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

Plan the feeder before you touch a knockout

Size the feeder to the calculated load, not to the main panel's rating. Run a load calc per NEC 220, then pick conductors that land inside the subpanel's main lug or main breaker rating. Apprentices burn hours rerunning wire because they guessed at 60A when the job needed 100A.

Distance matters. If the subpanel sits more than about 100 feet from the source, check voltage drop. NEC 210.19(A) Informational Note No. 4 recommends keeping drop at 3% or less on branch circuits and 5% total. On a detached garage or a long basement run, bump the feeder up a size before you pull anything.

Confirm the grounding path before you buy parts. Same structure or separate structure changes everything about how you land the neutral and ground.

Same building vs. separate structure

Inside the same building, a subpanel is fed with a 4-wire feeder: two hots, an insulated neutral, and an equipment grounding conductor. The neutral bar floats. The ground bar bonds to the enclosure. Never tie them together downstream of the service disconnect, NEC 250.24(A)(5) is specific about this.

A detached structure fed by a single feeder used to allow a 3-wire setup with the neutral re-bonded. That exception was removed. Per NEC 250.32(B), new installations require an equipment grounding conductor run with the feeder, and the neutral stays isolated at the remote panel. Drive ground rods at the detached structure per 250.32(A), bond them to the ground bar, not the neutral.

If you see a green bonding screw sitting in the bag taped to the inside of a subpanel door, it stays in the bag. That screw belongs in a service panel only.

Mounting, clearances, and working space

NEC 110.26 is non-negotiable. Give yourself 36 inches of depth in front, 30 inches of width or the panel width (whichever is greater), and 6 feet 6 inches of headroom. Dedicated equipment space above the panel extends 6 feet up or to the ceiling, with nothing foreign in that zone, per 110.26(E).

Mount the panel plumb and at a height where the highest breaker handle sits no more than 6 feet 7 inches off the floor, per 240.24(A). On finished walls, set the can so the front edge finishes flush with the drywall, 312.3 wants the front within 1/4 inch of the finished surface in non-combustible material, flush in combustible.

• Mark stud locations before you cut the hole for a flush-mount can.
• Leave at least 6 inches of free conductor inside the box, measured from where it enters the cabinet, NEC 300.14.
• Keep the bottom of the panel above any expected flood line. Basements off a sump get wet.

Landing conductors cleanly

Terminations fail more subpanels than anything else. Torque every lug with a calibrated driver to the value printed inside the panel door. NEC 110.14(D) now requires calibrated torque tools for terminations that have a listed torque spec, not a crescent wrench and a guess.

Strip to the gauge shown on the strip gauge, not by eye. Aluminum SER feeders need anti-oxidant compound on the strands and a retorque check the next day. Keep line and load sides separated inside the cabinet, dress neutrals to the neutral bar, grounds to the ground bar, and leave service loops long enough that a future electrician can swap a breaker without fighting the bundle.

On a 100A aluminum feeder, come back at the end of the day and hit each lug again. You will feel the wrench move. That movement is why the panel would have cooked in two years.

Breakers, bonding, and the finish

Use breakers listed for that panel. A classified breaker is not the same as a listed breaker, and the AHJ may reject it. Check the label inside the door for accepted types.

Identify the neutral on multiwire branch circuits and handle-tie or common-trip them per 210.4(B). GFCI and AFCI requirements follow the circuit, not the panel, so a basement subpanel feeding bedroom receptacles still needs AFCI per 210.12, and any 125V, 15 and 20A receptacles in the locations listed in 210.8 need GFCI protection.

• Fill unused openings with listed knockout seals, 408.7.
• Label the panel as a subpanel and mark the source, 408.4(B) requires identifying the feeder source.
• Update the directory with specific room and circuit descriptions, not "lights."

Before you energize

Meg the feeder if it ran through rough areas or got pulled hard. Verify neutral-to-ground continuity reads open at the subpanel and closed only at the service. A closed reading means the neutral is bonded where it should not be, find it before you close the main.

Torque check one more time, breakers seated, dead front on, doors clear. Turn on the feeder breaker at the source first, then bring up subpanel breakers one at a time while someone watches for smoke or buzz. Confirm voltage L-L and L-N at the bus, then at the farthest device on each circuit.

Write the install date, your initials, and the torque values on the inside of the door in pencil. The next electrician in that panel will thank you, and on a callback, so will you.