Field guide: installing a subpanel, step-by-step (edition 5)

Plan the load and feeder before you touch a tool

Start with a load calculation per NEC 220. Work out the total VA, apply demand factors, and size the feeder to carry the calculated load with a minimum 125% of continuous loads per 215.2(A)(1). Never guess the feeder size off the subpanel's rating alone. A 100A subpanel fed by a 60A breaker is fine if the math says so.

Confirm the main panel has capacity. Check the busbar rating and the feeder breaker you intend to add. If the existing service is already tapped out, you are looking at a service upgrade before anything else. Pull the deadfront and count the existing load, including any 240V circuits you might have forgotten.

Decide on the location. Working clearance per 110.26 is non-negotiable: 36 inches deep, 30 inches wide, 6.5 feet high. No storage, no laundry hampers, no water heaters crowding the panel. If the customer wants it in a closet, read 240.24(D) and push back.

Pick the right subpanel and feeder conductors

Main lug only (MLO) or main breaker (MB)? If the subpanel is in a separate structure, 225.31 and 225.36 require a disconnect at the building, so a main breaker panel is the cleaner answer. Same structure, you can run MLO, but a main breaker gives you a local shutoff and simplifies future work.

Size conductors from Table 310.16 using the 75°C column for most modern terminations. Don't forget the equipment grounding conductor per Table 250.122, sized to the overcurrent device. For a 100A feeder, that's typically #3 copper THHN/THWN or #1 aluminum, with a #8 copper EGC.

• 4-wire feeder required: two hots, neutral, separate EGC per 250.32(B)
• Neutral and ground bars must be isolated in the subpanel
• Remove the bonding screw or strap at the subpanel
• Use listed lugs rated for the conductor material, torque to spec

Bonding, grounding, and the neutral rules that trip everyone up

This is where most failed inspections happen. In a subpanel, the neutral is insulated from the enclosure. The EGC bonds to the enclosure. Never combine them downstream of the service disconnect, full stop. 250.24(A)(5) is explicit: no neutral-to-ground connection on the load side of the service.

If the subpanel is in a separate building, drive ground rods per 250.50 and 250.52, bond them to the EGC landing in the subpanel. Two rods, 6 feet apart minimum, unless you can prove a single rod hits 25 ohms or less. Nobody tests that, so drive two and move on.

Inspector tip from a 22-year journeyman: "Before you close the deadfront, put your finger on the neutral bar and the ground bar. If you feel a bonding screw between them, you just failed. Pull it out."

Physical install and conductor management

Mount the panel plumb and square. Use the KO sizes that match your connectors, no Swiss cheese. Romex connectors for NM-B, proper fittings for EMT or flex. Every unused KO gets a filler. If you are coming in with SER or SEU for the feeder, check 338.10(B)(4) for interior installation rules and protect it where it leaves the panel.

Strip carefully. Nicks in the neutral or hot will cause failures months later when the copper work-hardens and cracks. Leave enough slack to pull the deadfront without stressing the terminations. Stack your grounds and neutrals one per hole, this is in 408.41 and inspectors actually check it now.

1. Mount panel, verify clearances
2. Pull feeder, maintain bend radius per 312.6
3. Land feeder hots on lugs or main breaker, neutral on insulated bar, EGC on ground bar
4. Install branch circuit breakers and conductors
5. Torque all connections to the label specs
6. Label every breaker clearly per 408.4(A)

Branch circuits, breakers, and required protection

AFCI and GFCI requirements follow the circuit, not the panel. 210.12 for AFCI covers most dwelling unit circuits now. 210.8 for GFCI on bathrooms, kitchens, garages, outdoor, basements, laundry, and anywhere within 6 feet of a sink. If you are feeding a detached garage, everything in that structure needs GFCI per 210.8(A)(2).

Match breaker brand and series to the panel. A classified breaker might be UL listed for cross-use, but most inspectors want the panel manufacturer's breaker. Don't mix brands to save a few bucks, it is the fastest way to a red tag.

Field note: "I keep a torque screwdriver on the truck and use it on every lug. Aluminum feeders especially. Loose connections are the number one cause of callbacks in my 15 years."

Final checks before energizing

Meg the feeder if you have any doubt about the run, especially if it was pulled through wet conduit or a long underground. Verify rotation and phase-to-neutral voltage with the main breaker off, then on. Check voltage at the farthest device on each branch circuit for voltage drop under load, 210.19(A) informational note gives you the 3% target.

Button up the deadfront, confirm every breaker is labeled with circuit and room, and leave the load calculation and a panel schedule with the customer or in the panel pocket. Inspector shows up, hands them the paperwork, and you are done.