Field guide: installing a subpanel, material list (edition 4)

What the job actually needs

A subpanel install lives or dies on the material list. Miss one lug, one bonding screw, or the wrong breaker, and you are burning a second trip. This is the list working hands actually pull off the truck for a standard 100A feeder off a 200A main, fed with copper or aluminum, mounted in an attached garage or basement. Adjust for your AHJ, but the bones do not change.

Before you load out, confirm the load calc per NEC 220 Part III and the feeder ampacity per NEC 215.2. Subpanel must be rated equal to or greater than the feeder overcurrent device. If you are feeding a detached structure, you are now in NEC 225 Part II and the rules shift, especially around the disconnect at the second building and the grounding electrode system.

The panel and its guts

Pick a main lug only (MLO) panel if the feeder breaker at the source serves as the disconnect and the panel is in the same structure within sight or nipple-close. Pick a main breaker panel if the run is long, if you want local disconnect, or if you are crossing into a detached building where NEC 225.31 requires a disconnect. Do not mix brands on breakers. Classified breakers exist, but if the panel label does not list them, you are creating a listing violation under NEC 110.3(B).

Count your spaces honestly. A 20-space panel fills faster than you think once AFCI and GFCI breakers land. Buy the 30 or 40 space if the wall allows.

• Subpanel enclosure, 100A, 20 to 40 space, matched to source breaker brand
• Main breaker kit if required, or main lug kit
• Isolated neutral bar and separate equipment ground bar (critical, see below)
• Branch breakers sized to the load calc, AFCI and GFCI per NEC 210.8 and 210.12
• Filler plates for unused openings, NEC 408.7
• Panel directory, filled out legibly, NEC 408.4(A)

Feeder conductors and the four-wire rule

Modern subpanels take four wires. Two ungrounded conductors, one grounded (neutral), one equipment grounding conductor. The neutral and ground are bonded only at the service, never at the subpanel. Pull the green bonding screw or bonding strap out of the neutral bar and set it aside in the panel box for the inspector to see. This is NEC 250.24(A)(5) and 250.142(B) territory and is the single most common failure on a rough inspection.

Size the EGC per NEC 250.122 based on the feeder OCPD, not the conductor. A 100A feeder wants a minimum 8 AWG copper or 6 AWG aluminum EGC. Upsize the EGC proportionally if you upsized the ungrounded conductors for voltage drop per NEC 250.122(B).

• 3 AWG copper THHN/THWN-2 or 1 AWG aluminum XHHW-2 for a 100A feeder at 75C terminations, verified against NEC 310.16 and the 83% rule in NEC 310.12 if this is a dwelling feeder
• 8 AWG copper or 6 AWG aluminum EGC, green
• Anti-oxidant compound if you are on aluminum
• Listed lugs rated for the conductor material, torqued to the label spec

If you only remember one thing from this list: pull the bond screw at the subpanel. Tape it to the inside of the dead front. Inspector sees it, you pass.

Raceway, connectors, and support

Most of the misses on a punch list are not the panel, they are the connections to it. Have the right connector for the cable or raceway, in the right size, rated for the location. EMT in a dry basement is clean and cheap. Liquidtight flexible metal conduit (LFMC) for the last few feet into a panel on a vibrating wall or across an expansion point. SER cable is common for interior feeders, but check local amendments, some jurisdictions will not let SER pass through unconditioned space.

1. 1 inch EMT or 1-1/4 inch if pulling 1 AWG aluminum, sized per NEC Chapter 9 Table 1 fill
2. Set screw or compression connectors, one per termination plus spares
3. Minerallac or one-hole straps, supported per NEC 358.30 for EMT (10 ft max, within 3 ft of boxes)
4. Insulating bushings on raceways 1 inch and larger containing 4 AWG and larger conductors, NEC 300.4(G)
5. Cable staples and NEC 334.30 supports if running NM into the subpanel from above
6. Fire caulk, 3M CP25WB+ or equivalent, for any fire-rated wall penetrations

Grounding and bonding extras

If this subpanel is in a separate building, you are installing a grounding electrode system at that building per NEC 250.32. Two ground rods, 6 ft apart, 6 AWG copper GEC if rods are the sole electrode, irreversible crimp or listed acorn clamps. No neutral-ground bond at the separate building panel either, assuming you pulled an EGC with the feeder, which you should.

Inside the same structure, you still want a clean ground bar, properly torqued. Do not double-lug equipment grounds on a bar unless the bar is listed for it.

Torque screwdrivers are not optional anymore. NEC 110.14(D) has been explicit since 2017. Buy the Klein or Wera, log the readings, and you will never lose a callback argument.

The small stuff that ends the day

These are the items that send apprentices back to the van at 4:30 PM. Stock them in the gang box and the job closes clean.

• Red, black, white, green phase tape (re-identify neutrals over 6 AWG per NEC 200.6(B))
• Sharpie, silver paint pen for black jacket
• Spare KO seals and reducing washers
• Wire pulling lube for the feeder
• Label maker or pre-printed panel schedule
• Arc-flash label for the new panel, NEC 110.16
• Working space verified, 30 in wide, 36 in deep, 6.5 ft high, NEC 110.26

Edition 4 of this list reflects the 2023 NEC cycle. If your jurisdiction is still on 2020 or 2017, cross-check AFCI expansion in 210.12 and the surge protection requirement in 230.67, which applies at services but shapes how customers ask about subpanel protection too.