permit before you cut drywall. Most AHJs want a load calc submitted with the application for anything over a certain amperage, and some require a utility notification if the service is being re-tapped or the meter is being pulled. Don't assume. Call.

If you need to open the meter, coordinate the cutout with the POCO. A same-day reconnect is usually fine if you schedule it, but showing up Friday afternoon expecting a tag pull is a way to spend the weekend with an unhappy homeowner. Line-side taps ahead of the main are an inspector magnet, do them right or don't do them.

Rough-in inspection before drywall, final after trim. Photograph every splice and termination before you close the wall. When the inspector asks about a junction you buried, you either have the photo or you open the wall.

Final checks before you energize

Torque every lug to the manufacturer's spec, not by feel. 110.14(D) made the torque wrench a required tool, not a suggestion. Verify breaker compatibility with the panel listing. Classified breakers exist but the panel label is the last word.

Meg the feeder if the run is long or pulled through damp conduit. Confirm phase rotation if you're on 120/240 single phase it matters less, but on a 120/208 feed from a multi-family service, get it right. Close the main breaker last, then the sub main, then branch circuits one at a time while you watch the amp clamp.

Update the panel schedule. Handwritten is fine if it's legible. A subpanel with no directory is a callback waiting to happen, and 408.4 requires it anyway.