Field guide: installing a subpanel, step-by-step (edition 4)

Plan the load and pick the feeder

Start with a real load calc per NEC Article 220. Don't eyeball it. Add up the continuous and non-continuous loads on the subpanel, apply the 125% factor on continuous loads, and size the feeder from there. A 100A subpanel is the common answer for a detached garage or finished basement, but run the numbers before you buy wire.

Feeder conductors size per NEC 310.16 for standard installs, or 310.12 if you're feeding the entire dwelling (the 83% rule). For a 100A feeder on a dwelling unit subpanel under 310.12, #4 copper or #2 aluminum SER is typical. For non-dwelling or pure 310.16 ampacity, step up to #3 copper or #1 aluminum. Check the breaker and lug temperature ratings before you commit.

Voltage drop matters on long runs. Keep feeder drop under 3% per NEC 210.19 Informational Note No. 4. For a 100A run past 100 feet, price out the next size up before you pull.

Pick the panel and the location

Main lug or main breaker? If the subpanel is in a separate structure, NEC 225.31 and 225.32 require a disconnect at the building served, so you want a main breaker panel there. Same building, same structure, main lug is fine as long as the feeder breaker at the source is your OCPD.

Working clearance is not optional. NEC 110.26(A) wants 36 inches in front, 30 inches wide, and 6.5 feet high, clear. No water heaters crammed in front, no shelving, no stored paint. The panel also can't live in a bathroom (240.24(E)) or a clothes closet (240.24(D)).

Tip from the field: if the homeowner is finishing the basement later, mark the 110.26 working space on the slab with a paint pen before drywall goes up. Saves the argument when the GC tries to frame a utility closet around your panel.

Run the feeder correctly

Four wires to a separate-structure subpanel or a same-structure subpanel: two hots, a neutral, and an equipment grounding conductor. Three-wire feeders to detached structures were killed in the 2008 NEC. If you're working on an old three-wire feed, NEC 250.32(B) requires you to replace it with a four-wire on any significant rework.

Conductor protection: NEC 300.4 for holes through framing, keep the edge 1.25 inches back or use a nail plate. Support intervals per 334.30 for NM, or 336.30 for SE/SER. Derate for conduit fill (Chapter 9, Table 1) and ambient (310.15(B)) when you're in an attic above 86F.

• 100A feeder, dwelling: 2-2-2-4 aluminum SER or 4-4-4-6 copper SER
• Terminate in listed lugs only, torque to the label spec
• No splices inside the feeder unless you're in a legit junction box
• Anti-oxidant on aluminum terminations per the listing

Bond and ground the subpanel

This is where most failed inspections happen. The neutral and ground must be separated at the subpanel. Remove the main bonding jumper (green screw or strap) that ships installed from the factory. Neutrals land on the isolated neutral bar, grounds land on a separate ground bar that's bonded to the enclosure.

If the subpanel is in a separate structure, NEC 250.32(A) requires a grounding electrode system at that building. Two ground rods 6 feet apart, or one rod plus a verified sub-25-ohm reading. The GEC from those rods lands on the ground bar, not the neutral bar. Never re-bond at the subpanel.

Tip from the field: pull the green bonding screw out and tape it to the inside of the dead front with the panel directory. Inspector can see it, future you can find it, nobody gets creative.

Breakers, circuits, and labeling

Use breakers listed for that panel. Classified breakers get people in trouble, stick to the manufacturer's listing unless you've verified the classification in writing. Double check AFCI requirements under 210.12 for dwelling unit circuits and GFCI under 210.8 for the specific locations served.

1. Verify power is off at the source, lock out, test with a known-good meter
2. Mount the panel plumb, torque feeder lugs to spec
3. Land hots on the main breaker or lugs, neutral on neutral bar, EGC on ground bar
4. Install branch breakers, balance the loads across both legs
5. Fill out the directory with specific room and circuit descriptions per 408.4

Balanced legs matter more than people think. A 100A panel with 80A on A phase and 20A on B is asking for nuisance trips and neutral heat. Walk the load list before you set breakers, not after.

Final checks before you close it up

Torque every termination with a calibrated driver, not a guess. NEC 110.14(D) requires it, and most AHJs now want to see the torque tool on site. Voltage test leg to leg, leg to neutral, leg to ground. Confirm neutral to ground reads zero volts and near-zero ohms from the ground bar back to the main panel's bonded neutral.

Label the subpanel with the feeder source per 408.4(B), fill the directory, and note any AFCI/GFCI protection at the breaker. Inspector will look at the bonding, the working clearance, the directory, and the grounding electrodes at a separate structure. Hit those four and you're closing permits instead of chasing corrections.