Field guide: installing a subpanel, retrofit version (edition 2)

Scope and load math before you cut drywall

Retrofit subpanels live or die on the load calc. Before you touch a conductor, run Article 220 on the existing service and the proposed new loads. Confirm the main service has headroom, and size the feeder for the calculated load plus any realistic growth the homeowner is hinting at (EV charger, mini-split, heat pump water heater).

Pull the meter tag, confirm service size, and check the main breaker rating against the nameplate. A 100A service with a 90A existing demand does not get a 60A subpanel without a hard conversation. Document the calc on paper. If the AHJ asks, you hand it over without flinching.

Common feeder sizes for residential retrofits:

• 60A subpanel: 6 AWG Cu THHN or 4 AWG Al, per 310.16 and 75C terminations
• 100A subpanel: 3 AWG Cu or 1 AWG Al
• 125A subpanel: 1 AWG Cu or 2/0 Al

Panel location, working space, and the stuff nobody photographs

110.26 is where retrofits get ugly. You need 36 inches of depth, 30 inches of width, and 6 ft 6 in of headroom in front of the panel. That laundry shelf, the water heater sitting 18 inches off the wall, the homeowner's chest freezer... all of it has to move or the panel does.

Dedicated equipment space above the panel per 110.26(E): no piping, ducts, or foreign systems in the zone extending from the floor to 6 ft above the panel or to the structural ceiling. Sprinkler piping is the one exception. Everything else gets rerouted or you find a new wall.

If the only clear wall is in a clothes closet, stop. 240.24(D) prohibits overcurrent devices in clothes closets. Bathrooms are out too under 240.24(E). Find another wall before you promise the customer a Tuesday install.

Feeder routing and the four-wire rule

Separately derived this is not. A subpanel in the same structure gets a 4-wire feeder: two ungrounded, one grounded (neutral), one equipment grounding conductor. The neutral and ground are isolated at the subpanel. Remove the bonding screw or strap. Add a separate ground bar kit if the panel did not ship with one.

If you are feeding a detached structure, 250.32 applies. Since the 2008 NEC, you run the EGC with the feeder to the detached building and keep neutral isolated there too. No more re-bonding at the barn. Grounding electrodes at the detached structure are still required per 250.32(A).

Route choices in a retrofit usually come down to:

1. Fish through wall cavities with SER or NM-B where permitted (334.10, 338.10)
2. Surface EMT or MC along a basement ceiling
3. PVC underground between structures, 18 inches deep per 300.5 Table

Grounding electrode system, revisited

The subpanel does not get its own grounding electrode system when it is in the same building as the service. The EGC run with the feeder handles it. Do not drive a rod at the subpanel and call it good. That is a common retrofit mistake that creates parallel neutral paths the first time the main bonding jumper loosens up.

For a detached structure subpanel, you need a grounding electrode system at that building per 250.50. Ground rods, Ufer, metal water pipe within 5 ft of entry, whatever is present. Two rods 6 ft apart unless you measure 25 ohms or less on one, and nobody carries a ground resistance tester on a Tuesday, so drive two.

Breaker selection, AFCI, GFCI, and the current code cycle

Know which code cycle your AHJ is on. 2020, 2023, and 2026 all shuffled the AFCI and GFCI requirements. Under 2023 NEC 210.8(F), outdoor outlets on dwelling units require GFCI protection. 210.12 still requires AFCI on most dwelling branch circuits in habitable spaces.

When you move existing circuits into a new subpanel, extension and modification rules can trigger AFCI retrofit on those circuits per 210.12(D). If you lengthen, splice, or replace the first outlet, the circuit now needs AFCI protection. Price the dual-function breakers up front, not at rough inspection.

Match the breaker brand to the panel listing. A Siemens breaker in a Square D QO panel is a violation per 110.3(B), even if it physically clicks in. Inspectors look for this.

Terminations, torque, and labeling

110.14(D) requires you to torque every lug to the manufacturer's spec. Buy the torque screwdriver, use it, and document. On aluminum feeders, use an antioxidant compound rated for the lug, and retorque after 24 hours if the lug manufacturer calls for it.

Label the subpanel per 408.4(A): every circuit identified by purpose, legible, not fading marker on masking tape. Mark the feeder disconnect location on the subpanel dead front. If the feeder is more than 10 ft from the subpanel or behind a finished wall, 408.4(B) requires the source location to be identified.

Before you leave, verify:

• Neutral and ground are isolated in the subpanel
• Bonding screw removed, ground bar bonded to enclosure
• All torque values hit, including the lugs at the main
• Panel schedule filled out, dead front back on, cover screws tight
• Voltage and rotation verified under load