Field guide: installing a subpanel, material list (edition 3)

Scope and load math first

Before you pull a material list together, confirm the subpanel's purpose and calculated load. A 100A feeder to a detached garage is a different animal than a 60A feed to a kitchen remodel. Run the calc per NEC Article 220 and verify the existing service has headroom. Main panel bus rating and existing breakers dictate what you can feed.

For a detached structure, NEC 225.30 limits you to a single feeder unless conditions in 225.30(A) through (E) apply. Also review 225.32 for disconnect location: readily accessible, nearest the point of entrance. If the subpanel is inside the same building, 408.36 governs overcurrent protection at the supply end.

Grounding rules split based on location. Same structure: four-wire feed, neutral and ground isolated at the subpanel, bonding screw removed (NEC 250.24(A)(5) and 408.40). Detached structure: still four-wire per 250.32(B)(1), with a separate grounding electrode system at the outbuilding per 250.32(A).

The panel and breakers

Size the enclosure for future capacity, not just today's circuits. A 20-space panel for a 100A feeder is the common floor. Match the manufacturer on breakers; no UL-classified breaker substitutions unless the panel is listed for them. Check for plug-on neutral compatibility if the jurisdiction requires AFCI or dual-function breakers in dwelling units per 210.12.

For the feeder breaker in the main panel, size per the ampacity of the feeder conductors and the subpanel main. A 100A subpanel on a 100A breaker with properly sized copper or aluminum feeders, confirmed against 310.16 and the 60/75/90 degree column rules in 110.14(C).

• Main lug or main breaker subpanel enclosure, NEMA 1 indoor or NEMA 3R outdoor
• Feeder breaker sized to the calculated load (common sizes: 60A, 100A, 125A)
• Branch breakers: standard, AFCI, GFCI, or dual-function per circuit location
• Ground bar kit (isolated from neutral) and any filler plates

Feeder conductors and raceway

Pick the conductor based on ampacity, voltage drop, and install method. For a 100A feed in conduit, 3 AWG copper or 1 AWG aluminum THHN/THWN-2 is typical. For a longer run to an outbuilding, recalculate voltage drop; keep it under 3% on the feeder per 210.19(A) Informational Note No. 4. SER cable is common for interior feeds but watch 338.10(B)(4) for ampacity derating when run through thermal insulation.

For buried runs, NEC 300.5 gives minimum cover: 24 inches for direct burial, 18 inches for PVC conduit, 6 inches under a 4-inch concrete slab. Use Schedule 40 PVC for most buried work, Schedule 80 where subject to physical damage per 352.10(F). Transition to EMT or rigid at the panel with the correct LB or fitting.

Tip from the field: pull an extra #6 THHN green in the conduit when you're feeding a detached structure. Running short on equipment grounding conductor because you trusted the cable markings is a bad day.

Grounding and bonding hardware

For a detached structure, install a grounding electrode system per 250.50. Two ground rods spaced at least 6 feet apart, bonded with #6 copper, unless a single rod tests at 25 ohms or less (250.53(A)(2) exception). Ufer, if accessible, satisfies 250.52(A)(3). Bond the grounding electrode conductor to the ground bar in the subpanel, not the neutral.

Inside the same building, the equipment grounding conductor rides with the feeder. Neutral lands on the insulated neutral bar; ground lands on the bonded-to-enclosure ground bar. Confirm the main bonding jumper is removed from the subpanel. It is the number-one inspection failure on this work.

• (2) 5/8 inch x 8 ft copper-clad ground rods
• Acorn clamps listed for direct burial
• #6 bare copper GEC, sized per 250.66
• Intersystem bonding termination if none exists (250.94)

Consumables, fittings, and labeling

This is where jobs get delayed. Order the small stuff with the big stuff. Every connector, bushing, and anti-short counts toward pass or fail at rough-in.

1. Conduit bodies (LB, T) sized to the raceway
2. Set screw or compression connectors, listed for wet location outdoors
3. Insulating bushings for conductors 4 AWG and larger (300.4(G))
4. Anti-oxidant compound if terminating aluminum
5. Strut, unistrut, or panel standoffs for surface mounts
6. Panel schedule, permanent marker, and circuit directory per 408.4(A)
7. Arc-flash and feeder source labels per 110.21(B) and 408.4(B)

Torque is not optional. Use a calibrated torque screwdriver and a torque wrench for the lugs, values per the panel label and 110.14(D). Document if the AHJ requires it. Photos of torque marks save callbacks.

Tip from the field: bring two rolls of phase tape in each color. Re-identifying conductors is routine, and running back to the truck for red tape at 4pm is how overtime happens.

Quick-pick material checklist

Before you load the van, walk this list. Miss one item and you are making a second trip.

• Subpanel enclosure with correct amp rating, space count, and NEMA rating
• Feeder breaker for the main panel
• Branch breakers (count AFCI, GFCI, and dual-function separately)
• Feeder conductors: 2 hots, 1 neutral, 1 equipment grounding conductor, correctly sized
• Raceway and fittings, or cable and staples rated for the method
• Grounding electrodes, clamps, and GEC for detached structures
• Ground bar kit, filler plates, and cable connectors at the panel
• Torque tools, labels, circuit directory, and permit card

Verify the AHJ's local amendments before final. Some jurisdictions require a whole-structure surge protective device at the service per 230.67 and will push the same requirement to feeder-supplied structures. Others have specific rules for outbuilding disconnects and ground rod testing. Call the inspector before the rough, not after.