Field guide: installing a subpanel, material list (edition 1)

Planning the install

Before you pull a single wire, walk the job. Measure the distance from the main to the subpanel location, note every wall penetration, and decide whether you're running in EMT, flex, SER, or direct burial. The feeder method drives half your material list, so lock it in first.

Size the subpanel to the calculated load, not the existing circuits. NEC 220 load calc first, then pick the panel. A 100A subpanel in a detached garage with a welder and a mini-split is not the same animal as a 60A subpanel feeding a finished basement with lights and receptacles.

Confirm the grounding scheme. If the subpanel is in the same structure, neutrals and grounds stay separate, and you remove the bonding screw. If it's a separate structure, you still keep them separate per NEC 250.32(B), and you drive ground rods at the subpanel location. Get this wrong and the whole job gets pulled.

The panel and breakers

Pick the panel first, then the breakers, because breaker availability varies by brand. Square D QO, Eaton BR, and Siemens are the usual suspects. If the main is one brand, match it if you can, but a subpanel doesn't have to match the service panel.

Count your circuits, then add 25 percent headroom. A 20-space panel fills up faster than you think once AFCI and GFCI breakers start eating double-slots. NEC 408.4 requires every circuit to be legibly identified at the panel, so grab a fresh directory card while you're at the supply house.

• Main lug or main breaker subpanel, sized to the feeder
• Branch breakers: mix of 15A, 20A, 30A, 2-pole as needed
• AFCI or dual-function breakers per NEC 210.12
• GFCI breakers where receptacle-level protection won't be used, per NEC 210.8
• Ground bar kit (most panels ship without one)
• Filler plates for unused openings, NEC 408.7

Feeder conductors and conduit

The feeder is where most material-list mistakes happen. For a 100A feeder in conduit at 75C terminations, 3 AWG copper or 1 AWG aluminum per NEC Table 310.16. For SER cable feeding a subpanel in the same building, 2/0-2/0-2/0-1 aluminum is the standard 100A run. Always check the termination temperature rating on both ends, because that's what actually limits you.

Four conductors to the subpanel: two hots, a neutral, and a separate equipment grounding conductor. Do not reuse the neutral as a ground, and do not land them on the same bar at the subpanel. NEC 250.142(B) is explicit.

Buy 10 percent more feeder than your measurement. Every time you try to land exactly to the inch, you end up 18 inches short after the last 90. Scrap copper pays better than an extra trip.

Conduit: 1-1/4 inch EMT handles 3 AWG THHN comfortably with room for the ground. If you're in flex for the last few feet into the panel, sleeve it properly and don't exceed the bending radius.

Grounding and bonding hardware

If the subpanel is in a separate structure, you need a grounding electrode system at that structure per NEC 250.32(A). Two 8-foot ground rods spaced at least 6 feet apart, bonded with 6 AWG copper, acorn clamps listed for direct burial.

Inside the panel, the equipment ground lands on the ground bar. The neutral bar stays isolated from the can. Check that the bonding screw or strap that ships installed in most panels is backed out and bagged, not left in place. This is the single most common inspection fail on subpanel work.

• (2) 8-foot copper-clad ground rods
• (2) acorn or listed rod clamps
• 6 AWG bare copper GEC, length to suit
• Separate ground bar kit for the subpanel
• Anti-oxidant compound for aluminum feeder terminations

Supports, boxes, and fittings

NEC 314.23 covers box support, NEC 358.30 covers EMT support. Within 3 feet of the panel and every 10 feet after, plus every box. Use minerallac or two-hole straps on the panel risers, not single-screw clamps on a vertical run that's going to see vibration.

Every penetration into the panel needs a proper connector. Romex connectors for NM, SER connectors for SER, insulated throat connectors for THHN in EMT. Reducing washers if the knockout is oversized. Bushings on any raceway carrying 4 AWG or larger per NEC 300.4(G).

Keep a small parts bin of reducing washers, 1/2 and 3/4 insulated bushings, and grounding bushings on the truck. Nothing kills a Friday afternoon like driving back to the supply house for one fitting.

Final material checklist

Before you leave the shop, walk the list one more time against the drawing. This is the baseline list for a typical 100A same-structure subpanel run in EMT, about 40 feet, 12 branch circuits. Adjust for your actual job.

1. Subpanel, 100A main-lug, 20-space minimum
2. 100A 2-pole feeder breaker at the main panel
3. Branch breakers per circuit schedule, plus 2 spares
4. Feeder: 3 AWG THHN copper, black, red, white, plus 8 AWG green
5. 1-1/4 inch EMT, couplings, connectors, straps
6. Ground bar kit for the subpanel
7. Anti-ox, wire nuts, labels, panel directory
8. Fire-rated caulk for wall penetrations per NEC 300.21

Torque every lug to the manufacturer spec, NEC 110.14(D). A calibrated torque screwdriver is cheap insurance against a callback for a loose neutral six months down the road.