Field guide: installing a subpanel, new construction version (edition 4)

Plan the feeder before you cut a single hole

New construction is the easy version of this job, but only if you think it through before the rough. Size the feeder to the calculated load, not the panel rating. A 100A subpanel fed from a 60A breaker is fine if the load calc supports it, and you will see this all day long on detached garages and ADUs. Run the numbers per NEC 220, pick your conductor from 310.16, and confirm the terminal temperature rating on both ends before you buy wire.

Distance matters. Voltage drop is a recommendation in 210.19 and 215.2 informational notes, not a hard rule, but 3% on the feeder and 5% total is the standard every inspector and GC expects. On long runs to a shop or barn, bump the conductor one size and stop arguing with the HO about flickering lights later.

Confirm whether the subpanel is in the same structure or a separate building. That one question changes your grounding, your disconnect requirements, and your wire count.

Four wires, always, and know why

Since the 2008 cycle, feeders to subpanels require a separate equipment grounding conductor. Two hots, a neutral, and a ground. The neutral and ground bond only at the service, never at the sub. This is 250.24(A)(5) for the service side and 250.32 for separate structures. If you bond the neutral at the sub, you put objectionable current on the grounding path and every metal box downstream becomes a return conductor.

Pull the green bonding screw. Remove the bonding strap. Add a separate ground bar, kit listed for that panel, and land every EGC there. Neutrals stay on the factory neutral bar, isolated from the can.

If you find a bonded sub on a remodel next to your new work, fix it while you are there. Takes ten minutes and keeps you off the callback list when someone gets bit off a dryer frame.

Separate structure rules are their own animal

Feeding a detached garage, shop, or ADU triggers 225 and 250.32. You need a disconnect at or inside the separate structure per 225.31 and 225.32, grouped and marked, not more than six throws. The subpanel with a main breaker usually satisfies this if it is the first means of disconnect.

Grounding electrodes are required at the separate structure per 250.32(A). Two ground rods 6 feet apart, or one rod plus a proof of 25 ohms or less (good luck), or a Ufer if the slab is not yet poured. Tie them to the ground bar in the sub with a 6 AWG copper GEC sized per 250.66.

• Feeder: 2 hots, 1 neutral, 1 EGC, all sized per 310.16 and 250.122
• Disconnect at the structure: main breaker sub counts
• GES at the structure: rods, Ufer, or qualifying metal water
• No neutral-to-ground bond at the sub, period

Rough-in the cabinet like you mean it

Set the can so the top breaker is no higher than 6 feet 7 inches to the operating handle, per 240.24(A). Working space per 110.26: 36 inches deep, 30 inches wide or the width of the panel, 6.5 feet of headroom, and dedicated space above to the structural ceiling per 110.26(E). Do not let the framer put a shelf over your panel. Mark the dedicated space on the studs with a sharpie and a note.

Set the can flush to the finished drywall plane. Quarter inch recess max in combustible walls per 312.3. Nothing ruins trim-out faster than a can set to half-inch rock when the GC switched to five-eighths. Ask once, measure twice.

Knockouts: bring feeders in the top or bottom based on the run, and leave yourself real estate on the opposite end for branch circuits. If you are running EMT or PVC, stub up with a bushing or a listed fitting, not a chewed-up KO and a locknut on bare THHN.

Load the branch circuits with trim-out in mind

Balance the phases as you land breakers. Big motor loads (HVAC, well pump, EV charger) on their own two-pole, sized per nameplate and 430 for motors or 625 for EVSE. AFCI for most dwelling-unit circuits per 210.12, GFCI per 210.8. Kitchen, laundry, bath, garage, outdoor, basement, all require GFCI in the 2023 cycle, and dual-function breakers are the cleanest way to handle it in a sub.

1. Land neutrals and grounds first, one conductor per terminal unless listed otherwise
2. Torque every lug to the label, every time, and initial your panel schedule
3. Fill in the directory with room names, not "lights 1, lights 2"
4. Label the feeder breaker at the main: "Feeds garage sub" or similar per 408.4

Torque specs are code now. 110.14(D). Buy the click wrench, use it on every lug, and your AHJ will stop finding reasons to fail you.

Final checks before you call for inspection

Walk the panel cold. Verify no neutral-ground bond, verify the EGC is landed and the bar is bonded to the can, verify the feeder breaker matches the conductor ampacity after adjustment and correction per 310.15. Check that every AFCI and GFCI trips on the test button. Pull the dead front and photograph the interior for your records, because the HO will ask about that breaker in two years and you will not remember.

Confirm the panel directory is filled out, legible, and accurate. 408.4(A) is not optional. An inspector who finds a blank directory assumes the rest of the work is the same quality.

Button it up, set the cover true, and move on. New construction subpanels are a grind only when you skip the planning. Do it once, do it right, and the trim-out takes an afternoon.