Field guide: installing a subpanel, during the job (edition 1)

Before you pull the main

Walk the job once with the panel schedule in hand. Confirm the feeder conductors you pulled match the breaker you are landing on in the main, and that the subpanel bus rating is equal to or greater than the overcurrent device protecting the feeder. NEC 408.36 is the one that bites guys who grab whatever panel was on the truck.

Check the grounding electrode situation at the subpanel location before you open anything. If this sub is in a separate structure, you are running a 4-wire feeder and driving rods or bonding to an existing electrode per NEC 250.32. Same building, 4-wire feeder, no new electrode required, but the equipment ground and neutral stay separate. That separation is the single most common callback on sub installs.

Landing the feeder

Torque matters more than speed here. Use a calibrated screwdriver or click wrench and hit the value stamped inside the can, not what you remember from the last job. NEC 110.14(D) made field torquing explicit, and inspectors are checking for the tool on site, not just the result.

Neutral to the isolated neutral bar. Equipment grounding conductor to the ground bar that is bonded to the enclosure. Do not install the green bonding screw or strap in a subpanel. If the panel shipped with it driven, back it out before you energize. The bond belongs only at the service disconnect per NEC 250.24(A)(5).

• Strip length matching the lug window, no copper showing past the lug
• Anti-ox on aluminum feeders, wiped, not globbed
• Phase tape on the ungrounded conductors if you used SER or reidentified a white
• Neutral torqued to spec, then marked with a paint pen so the next guy knows

Old timer on a commercial retrofit told me: "If you can't see the torque mark, assume it's loose." Paint pen on every lug after you torque it. Takes ten seconds, saves a callback.

Bonding and grounding, sorted out

This is where the rework happens. In a subpanel fed from the same structure, the neutral bar floats. The ground bar is bonded to the can. The two never touch downstream of the service. If you find a neutral landed on the ground bar from a previous hack, pull it and move it before you close the cover.

Separate structure sub, you still keep the neutral and ground separated in the sub, but you need a grounding electrode system at that structure per NEC 250.32(A). Two ground rods 6 feet apart, or an existing qualifying electrode, tied to the ground bar with a conductor sized per NEC 250.66. If there is a metallic path back to the main building, that does not replace the electrode, it adds to it.

Breakers, circuits, and the schedule

Install breakers only after the feeder is landed and torqued. Match the manufacturer and series, no classified breakers unless the panel is listed for them. NEC 110.3(B) is the citation, but the bigger issue is warranty and inspection acceptance.

Balance the load across both phases as you land circuits. A sub feeding a kitchen remodel with everything on A phase will trip the feeder breaker under real use even if the math pencils out cold. Pull your schedule, alternate high-draw circuits, and log amperage by phase before you energize.

1. Verify breaker type and AIC rating against the panel label
2. Land AFCI and GFCI per NEC 210.12 and 210.8 for the occupancy
3. Torque branch breakers to the panel spec, not the breaker body spec
4. Fill the directory with circuit numbers, room, and device, in pen
5. Install fillers in every unused opening before the cover goes on

Working space and cover

Before you close it up, look at the working space. NEC 110.26 requires 36 inches depth, 30 inches width or the width of the panel, and 6.5 feet of headroom. If the homeowner built a shelf over the sub after rough-in, that shelf comes down or the panel moves. Do not sign off on a panel you cannot legally stand in front of.

Dead front on, screws tight, cover flush. Label the panel with the feeder source, the breaker number at the main, and the date. If this is a separate structure, add the disconnect location required by NEC 225.31 and 225.32. A sharpie on the inside of the door is fine, a printed label is better.

Journeyman rule: if the next electrician opens this panel in ten years and cannot figure out where the feeder comes from in under thirty seconds, you labeled it wrong.

Energizing and verification

Meter before you flip. Verify the feeder is dead at the sub lugs, then energize at the main. At the sub, check voltage phase to phase and phase to neutral. Numbers should match the service within a volt or two under no load. If phase to neutral is off and phase to phase is right, you have a neutral problem, stop and find it.

Walk every circuit with a plug tester or a meter, depending on what is on the circuit. GFCI and AFCI breakers get tested with the test button and a tester that trips them on a real fault, not just the button. NEC 210.8 and 210.12 both require functional devices, a breaker that will not trip on a plug-in tester is a failed install even if the button works.

Last thing: thermal scan if you have the camera, or hand-check the cover for hot spots after an hour under load. A loose lug will show up as heat before it shows up as a fault. Catch it on the install, not on a callback at 11 PM.