Field guide: installing a subpanel, step-by-step (edition 1)

Adding a subpanel is one of the most common jobs you'll pull a permit for, and also one of the most common places inspectors find violations. This guide walks the install start to finish, with the code cites you need to defend every decision on site.

Size the feeder and the panel

Start with the calculated load per NEC Article 220. Don't guess from the existing panel's main breaker, run the numbers. For a detached structure or a remodel adding a kitchen, laundry, or shop circuits, the feeder calc is what keeps you out of trouble at rough-in.

Pick a panel with enough spaces for the present load plus reasonable spares. A 30/40 tub is cheap insurance. For feeder conductors, NEC 215.2 sets the minimum ampacity, and 310.16 gives you the ampacity table. Don't forget the neutral per 220.61 if you're pulling a 4-wire feed.

• Calculate load: NEC 220, Parts III or IV
• Feeder ampacity: NEC 215.2, 310.16
• Neutral sizing: NEC 220.61
• EGC sizing: NEC 250.122 (based on OCPD ahead of feeder)

If the subpanel feeds a dwelling unit and the feeder is the service for that unit, treat the neutral like a service neutral for sizing. That catches a lot of guys on ADU jobs.

Pull permits and plan the route

Permit first, always. Most AHJs want a one-line, load calc, and panel schedule before they'll issue. Call 811 before any trenching for a detached structure feed.

Plan the raceway or cable route before you cut anything. Measure twice for bending radius on larger feeders, 2/0 aluminum in 1-1/4 inch PVC doesn't turn on a dime. If you're running NM through framing, remember NEC 334.15 and 300.4 for protection from nails and screws.

Mount the panel and land the feeder

Working space is non-negotiable. NEC 110.26(A) gives you 36 inches of depth, 30 inches of width (or the width of the equipment if wider), and 6 feet 6 inches of headroom. No storage, no shelving, no water heater crowding the panel. Inspectors measure.

Mount height: the highest breaker handle can't exceed 6 feet 7 inches above the floor per 240.24(A). Strap the feeder within 12 inches of the panel for EMT, or per 334.30 for NM. Use the right connectors, no crushed cables under clamps.

1. Verify working space per 110.26 before drilling mounting holes
2. Install the panel plumb and level, flush to finished wall if recessed
3. Land the feeder with a listed connector, torque per manufacturer
4. Strip back the jacket only inside the enclosure

The four-wire rule and bonding

This is where most subpanel jobs go sideways. A subpanel needs a four-wire feed: two hots, an insulated neutral, and a separate equipment grounding conductor. The neutral bar must be isolated from the enclosure. The ground bar bonds to the enclosure. NEC 250.24(A)(5) and 408.40 are the cites.

Remove the main bonding jumper, that green screw or strap, in the subpanel. Leave it in the service disconnect only. If the subpanel is in a separate structure, NEC 250.32 governs, and since the 2008 code you're pulling an EGC with the feeder, no more bonding neutral to ground at the second building.

Pop the cover on the service panel and confirm the bonding jumper is there before you leave. Nothing worse than finding a floating neutral on a call-back six months later.

• Neutral bar: isolated, insulated from can
• Ground bar: bonded to can, green screw or listed kit
• Main bonding jumper: removed in subpanel
• Separate structure: EGC required per 250.32(B)

Branch circuits, labeling, and AFCI/GFCI

Land branch circuits with neutrals and grounds on their respective bars, not mixed, not doubled under one lug unless the bar is listed for it. Torque every lug to the label inside the door. NEC 110.14(D) makes torque a code requirement now, not a suggestion, use a calibrated screwdriver.

Protection requirements ride on the circuit, not the panel. Dwelling unit bedrooms, living rooms, kitchens, and most other habitable spaces need AFCI per 210.12. Kitchens, bathrooms, garages, outdoors, laundry, and unfinished basements need GFCI per 210.8. Don't skip the dishwasher, 210.8(D) catches a lot of rough-in inspections.

Final inspection checklist

Before you call for inspection, walk the job with the code book or the app open. Cover the big stuff the inspector will hit first: working clearances, conductor fill, terminations, bonding, and labeling per 408.4.

Every circuit needs a description that's legible and specific. "Lights" is not a description. "Kitchen counter SABC 1" is. Panel directory gets updated as the final step, not from memory in the truck at the end of the day.

1. Working space clear, 110.26
2. Feeder and EGC sized correctly, 215.2 and 250.122
3. Neutral isolated, ground bonded, main bonding jumper removed
4. All terminations torqued, 110.14(D)
5. AFCI and GFCI per 210.12 and 210.8
6. Directory complete and legible, 408.4
7. Cover on, deadfront secured, breakers labeled

Subpanels aren't hard, but they're unforgiving. Get the bonding right, size the feeder honestly, and document every circuit. The inspector's job gets easier, and so does yours on the next service call.