Field guide: installing a subpanel, step-by-step (edition 2)

Plan the load and pick the panel

Start with a load calc per NEC Article 220. Size the subpanel to the calculated load plus reasonable headroom, not to the feeder breaker. A 100A subpanel fed by a 60A breaker is fine; a 60A subpanel fed by a 100A breaker is not.

Count circuits you actually need, then add 25% spare spaces minimum. Detached structures follow 225.30 through 225.40: generally one feeder, one disconnect. If the subpanel is in a separate building, plan for a grounding electrode system at that building per 250.32.

Confirm panel location meets 110.26 working space: 3 ft depth, 30 in width, 6.5 ft headroom. No panels in clothes closets (240.24(D)), bathrooms in dwellings (240.24(E)), or over steps.

Size the feeder and overcurrent protection

Feeder ampacity comes from 310.16 with adjustments from 310.15. For dwelling services and feeders supplying the main power to a dwelling, 310.12 still allows the reduced conductor sizes, but a subpanel feeder inside the same structure is not a "main power feeder" unless it carries the entire dwelling load.

Typical picks for copper at 75C terminations:

• 60A feeder: #6 THHN/THWN-2
• 100A feeder: #3 THHN/THWN-2 (or #4 under 310.12 when it qualifies)
• 125A feeder: #1 THHN/THWN-2
• 200A feeder: 3/0 THHN/THWN-2 (or 2/0 under 310.12 when it qualifies)

Four-wire feeder: two hots, a neutral sized per 220.61, and an equipment grounding conductor sized per 250.122. No exceptions for separate structures anymore, the old 250.32(B) bonded-neutral allowance is gone.

Pull the feeder and land it right

Raceway fill per Chapter 9 Table 1. For three or four current-carrying conductors in a raceway, no ambient derate needed until you exceed four CCCs per 310.15(C)(1). The neutral on a 240/120 single-phase feeder is not counted as a CCC unless it carries nonlinear load (310.15(E)).

Torque every lug to the listed value. This is not optional, 110.14(D) requires a calibrated torque tool. Loose lugs are the number one cause of subpanel failures that come back as warranty calls.

Mark the torque value on the dead front with a paint pen after you set it. Next guy in the panel knows it was done, and you know if someone else backed it off.

Bond and ground correctly

This is where most subpanels get installed wrong. At a subpanel, the neutral bar and the ground bar must be separate. Remove the main bonding jumper, the green screw, or the bonding strap, whatever the manufacturer used. Neutrals land on the isolated neutral bar. Grounds land on a ground bar bonded to the enclosure.

If the subpanel didn't ship with a ground bar, add a listed accessory kit. Do not land EGCs under the enclosure screws or on the neutral bar.

For a detached structure, drive the grounding electrodes at the subpanel per 250.32(A) and bond them to the EGC that came with the feeder. Do not re-bond neutral to ground at the detached building. Single grounding rod requires a second unless you can prove 25 ohms or less per 250.53(A)(2).

If the inspector asks you to show the bond, the green screw should be in your pocket or in the panel's literature pouch, not in the panel.

Circuits, AFCI, GFCI, and labeling

Populate circuits per the usual rules. 210.8 for GFCI, 210.12 for AFCI, 210.52 for dwelling receptacle placement. A subpanel doesn't change any of that. If the subpanel feeds a kitchen, laundry, bath, garage, outdoor, or unfinished basement area, the relevant GFCI and AFCI requirements apply at the branch circuit.

Balance the legs. Alternate 120V loads between A and B phase as you fill the bus. A wildly unbalanced subpanel runs hot on one leg and wastes neutral capacity.

1. Fill from the top down, 240V loads first
2. Distribute 120V loads to balance per-leg amperage
3. Leave at least two adjacent spaces for future 240V
4. Label every circuit with the room and device, not "lights"

Directory per 408.4(A): legible, specific, at the panel. "Bedroom 2 receps" beats "bedroom." Handwritten is fine if it's readable. Print it if the AHJ is picky.

Final checks before you button up

Before the dead front goes on, verify: neutral and ground bars separated, bonding screw removed, EGC landed on ground bar, torque marks on every lug, no double-tapped breakers unless the breaker is listed for it, and no damaged insulation at the cable clamps.

Energize and check voltage leg to leg (240V nominal), leg to neutral (120V each), and leg to ground (120V each). Neutral to ground should read close to zero volts at the subpanel. If you see more than a volt or two, you have a bond somewhere you shouldn't, go find it before you leave.

Write the feeder size, breaker size, and install date inside the dead front in permanent marker. Future-you, or the next electrician in there, will thank present-you.