Field guide: installing a subpanel, safety checklist (edition 2)

Plan the load before you touch a knockout

Every subpanel job starts with math, not conduit. Pull the feeder calculation per NEC 220, confirm the largest motor or continuous load, and size the feeder conductors to 125 percent of continuous plus 100 percent of non-continuous per 215.2(A)(1). If this subpanel lands in a detached structure, remember 225.39 sets the disconnect rating floor at 60 amps for a one-family dwelling with one branch circuit feeding it, and you need a single disconnecting means grouped at the supply side per 225.36.

Verify the available fault current at the line side. Panels stamped with an AIC rating below what the service delivers are a code violation under 110.9 and a real hazard. If you inherit an older service, get a letter from the utility or run the calc yourself.

Document your load calc on paper before you cut anything. Inspectors ask for it more often now, and a clean worksheet saves a return trip.

Feeder, grounding, and the neutral rule

The rule that still trips seasoned hands: in a subpanel, the neutral bar floats and the equipment grounding conductor lands on a bonded ground bar. 250.24(A)(5) prohibits a neutral-to-ground bond anywhere downstream of the service disconnect. Pull the green bonding screw. Remove the factory bond strap. Separate the bars with a listed isolated neutral kit if the panel did not ship with one.

For the feeder itself, four wires: two hots, one insulated neutral, one equipment ground. Three-wire feeders to separate structures were retired in the 2008 NEC cycle, and 250.32(B) is explicit now. If you find a legacy three-wire feed during a remodel, price the upgrade into the change order before you energize.

• Feeder EGC sized per 250.122 based on the upstream OCPD, not the conductor ampacity.
• Separate structure: drive ground rods per 250.50 and bond per 250.32(A), even with a four-wire feeder.
• Neutral conductor sized for the maximum unbalanced load per 220.61, never smaller than the EGC.
• Torque every lug to the label value. A calibrated driver is not optional under 110.14(D).

Location, working space, and the items nobody photographs

110.26 is the article that fails more rough inspections than any other. Thirty inches of width, three feet of depth for a 120/240V panel on grounded concrete, and 6.5 feet of headroom. No storage in that envelope, no water heater crowding the side, no shelving above that blocks the dedicated space defined in 110.26(E).

Do not mount a subpanel in a clothes closet (240.24(D)), a bathroom of a dwelling (240.24(E)), or above stairs where the working space lands on a tread. Garages and unfinished basements are fine. Attics are legal but miserable; if you must, confirm the passageway and working platform meet 110.26(A) at the equipment.

Before you set the can, stand where the panel will live and open an imaginary door 90 degrees. If your shoulder hits a stud, the stud moves or the panel does. Fix it now, not after drywall.

Circuit protection, AFCI, and GFCI at the breaker

The subpanel is where most of your AFCI and GFCI protection lands now. 210.12 requires combination AFCI on nearly every 120V, 15 and 20 amp branch circuit in dwelling units: bedrooms, kitchens, living rooms, laundry, hallways, closets. 210.8(A) adds GFCI to kitchens, bathrooms, garages, outdoor receptacles, laundry, and any receptacle within six feet of a sink.

Dual-function breakers cost more but save a J-box and a homeowner callback. Label every branch circuit at both ends per 408.4(A), with a description specific enough that the next electrician does not have to guess. "Bedroom 2 receptacles" beats "bedroom."

1. Map circuits on the directory in pencil first, then ink when the rough is signed off.
2. Stagger your double-pole breakers so the panel balances across both legs.
3. Leave at least two spare positions for future load, 408.4(B) requires an accurate legend regardless.
4. Keep a torque log with the job file, not loose in the truck.

Safety checklist before you throw the handle

Lockout before anything else. A proper LOTO on the upstream disconnect, personal lock and tag, and a verified zero-voltage reading with a meter you proved on a known source before and after. OSHA 1910.333 covers this, and it is not negotiable on a live service.

Wear the PPE your arc flash label calls for. If the panel has no label, treat the working space as Category 2 minimum until you have calculated incident energy. Rubber gloves with leather protectors, FR shirt and pants, face shield with balaclava for any energized work.

The last thing you do before closing the cover: run your fingers across every lug. Warm means loose. Loose means you are coming back.

• Verified de-energized state with a rated meter, tested on a known source.
• All knockouts filled or closed with listed fittings per 110.12(A).
• Cable clamps tight, no exposed conductor inside the cabinet.
• Grounding electrode conductor continuous to the electrode, no splices unless irreversible per 250.64(C).
• Panel directory complete, legible, and accurate.
• Cover screws all present, dead front flush, no gaps at the wall.

Energize one breaker at a time. Meter each circuit at the farthest device. Photograph the finished panel with the cover off and on, filed with the job number. That folder is your defense when something goes sideways two years from now.