Field guide: installing a subpanel, code-compliant approach (edition 2)

Plan the feeder before you cut drywall

A subpanel lives or dies by the feeder calculation. Run the load on the subpanel side first, then size the feeder conductors and overcurrent device to match. NEC 215.2(A)(1) requires feeder conductors to have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load. Don't guess. Write it down.

Separate the grounded (neutral) and grounding conductors at the subpanel. That means a 4-wire feeder for any subpanel in a separate structure or in the same structure when the subpanel is downstream of the service disconnect. NEC 250.32(B) and 408.40 are the citations you'll get cited for if you miss this.

Common feeder sizes worth memorizing for dwelling subpanels:

• 60A feeder: 6 AWG Cu THHN, 10 AWG EGC
• 100A feeder: 3 AWG Cu or 1 AWG Al, 8 AWG Cu EGC
• 125A feeder: 2 AWG Cu or 1/0 Al, 6 AWG Cu EGC
• 200A feeder: 2/0 Cu or 4/0 Al, 4 AWG Cu EGC

Pick the right panel and location

Match the panel bus rating to the feeder breaker, not the other way around. A 125A panel fed by a 100A breaker is fine. A 100A panel fed by a 125A breaker is a violation and a fire waiting to happen. Check the label inside the dead front before you energize anything.

Working clearances are non-negotiable. NEC 110.26(A) gives you 36 inches of depth, 30 inches of width, and 6.5 feet of headroom in front of the panel. That space stays clear, forever. Customers will try to put a washer, a shelving unit, or a water heater in front of it. Document the clearance in your photos before you leave.

If the homeowner says "we'll just move that dresser when we need to get in there," put it in writing that the location doesn't comply. Either they move it permanently or you relocate the panel.

Bonding, grounding, and the neutral bar

This is where most failed inspections happen. The neutral bar in a subpanel must be isolated from the enclosure. Remove the bonding screw, bonding strap, or bonding jumper that ties the neutral bar to the can. NEC 250.142(B) prohibits re-grounding the neutral on the load side of the service disconnect.

Install a separate equipment grounding bar and bond it to the enclosure. All EGCs from branch circuits land on that grounding bar. All neutrals land on the isolated neutral bar. One wire per hole on the neutral bar, every time, per UL listing and NEC 408.41.

For subpanels in separate structures, you'll also need a grounding electrode system at that structure per NEC 250.32(A). Ground rods, Ufer, or whatever the site offers, bonded to the EGC bar, not the neutral.

Feeder routing and overcurrent protection

The feeder overcurrent device lives at the supply end, in the main panel. Size it to protect the feeder conductors, not the subpanel bus. Use the 60°C column for terminations on equipment rated 100A and below unless the equipment and conductors are both listed for 75°C (most modern gear is). NEC 110.14(C) governs this.

Support and protect the feeder per NEC 334.30 for NM cable or NEC 300.11 for raceways. NM through a stud bay needs to be 1.25 inches from the face of the stud or use a nail plate. In finished spaces with exposed conductors, you need a chase, conduit, or listed wiring method. Romex stapled to a basement ceiling joist is fine; Romex stapled to the face of drywall is not.

1. Calculate load and size feeder
2. Size feeder OCPD to protect conductors
3. Verify panel bus rating meets or exceeds OCPD
4. Pull feeder with proper EGC, separate from neutral
5. Terminate at correct torque spec (check the label)

Torque, labeling, and the dead front

Torque every lug. NEC 110.14(D) makes this explicit: use a calibrated torque tool and match the manufacturer's spec printed inside the panel. Hand-tight is not a spec. An under-torqued lug on a 100A feeder will cook itself inside a year.

Label every breaker with the specific circuit it serves. "Lights" is not a label. "Kitchen counter SABC east wall" is a label. NEC 408.4(A) requires legible, specific identification. While you're at it, mark the feeder source on the subpanel directory so the next electrician knows where the upstream breaker lives.

Tape a photo of the completed panel, with the dead front off, inside the door before closing up. Future you, or the next guy, will thank you when troubleshooting at 9pm.

Final checks before energizing

Megger the feeder if it's a long run or you're at all suspicious of the pull. At minimum, ring out each conductor for continuity and verify no phase-to-ground or phase-to-neutral shorts. Check the neutral bar is isolated with an ohmmeter between the neutral bar and the can, should read open.

Energize the feeder breaker first with all subpanel branch breakers off. Verify voltage at the subpanel bus: 240V across the hots, 120V hot-to-neutral, 120V hot-to-ground, 0V neutral-to-ground. Any voltage between neutral and ground means your bonding is wrong, shut it down and find it.

Turn on branch breakers one at a time. Check for unexpected draw, GFCI and AFCI function per NEC 210.8 and 210.12, and correct polarity at a few receptacles. Close the dead front, label it, and get your inspection scheduled.