Field guide: installing a subpanel, inspector tips (edition 1)

Subpanels are bread and butter, but they fail inspection more than they should. Usually for the same handful of reasons. This is a field guide from electricians who have eaten those corrections, plus what inspectors actually flag when they walk up to the can.

Sizing the feeder and the panel

Start with the load calc, not the panel you have in the truck. NEC 220 Part III gets you the feeder demand. Round up to the next standard breaker per 240.6(A), then pick conductors that match per 310.16 and the terminal temperature rating at 110.14(C). Dwelling services and feeders can use 310.12 for 120/240 single phase, which will save you a size on most house subs.

Panel bus rating has to meet or exceed the feeder overcurrent device. A 100A feeder into a panel labeled 125A bus is fine. A 100A feeder into a panel whose label shows a 100A main lug rating is also fine, but do not stuff a 125A breaker upstream of a 100A bus just because the lugs accept the wire.

Derate for continuous loads at 125 percent per 215.2(A)(1). EV chargers, shop dust collectors running all afternoon, and grow lights are continuous. Treat them that way on paper so the inspector does not have to ask.

Grounding and bonding, the number one fail

This is where most subpanels get red tagged. A subpanel is not a service. Neutrals and grounds must be separated at every panel downstream of the service disconnect per 250.24(A)(5) and 408.40. Pull the bonding screw or strap. Install a separate equipment grounding bar, bolted to the enclosure. Neutrals land on the isolated neutral bar, which must be floated off the can.

Four wire feeder only. Two hots, a neutral, and an equipment grounding conductor sized per 250.122 against the feeder OCPD. Detached structures used to allow a three wire feeder under the old 250.32(B) exception. That exception is gone. New work is four wire, full stop, with a grounding electrode system at the detached building per 250.32(A).

"If I can touch a neutral and a ground on the same bar in a subpanel, I am writing it up before I open my clipboard." ... county inspector, 22 years on the job.

Feeder conductors, conduit, and terminations

Aluminum SER is common for 100A and 125A house subs. It is legal, but read the panel label. Many listed panels require anti-oxidant compound on aluminum and a specific torque, usually printed on the label or inside the dead front. Torque every lug with a calibrated driver per 110.14(D). Inspectors are checking for the torque mark now, not just a snug lug.

If you are running in conduit, pay attention to conductor fill at 314.16 for boxes and Chapter 9 Table 1 for raceway fill. Ambient and bundling adjustments from 310.15(B) and (C) matter in attics and exterior runs. A 100A feeder in a hot attic can lose a full ampacity step.

• Strip length matches the lug, no copper showing above the barrel.
• Torque to label value, mark the lug after.
• Anti-ox on aluminum, wiped clean off insulation.
• Bushings on metal raceways entering the can per 300.4(G).
• Support the feeder within 12 inches of the cabinet if using NM or SER per 334.30.

Working space and mounting

110.26 is non negotiable. 36 inches of depth in front, 30 inches of width, 6 feet 6 inches of headroom, and a clear path out. No water heaters, no shelving, no stored paint cans in the working space. If the panel is in a closet with clothes hanging, you already failed. Dedicated equipment space above the panel per 110.26(E), 6 feet up or to the structural ceiling.

Mount height for the highest breaker handle tops out at 6 feet 7 inches per 240.24(A). In a garage, keep the panel off the floor and make sure the working space does not sit in the swing of a vehicle door. Inspectors measure. Bring a tape.

Circuit makeup inside the can

Two pole breakers for multiwire branch circuits, or handle ties that are identified for the purpose per 210.4(B). AFCI and GFCI as required by 210.8 and 210.12, which now cover most of the house. If the sub feeds a kitchen, laundry, garage, basement, or outdoor receptacles, the protection travels with the circuits, not the panel.

Neutrals cannot share under one lug with a ground, and no more than one neutral per terminal unless the bar is listed for it. Most are not. Landing two neutrals under one screw is an automatic correction.

1. Land grounds first, keep them short and neat.
2. Land neutrals on the isolated bar, one per hole.
3. Dress hots up the sides, leave service loops.
4. Label every circuit per 408.4(A), no "spare" on an energized breaker.

"Clean panels pass. If the inside looks like a bird nest, I am going to look harder at everything else." ... AHJ in a mid size city.

Before you call for inspection

Walk it like the inspector will. Cover plate off, flashlight in hand, label readable, torque marks on every lug, bonding screw removed and bagged inside the panel for the next guy. Verify the feeder OCPD matches the panel label, the grounding electrode conductor at a detached structure is present and sized per 250.66, and the panel schedule reflects what is actually on the breakers.

Photos of the open panel before the cover goes on have saved more callbacks than any tool in the bag. Keep them with the job file. When an inspector asks about a torque value or a wire size three weeks later, you have the answer on your phone.