Field guide: installing a subpanel, safety checklist (edition 6)

Plan the feeder before you pull a single wire

Sizing starts at the load calc, not the breaker aisle at the supply house. Run NEC 220 Part III or IV, pick the larger result, then size the feeder conductors per 215.2(A)(1) and the OCPD per 215.3. For a detached structure, remember 225.39 caps the disconnect at six handles and the feeder must land ahead of any branch circuits.

Grounded conductor (neutral) on a subpanel is always isolated from the equipment ground. That bonding screw belongs at the service, not downstream. 250.24(A)(5) and 250.142(B) are the articles that get cited on every failed inspection we have seen.

Verify available fault current at the supply side. If the existing service can push more than the subpanel SCCR, you need a current-limiting OCPD or a panel rated for the fault. 110.24 requires the marking, and AHJs are starting to enforce it.

Grounding, bonding, and the four-wire rule

Every subpanel fed from the same structure gets four wires: two ungrounded, one grounded, one equipment grounding conductor. No exceptions since the 2008 cycle killed the old three-wire allowance in 250.32(B). If you are replacing a legacy subpanel, plan on pulling a new EGC or running a separate grounding conductor per 250.130(C) where permitted.

For a detached building, drive the ground rods per 250.50 and 250.53, bond them to the subpanel enclosure with a 6 AWG copper minimum (250.66), and keep that electrode conductor continuous or irreversibly spliced. The EGC from the feeder still comes back to the main service ground.

• Neutral bar: floating, bonding screw removed or backed out.
• Ground bar: bonded to enclosure, all EGCs and GECs land here.
• Feeder EGC: sized per 250.122 based on feeder OCPD.
• GEC at detached structure: sized per 250.66, bonded to local electrodes.

Working space and mounting, before the lid comes off

110.26 is not negotiable. 30 inches wide, 36 inches deep at 120/240V, 6.5 feet of headroom. Measure the actual swing of the dead front and the panel door before you mount. A subpanel tucked behind a furnace or under a stair is a red tag waiting to happen.

Mount the enclosure plumb, with the top breaker no higher than 6 feet 7 inches to the highest operating handle (240.24(A)). In a garage or basement, keep it out of the splash zone and off the floor. If it is in a damp or wet location, the enclosure rating has to match (312.2, NEMA 3R minimum outdoors).

If you can't stand square in front of the panel with the door fully open and your tool pouch on, the working space is too tight. Move it now, not after drywall.

Feeder install, terminations, and torque

Pull the feeder in one continuous run where possible. Splices in a feeder are allowed but add failure points. Use listed conductors rated for the terminal temperature (110.14(C)), which on most residential panels means the 75 degree C column even if the wire is rated 90.

Torque every lug to the listed value. 110.14(D) now requires a calibrated torque tool, not a guess. Write the values on a sticker inside the dead front so the next person who opens it has a reference. Retorque nothing that is already set unless the manufacturer allows it.

1. Verify upstream is de-energized and locked out. Test, don't trust.
2. Land the two hots on the main lugs or back-fed main breaker.
3. Neutral to the isolated neutral bar, EGC to the ground bar.
4. Torque to spec, log the values, mark the dead front.
5. Install breakers, balance the load across phases.

AFCI, GFCI, and the circuits that leave the subpanel

The subpanel does not change the branch circuit protection rules. 210.8 still governs GFCI, 210.12 still governs AFCI. Kitchen, bath, laundry, garage, outdoor, unfinished basement, dishwasher, and anything within 6 feet of a sink gets GFCI. Most dwelling unit 15 and 20 amp, 120V circuits get AFCI.

If the subpanel feeds a detached garage or workshop, the entire branch circuit protection still applies at the subpanel, not at the point of use. Dual function breakers solve most of this in one slot, but watch the neutral, shared neutrals on MWBCs need a two-pole AFCI or handle-tied pair (210.4(B)).

Label every breaker with the actual room and device, not "kitchen" or "lights." The homeowner calls you at 11 p.m. because they cannot find the right circuit, and that label is the difference between a two minute call and a truck roll.

Pre-energize checklist and final walk

Before you flip the feeder on, walk the panel one more time. Dead front off, light in hand, eyes on every termination. Loose strand, nicked insulation, missed knockout, wrong size connector, neutral touching ground, any of these will come back on you.

Meg the feeder if the run is long or the environment is rough. Verify voltage phase to phase, phase to neutral, phase to ground. Check that the neutral to ground reads near zero at the main and something non-zero at the subpanel under load, that is the proof your bonding is correct.

• 110.26 working space clear and marked.
• Neutral isolated, ground bonded, EGC pulled.
• Torque values logged inside the dead front.
• Directory filled out, every breaker identified.
• AFCI/GFCI tested with the built-in button and a plug-in tester.
• Arc flash and available fault current labels applied per 110.16 and 110.24.

Subpanels fail inspection for the same five things every time: bonded neutral, missing EGC, wrong working space, untorqued lugs, and a blank directory. Hit those and the rest is just craftsmanship.