Field guide: installing a subpanel, for master electricians (edition 2)

Load calc and feeder sizing come first

Before you touch a knockout, run the load calculation per NEC 220. Standard method or optional method, pick one and document it. A 100A subpanel feeding a detached shop with a welder, compressor, and a mini-split is not the same animal as a 60A subpanel for a finished basement with LED cans and a few receptacles.

Size the feeder conductors to the calculated load, then confirm the OCPD at the source panel matches. Remember 310.16 ampacity tables apply, but terminations are governed by 110.14(C). Most residential breakers are 75C rated, so use the 75C column unless the equipment label says otherwise.

• Copper vs aluminum: SER aluminum is common for 100A feeders, and it is fine if you torque and antiox correctly.
• Voltage drop: over 100 feet, bump up a size. NEC 210.19(A) Informational Note No. 4 suggests 3 percent on feeders.
• Neutral sizing: per 220.61, the neutral carries unbalanced load only, but do not undersize it on a panel with heavy 120V loads.

Four wire feeder, always, on a separate structure or detached

Since the 2008 NEC rewrite, you run four wires to any subpanel: two hots, a neutral, and an equipment grounding conductor. The neutral and ground are isolated at the subpanel. This applies whether the sub is in the same building or a detached structure per 250.32(B).

Pull the bonding screw or strap from the subpanel. This is the number one callback on sub installs. If the neutral bar is bonded to the enclosure, you have parallel paths on the EGC and nuisance trips on GFCIs downstream.

Pop the green bonding screw and tape it to the inside of the dead front before you energize. If the next guy opens the panel, he sees immediately that the sub is floating the neutral on purpose.

Grounding electrodes at a detached structure

If the subpanel is in a separate building, you need a grounding electrode system at that structure per 250.32(A). Two ground rods 6 feet apart, or a single rod if you can prove under 25 ohms (nobody measures, so drive two). Bond the rods to the EGC bus in the subpanel, not the neutral.

For a detached garage or shop, this is non-negotiable. The inspector will look for it and the GEC sizing comes from 250.66. A 100A feeder typically gets a 8 AWG copper GEC to rods, or 4 AWG if you hit a metal water pipe or Ufer.

1. Drive rods first, before pouring concrete or setting the panel, so you are not fighting rocky ground later.
2. Use acorn clamps listed for direct burial, and leave the GEC continuous where possible per 250.64(C).
3. If there is a metal water line or structural rebar accessible, bond to those too. All electrodes present must be bonded together.

Panel placement and working space

NEC 110.26 working space is where most inspections go sideways. 30 inches wide, 36 inches deep, 6.5 feet high, clear and dedicated. The space in front of the panel is not storage. Do not mount a subpanel above a washer, behind a water heater, or in a clothes closet per 240.24(D) and (E).

Mount the panel so the highest breaker handle is no more than 6 feet 7 inches off the finished floor, per 240.24(A). In unfinished basements, think about where drywall might land later and leave yourself room for the dead front to swing.

Breakers, AFCI, GFCI, and the stuff that bites you at inspection

Match the breaker brand to the panel. No UL classified cross brand breakers unless the panel label specifically lists them. This is a 110.3(B) issue and an AHJ will fail it on sight in most jurisdictions.

Dwelling unit circuits from the subpanel still need AFCI per 210.12 and GFCI per 210.8 where applicable. A subpanel in a finished basement feeding bedrooms, living areas, laundry, and unfinished storage hits almost every protection requirement in the book. Plan your breaker budget before you order the panel, dual function breakers are not cheap and they eat slots fast.

• Laundry receptacle: GFCI per 210.8(A)(10), and a dedicated 20A circuit per 210.11(C)(2).
• Unfinished basement receptacles: GFCI per 210.8(A)(5).
• Bedrooms, living rooms, hallways: AFCI per 210.12(A).
• HVAC disconnect within sight per 440.14 if the sub feeds a condenser.

Torque, label, and close it up

Every lug, every breaker, every neutral and ground termination gets torqued to the manufacturer spec. 110.14(D) made this explicit in 2017. Use a calibrated torque screwdriver or click wrench, not your wrist memory. Aluminum especially, apply antioxidant and torque in the recommended pattern.

Fill out the circuit directory legibly, not "lights" and "plugs." Room by room, with amperage. Label the sub at the main panel disconnect, and label the main at the sub per 408.4(A) and 225.37 if applicable. The next electrician in this panel is you in five years on a service call, be kind.

Take a photo of the completed panel with the dead front off, torque marks visible, and the directory filled in. Store it with the job file. It has saved me more than one callback argument.