Field guide: installing a subpanel, new construction version (edition 6)

Sizing and planning before you pull a single wire

New construction gives you the luxury of choosing panel location, feeder path, and grounding strategy before drywall locks you in. Use it. Walk the job with the prints, confirm the load calc per NEC 220, and pick a subpanel that leaves at least 25% spare spaces for the inevitable adds.

Feeder sizing is where most rough-in mistakes start. Run the calculated load through NEC 215.2 and 310.16, then check voltage drop per the informational note in 210.19(A). For long runs in a big house, bumping one size up on the feeder is cheaper now than pulling it again later.

• Confirm available fault current at the service per NEC 110.24.
• Verify the subpanel SCCR meets or exceeds that value.
• Pick a location with working clearance per NEC 110.26 (30 in. wide, 36 in. deep, 6.5 ft high).
• Keep it out of bathrooms (240.24(E)) and clothes closets (240.24(D)).

Feeder conductors, conduit, and the neutral question

For a detached structure, NEC 225.30 limits you to one feeder (with exceptions). For an attached subpanel in the same building, run four wires: two hots, a neutral, and a separate equipment grounding conductor. The days of three-wire feeders to subpanels ended with the 2008 cycle, see 250.32(B).

Size the EGC from NEC 250.122 based on the upstream overcurrent device, not the feeder ampacity. If you upsized the feeder for voltage drop, upsize the EGC proportionally per 250.122(B). Miss that and an inspector will catch it every time.

Pull an extra 18 inches of each conductor into the subpanel can. Trimming is free. Splicing a short feeder conductor inside a wall is a callback waiting to happen.

Grounding and bonding the subpanel correctly

This is the single most failed item on rough-in inspections. At a subpanel fed from the same building's service, the neutral bar must be isolated from the enclosure. Remove the bonding screw, bonding strap, or bonding jumper that ships with the panel. Neutrals land on the isolated neutral bar, grounds land on a separate ground bar bonded to the can.

For a subpanel in a separate structure, NEC 250.32(A) requires a grounding electrode system at that structure. Drive two rods 6 feet apart unless you can prove 25 ohms or less with one, per 250.53(A)(2). The EGC from the main still runs with the feeder, and the neutral stays isolated at the remote subpanel.

• Main panel: neutral and ground bonded, one GEC to the electrode system.
• Attached subpanel: four-wire feeder, neutral isolated, ground bar bonded to can.
• Detached subpanel: four-wire feeder, neutral isolated, local grounding electrodes, EGC back to main.

Circuit layout and breaker selection

Balance the phases as you go. Heavy 240V loads (range, dryer, HVAC, EV charger) should be spread across the busbar, not stacked at the top. Keep AFCI and GFCI requirements straight per NEC 210.8 and 210.12, and remember that dwelling unit kitchens now require GFCI on all 125V through 250V receptacles up to 150V to ground (210.8(A)(6)).

Label as you land each circuit. A clear directory per NEC 408.4 is code, not courtesy. Use the actual room names and device types, not "lights 1" through "lights 12". The homeowner will thank you, and so will the next electrician troubleshooting at 11 p.m.

1. Land the feeder first, torque to the label value with a calibrated wrench per 110.14(D).
2. Install the main breaker or main lugs kit per the panel schedule.
3. Populate 240V loads, then 120V homeruns, balancing A and B phases.
4. Fill unused openings with listed knockout seals, not tape.

Rough-in details inspectors actually check

Box fill per NEC 314.16 trips up crews that are moving fast. Count every conductor, device, and internal clamp. If you're stuffing a 4 square with a mud ring and six 12 AWG conductors plus a duplex, do the math before you button it up.

Support conduit and cable per 334.30 for NM and 358.30 for EMT. NM cable needs support within 12 inches of the box and every 4.5 feet thereafter, and it must be stapled flat, not on edge. Staples through the jacket are a rejection.

Take photos of every stud bay before insulation. Receptacle heights, cable paths, and fire-caulk at top and bottom plates. That phone roll has saved more callbacks than any meter on my truck.

Final checks before you call for inspection

Megger the feeder if the run is long or the environment is wet. A quick insulation resistance test catches a nicked conductor before the panel is energized. Verify torque on every lug, including the neutral and ground bars, using the panel's printed torque values and a calibrated tool.

Walk the rough with the prints one more time. Confirm every required outlet per NEC 210.52, every smoke and CO location per the adopted building code, and every dedicated circuit the plans call for. A five-minute walkthrough now beats a two-hour return trip after the drywallers show up.