Field guide: installing a subpanel, after the job (edition 3)

Walk the install before you energize

Before the main breaker clicks on, do a slow walk of the subpanel. Start at the feeder, end at the last branch circuit. Eyes on every termination, every bushing, every label. The inspector is going to do this walk too, and the first time they see the work should not be the first time you have looked at it with fresh eyes.

Check that the neutral and ground bars are separated in the subpanel. The bonding screw or strap from the neutral bar to the enclosure must be removed or left out per NEC 250.24(A)(5) and 408.40. This is the single most common callback on subpanel work, and it will cook a ground path under fault conditions if you miss it.

Confirm the feeder ampacity matches the panel rating and the overcurrent device at the source. A 100A subpanel fed with #4 copper on a 100A breaker is fine at 75 degrees C per NEC 310.16. Undersized feeders hide until load climbs.

Torque, label, photograph

Every lug, every breaker, every bar screw gets torqued to spec. NEC 110.14(D) requires it, and manufacturer values are printed on the panel label or in the instructions. Use a calibrated torque screwdriver, not feel. Aluminum feeders especially will loosen over thermal cycles if under-torqued, and they will not forgive you.

Label every breaker with the circuit it serves. NEC 408.4(A) requires legible, specific identification. "Lights" is not specific. "Kitchen south wall receptacles" is. Do this while the circuits are still fresh in your head, not three months later on a callback.

• Torque all feeder lugs and branch breaker terminations to panel spec
• Apply anti-oxidant compound on aluminum conductors where manufacturer requires
• Photograph the open panel, the directory, and the feeder source for your records
• Note torque values and date on a sticker inside the dead front

Those photos save you on warranty calls. When a homeowner claims you left something loose, the timestamped image of a properly dressed panel ends the conversation.

"I keep a cheap torque screwdriver in every truck. Thirty bucks, and it has bailed me out of two insurance inspections. If it is not torqued, it is not done."

Test before you trust

Power up the feeder with all branch breakers off. Verify voltage at the main lugs, line to line and line to neutral. You are looking for balanced voltage and a solid neutral reference. If line to neutral drifts when you switch on a load later, the neutral is compromised somewhere upstream.

Turn on one branch at a time. Verify correct voltage, correct polarity, and GFCI or AFCI function where required by NEC 210.8 and 210.12. Test every GFCI receptacle with the button and with a plug-in tester. Test AFCI function using the breaker test button and by actually tripping a downstream fault simulator if you have one.

Measure the equipment grounding conductor continuity from the subpanel ground bar back to the service. A bonded ground that reads open will pass visual inspection and fail a fault. Use a low-resistance ohmmeter, not a DMM on the 200 ohm range.

Working clearance and the things inspectors actually check

NEC 110.26 clearances are non-negotiable. 36 inches depth, 30 inches width, 6.5 feet height, clear and dedicated. Not "clear except for the water heater." Not "clear when you move the shelving." Clear.

1. Verify 36 inch working depth from the face of the panel
2. Verify 30 inch width, centered or offset per the panel door swing
3. Verify dedicated electrical space per NEC 110.26(E), nothing foreign above the panel to the structural ceiling
4. Verify illumination at the working space per NEC 110.26(D) for panels over 1200A, but plan for it on smaller jobs too

Inspectors will also check that the panel is not in a clothes closet, bathroom, or over steps. NEC 240.24(D) and (E) kill those locations. If you inherited a questionable spot from the general contractor, document your objection in writing before the rough. After the fact, it is your problem.

Customer handoff and documentation

Walk the customer through the panel directory. Show them the main breaker, explain what a subpanel is and where the upstream disconnect lives. Point out the GFCI and AFCI test buttons and tell them to test monthly per manufacturer recommendation.

Leave a packet. Printed directory, photos of the panel interior, torque spec reference, and your permit and inspection paperwork. Homeowners lose this stuff, but the next electrician or home inspector will find it and thank you. Your name stays clean.

"The panel is the resume you leave behind. Every electrician who opens that door after you is grading your work. Label it like you are proud of it, because somebody is going to see it."

Callback-proofing the job

Come back in thirty days if the job is large or the load is heavy. Pop the dead front, thermal scan the lugs, re-torque if the manufacturer instructions call for it. Some lugs specify a retorque after initial load cycling, some do not. Read the sticker.

Log the install in your own records with the address, panel model, feeder size, date, and inspector. When that panel shows up on a service call in eight years, you will have the history. That is how you build a reputation that feeds itself.