Field guide: installing a subpanel, after the job (edition 2)

Walk the panel before you button it up

The subpanel is energized, the loads check out, and the homeowner is hovering. Slow down. The five minutes you spend before the cover goes on saves the callback. Pop every breaker off, then back on, and listen for chatter on the bus. A loose stab tab will tick under load.

Pull each conductor at the lug and verify torque with a calibrated screwdriver, not a cheater. NEC 110.14(D) requires you to torque to the manufacturer's spec, and inspectors are checking for the marked value on the panel label. If the label says 45 in-lb on the 100A feeder lugs, that is the number, not "tight enough."

Check that the neutral and ground are separated. This is the number one fail on a subpanel. The bonding screw or strap that came with the panel goes in the trash, or in your truck for the next service panel install.

Grounding and bonding, the part that fails inspection

NEC 250.32(B) is the rule that catches people. A subpanel in a separate structure fed by a four-wire feeder (two hots, neutral, equipment grounding conductor) keeps the neutral isolated. Same building, four-wire feeder, neutral isolated. The three-wire feeder with bonded neutral is gone for new work, full stop.

Verify the grounding electrode conductor at the subpanel if it is in a separate structure. Per 250.32(A), you need a grounding electrode system at that structure, sized per 250.66 from the feeder conductors. A 100A feeder with #4 copper feeders gets a #8 copper GEC to a ground rod, or #6 to the rebar in the slab if you can get to it.

If you are feeding a detached garage, drive the rod before you pull the feeder. Trying to drive a rod with the panel live and the homeowner's car in the bay is how backs get hurt.

Breaker fill, labeling, and the stuff inspectors actually read

Every circuit gets labeled. Not "lights," but "kitchen south recessed" or "garage door opener." NEC 408.4(A) requires the directory to be legible, specific, and durable. Pencil on the inside of the door is not durable. Use a label maker, or print and laminate.

Count your breakers against the panel rating. A 20-space panel means 20 poles, not 20 breakers. Tandems only count where the panel is listed for them, and only in the slots marked CTL. Mixing in tandems where the panel is not rated is an instant fail and a fire risk.

• Verify breaker brand matches panel listing. UL listings are panel-specific. Eaton CH breakers in a Square D QO panel is not a code question, it is a listing violation under 110.3(B).
• Check AFCI and GFCI requirements per 210.8 and 210.12 for the circuits landed in this sub. Bedrooms, kitchens, laundry, garages, basements all have rules.
• Confirm the handle ties on multi-wire branch circuits per 210.4(B). Two single-pole breakers held together with a nail is not a handle tie.

Document the feeder, write down the numbers

Before you close up, write down the actual measured values. Voltage L1 to N, L2 to N, L1 to L2. Voltage drop across the feeder under load if you can swing it. Take a photo of the panel interior with the directory visible. This is your insurance policy.

Note the feeder conductor size, type, and length on the panel directory or in your job file. If the feeder is 100 feet of #2 aluminum SER on a 100A breaker, write it down. Two years from now when the homeowner adds a hot tub, you or the next electrician will know what is upstream.

Photograph the torque marks. A dab of paint pen on each lug after you torque proves you did the work, and tells the next guy what has been touched.

The walkthrough with the customer

Show them the main disconnect for the subpanel. Show them which breaker kills which circuit. Hand them the panel directory photo on their phone. If you installed surge protection per 230.67 (now required on services, good practice on subs feeding sensitive loads), explain the indicator light and what it means when it goes out.

Tell them what not to do. No storing brooms in front of the panel, NEC 110.26 working space is 30 inches wide, 36 inches deep, 6.5 feet tall, and it stays clear. Tell them to call you, not their brother-in-law, when they want to add a circuit.

Close out your own paperwork

File the permit close-out the same day if you can. Inspectors remember the electricians who chase their own paperwork and the ones who do not. Update your job file with the as-built feeder run, the panel schedule, and the inspection sticker photo.

1. Permit signed off and filed.
2. Invoice sent within 48 hours while the work is fresh.
3. Warranty terms in writing, even if it is a single line on the invoice.
4. Calendar reminder at 11 months to check in before the one-year warranty closes.

The job is not done when the cover is on. It is done when the paperwork is filed, the customer knows how to use what you installed, and you have a record you can defend if anything comes up later.