Field guide: installing a subpanel, after the job (edition 5)

Walk the install before you energize

Before you flip the main back on, do a slow walk around the subpanel with a flashlight and your prints. You are looking for things that pass inspection but bite you later: an EGC landed under two screws, a neutral riding the bond, a knockout missing its bushing on a 1-1/4 inch pipe. The truck is still on site. Fix it now or fix it on a callback.

Pull the dead front. Verify the neutral and ground bars are separated per NEC 250.24(A)(5) and that the bonding screw or strap is removed. This is the number one finding on subpanel reinspections, and it is the easiest to miss when you were rushing to beat the inspector.

Check torque on every lug, including the feeder terminations and the main lugs or the backfed breaker hold-down. NEC 110.14(D) has required a calibrated torque tool since the 2017 cycle. Eyeballing it is not compliant and aluminum feeders will cold-flow on you within a year.

Verify the grounding and bonding path

Subpanels in a separate structure get their own grounding electrode system per NEC 250.32(A). If you ran a 4-wire feeder, which you should have, do not drive a ground rod and call it done. The rod ties to the EGC bar, the neutral stays isolated, and you still need to meet 250.53 for electrode resistance or install two rods.

For subpanels in the same structure as the service, no local electrode is required, but the EGC must be sized per NEC 250.122 based on the feeder overcurrent device, not the feeder ampacity. A 100 amp feeder on #3 copper still needs a #8 copper EGC minimum.

If the existing service has bootleg grounds upstream, your shiny new subpanel will inherit every one of them. Test a few receptacles on the feed side with a wiggy or a SureTest before you sign off, or that first GFCI nuisance trip becomes your problem.

Load test and balance

Energize one branch at a time and watch the amp clamp on each feeder leg. A subpanel that reads 38 amps on A phase and 4 amps on B phase is going to cook the high leg and trip on inrush when the AC kicks. Move circuits across the bus until you are within 20 percent leg to leg under typical load.

Check voltage at the furthest receptacle on the longest branch with a load applied. You want to see less than 3 percent drop on the branch and less than 5 percent total per NEC 210.19(A) Informational Note 4. If you are seeing 112 volts at a kitchen counter outlet under a hair dryer, your feeder is undersized or your terminations are loose.

• Clamp each feeder conductor at the lugs, not at the breaker
• Log voltage with no load, then with a 1500 watt heater plugged in at the worst-case outlet
• Trip-test every AFCI and GFCI breaker with the test button, then with a real fault tester
• Cycle each two-pole breaker on and off three times to seat the contacts

Label like the next guy is you at 2 a.m.

The directory glued to the dead front by the breaker manufacturer is not a panel schedule. Fill it out with circuit numbers that match the breaker positions, room names that match what the homeowner calls them, and the date you commissioned the panel. NEC 408.4(A) requires every circuit to be legibly identified as to its clear, evident, and specific purpose.

If the subpanel is fed from a service in another room or another building, NEC 408.4(B) requires you to mark the location of the supply on the panel. A label that says "Fed from main panel, garage south wall" takes ten seconds and saves the next electrician fifteen minutes of detective work.

Take a phone photo of the open panel with the directory visible and text it to the customer. They lose the paper copy. They do not lose the text thread with their electrician.

Document for the inspector and the file

Most jurisdictions want the inspection card signed and a final photo. Your own records should go further. Shoot the panel open with all breakers labeled, the feeder conduit entry with the bushing visible, the grounding electrode connection at the rod or water pipe per NEC 250.68(A), and the torque marks if you used a paint pen on the lugs.

If you used SER cable for the feeder, note the run length and the ampacity table you used. NEC 338.10(B)(4)(a) sends you to the 60 degree column for SER in residential interior runs, which trips up a lot of guys who default to the 75 degree column for THHN. A clear note in your file head off the argument when the AHJ calls.

Walk the customer through it

Spend five minutes with whoever is paying the bill. Show them where the feeder breaker is in the main panel, how to reset a tripped breaker, and which circuits are on AFCI or GFCI protection so they know to hit the test button monthly per the manufacturer instructions and NEC 210.8 / 210.12.

Tell them what not to touch. The neutral bar, the ground bar, and the bonding configuration are not user serviceable, and a homeowner with a YouTube education will absolutely add a ground screw "just to be safe" if you do not warn them. Leave a business card taped inside the dead front for the next person who opens it.