Field guide: installing a subpanel, material list (edition 2)

Plan the load before you pull the truck in

Subpanel jobs live or die on the load calc. Before you price a single lug, sit down with the homeowner's list: shop tools, EV charger, hot tub, mini split, kitchen rework. Run the calc per NEC 220 Part III. A 100A feeder covers most detached garages and finished basements. Step to 125A or 200A when the EVSE plus a welder or HVAC pushes you over.

Measure the feeder run before you buy wire. Voltage drop on a 100A feeder at 80 feet in aluminum will bite you. NEC 215.2(A)(1) Informational Note No. 2 recommends 3% max on feeders. Most inspectors will not cite you for 4%, but the customer will feel it when the compressor kicks.

Confirm the main panel has spare bus capacity and a breaker slot wide enough for a 2-pole feeder breaker. If the main is full, price a tandem swap or a main lug addition before you promise a date.

Panel, breakers, and the feeder breaker

Pick a panel rated for the feeder ampacity with room for future circuits. For a detached structure, 20 to 30 spaces is the sweet spot. Square D QO, Eaton BR, and Siemens PL all have good stock at supply houses. Avoid any panel with obsolete breaker lines unless you are matching an existing installation.

The feeder breaker goes in the main panel, sized to the conductor per NEC 240.4. If you are feeding a detached structure, the disconnect at the subpanel counts as the required disconnecting means under NEC 225.31 and 225.32. Label it clearly.

• Main lug or main breaker subpanel, 100A or 125A, 20 to 30 spaces
• 2-pole feeder breaker sized to the conductor, installed in the main
• Branch breakers: 15A and 20A single pole, 2-pole for 240V loads
• GFCI or dual-function breakers for any circuit covered by NEC 210.8
• Surge protective device at the subpanel per NEC 230.67 if this is the service, or as good practice otherwise

Feeder conductors and the grounding electrode

Four wire feeder to a subpanel in a separate structure. Two hots, a neutral, and an equipment grounding conductor. No more three-wire feeders to detached buildings; that allowance in NEC 250.32 went away in the 2008 cycle. Isolate the neutral bar from the can. Bond the ground bar to the enclosure.

Size per NEC Table 310.16 and the 83% rule in NEC 310.12 if this is a dwelling feeder carrying the entire load. For a 100A feeder in a dwelling: 1 AWG copper or 1/0 aluminum SER works for interior runs; 1/0 or 2/0 aluminum URD direct burial for outdoor. Verify terminal temperature ratings on both ends before you commit.

A detached structure needs its own grounding electrode system per NEC 250.32(A). Two 8 foot ground rods, 6 feet apart, or a single rod with a tested resistance under 25 ohms. The equipment grounding conductor in the feeder bonds back to the service; the rods are supplemental, not a substitute.

Tip from the field: buy your ground rod clamps listed for direct burial. The cheap ones rust off the rod in five years and you will fail a re-inspection on a house you thought was done.

Raceway, cable, and the physical path

Pick the wiring method before you dig or drill. Underground to a detached structure is usually PVC Schedule 40 at 18 inches cover, or Schedule 80 where exposed to physical damage per NEC Table 300.5. Direct burial URD at 24 inches works but leaves you no room to upsize later. Spend the extra on conduit.

Interior runs in finished space go in SER cable through studs with proper support per NEC 334.30 and protection from nails per NEC 300.4. In unfinished basements or garages, strap SER every 4.5 feet and within 12 inches of the panel. THHN in EMT is cleaner if the customer ever adds a second subpanel off the same run.

• PVC Schedule 40 or 80, sweeps, couplings, and bell ends for underground
• Expansion fitting where the PVC comes out of the ground into the panel
• SER cable and listed connectors for interior feeder runs
• Cable staples and stackers rated for the cable size
• Fire caulk for any penetration through a fire-rated assembly

The full material list, printable

This is the list I hand the apprentice before he heads to the supply house. Adjust quantities to the job but do not leave without any of these categories covered.

1. Subpanel with main breaker or main lug, correct amperage, correct space count
2. Feeder breaker for the main panel, 2-pole, sized to conductor
3. Feeder conductors: SER or individual THHN, sized per NEC 310.12 or 310.16
4. Conduit, fittings, sweeps, expansion joint, bell ends, PVC cement
5. Two 8 foot copper ground rods, listed acorn clamps, 6 AWG bare copper
6. Equipment grounding conductor sized per NEC 250.122
7. Antioxidant compound for aluminum terminations
8. Torque screwdriver and torque wrench, calibrated, per NEC 110.14(D)
9. Branch circuit breakers, including AFCI and GFCI where required
10. Panel schedule card, permanent marker, and a roll of red phase tape

Tip from the field: torque every lug to the label value and write the date on the panel schedule. When the insurance adjuster shows up after a fire three houses down, that note is worth an hour of billable defense.

Stage everything on the truck the night before. A subpanel install should be a one-trip day. If you are driving back to the supply house for a connector, you either skipped the walkthrough or trusted your memory. Neither is a habit worth keeping.