Field guide: installing a subpanel, material list (edition 6)

Planning the subpanel install

Before you pull the first length of wire, nail down three numbers: calculated load, feeder distance, and available fault current at the source. Those drive everything else on the list. NEC 220 governs the load calc, 215.2 sets feeder ampacity, and 110.24 is where the fault current marking lives if the main is service equipment.

Walk the run. Measure from the source panel to the subpanel location, add for drops, rises, and the working space you actually need at both ends per NEC 110.26. Note every penetration, fire-rated assembly, and wet location you cross. Those details change your connector count and conductor type before you ever touch the truck.

Confirm the feed. A 100A subpanel off a 200A main with existing 60A of demand is fine on paper, but if the homeowner added a heat pump and an EV charger last year, redo the calc. Article 220 Part III is your friend here.

The panel and its guts

Pick the enclosure for the environment, not the price tag. Indoor dry locations get a NEMA 1. Garages, barns, and unconditioned basements that sweat in summer get a NEMA 3R even if technically indoors. The listing matters under NEC 110.3(B).

Size the can for future circuits. A 30-space panel for a current 12-circuit load costs maybe forty dollars more and saves a full day when the customer calls back in two years for a hot tub.

• Main lug or main breaker subpanel (main breaker if required as service disconnect at a detached structure, NEC 225.31)
• Ground bar kit, isolated from neutral bar in all subpanel applications per NEC 250.24(A)(5)
• Branch breakers sized to the loads, AFCI per 210.12 and GFCI per 210.8
• Handle ties or two-pole breakers for multiwire branch circuits, NEC 210.4(B)
• Filler plates for every unused opening, NEC 408.7

Buy the ground bar kit with the panel, not after. Half the callbacks on a subpanel are because someone landed EGCs on the neutral bar when no isolated ground bar was installed.

Feeder conductors and raceway

Size feeders from NEC Table 310.16 after you apply 310.15 adjustments and the 125% continuous load rule in 215.2(A)(1). A 100A feeder in THHN copper at 75C terminations is 3 AWG; aluminum is 1 AWG. Do not mix rating columns, and do not forget the neutral sizing rules in 220.61 if the load is unbalanced or heavy on nonlinear equipment.

The equipment grounding conductor is sized from NEC Table 250.122 based on the upstream overcurrent device, not the feeder ampacity. For a 100A feed, that is 8 AWG copper or 6 AWG aluminum. Run it; do not rely on the raceway as the sole EGC unless the raceway itself qualifies under 250.118 and you are comfortable defending that choice at inspection.

• SER cable for interior routes where permitted by local amendment, NEC 338.10(B)
• PVC schedule 40 or 80 for underground, minimum 18 inches cover for 120/240V feeders per Table 300.5
• EMT with compression fittings for exposed indoor runs, set screws only in dry locations
• Bushings on every conduit termination in the panel, NEC 300.4(G) and 312.6

Bonding, grounding, and the neutral question

This is where rough installs fail inspection. At the subpanel, the neutral bar floats. The bonding screw or strap that came in the box stays in the bag. The EGCs land on the ground bar, and the neutrals land on the insulated neutral bar. Four-wire feeder, always, whether the subpanel is in the same building or another structure fed after 2008.

For a detached structure, re-read NEC 250.32. You need an EGC with the feeder, and if there is no metallic path between buildings (no water line, no phone, nothing), you do not drive new ground rods to replace the EGC, you drive them to supplement it. Two rods six feet apart, or one rod if you can prove 25 ohms, per 250.53(A).

Label the feeder disconnect at the source panel before you energize. NEC 408.4(A) is not optional, and inspectors will red-tag a panel with unlabeled breakers faster than any other single issue.

The material list, consolidated

Print this, tape it to the dash, and shop once. Adjust quantities for your specific run.

1. Subpanel enclosure, correct NEMA rating, 30+ spaces
2. Main breaker or main lug kit as required
3. Isolated ground bar kit
4. Branch breakers, AFCI and GFCI as required by 210.8 and 210.12
5. Feeder conductors: two hots, one neutral, one EGC, sized per 215.2 and 250.122
6. Raceway: EMT, PVC, or cable assembly appropriate to the route
7. Conduit fittings: connectors, couplings, bushings, LBs as needed
8. Anti-oxidant compound for all aluminum terminations
9. Ground rods, acorn clamps, and 6 AWG bare copper for detached structures
10. Panel labels, circuit directory, and arc flash marking per 110.16 if applicable
11. Fire caulk for every rated penetration, NEC 300.21
12. Staples, straps, and standoffs at the spacing required by the cable method used

Torque every lug to the manufacturer spec listed inside the panel door. NEC 110.14(D) made that a code requirement, not a best practice. A calibrated screwdriver pays for itself the first time an inspector asks to see it.