Field guide: installing a subpanel, for journeymen (edition 1)

Plan the feeder before you open the main

Size the feeder for the actual calculated load, not the subpanel's busbar rating. Run Article 220 Part III, apply demand factors, and don't forget continuous loads at 125%. A 100A subpanel fed by #4 copper THHN at 75°C is a common answer, but only after you've verified ampacity per 310.16 and voltage drop on the run.

Voltage drop kills more subpanel installs than ampacity. For runs over 100 feet, check NEC 210.19(A) Informational Note 4, target 3% on the feeder. If the panel feeds a detached structure, Article 225 Part II applies and you're likely pulling four wires with an isolated neutral.

Label the conductors on paper before you cut: two hots, one neutral sized per 220.61, and an equipment grounding conductor sized per 250.122 based on the upstream OCPD, not the feeder ampacity.

Grounding and bonding, the part everyone gets wrong

In a subpanel inside the same structure, the neutral and ground are separated. Remove the main bonding jumper, remove the green bonding screw, and land neutrals on the isolated neutral bar. Grounds land on a separate ground bar that is bonded to the enclosure. This is 250.24(A)(5) and 408.40 territory.

For a subpanel in a detached building, follow 250.32. Run an EGC with the feeder, keep neutral and ground separate, and drive a grounding electrode at the detached structure per 250.32(A). The old three-wire feeder to outbuildings is gone under current code, don't re-create it.

If you pull the cover off an existing subpanel and find neutrals and grounds sharing a bar with the bonding screw in place, that's a parallel neutral path through every metal raceway and ground rod on site. Fix it before you energize anything new.

OCPD, panel selection, and physical install

Match the feeder breaker to the conductor ampacity and the panel busbar rating, whichever is lower. A 100A panel gets a 100A max feeder breaker at the source, sized per 240.4 and 408.36. Don't install a 200A panel fed by a 100A breaker unless you want confusion on the next service call, and make sure the panel is listed for use as a subpanel (most are, but verify).

Working clearances are not optional. NEC 110.26 requires 36 inches of depth, 30 inches of width or panel width (whichever is greater), and 6.5 feet of headroom. If the customer wants the panel behind the water heater, move the water heater or move the panel.

• Mount height: breaker handle no higher than 6 feet 7 inches per 240.24(A)
• Dedicated electrical space above the panel per 110.26(E), no plumbing or ductwork
• Illumination at the working space per 110.26(D)
• Panel directory filled out legibly per 408.4(A), circuit by circuit

Conductor entry, fill, and terminations

Pick your raceway or cable method based on the environment. EMT with THHN is the workhorse indoors. For a detached garage feed, PVC schedule 40 buried at 18 inches with a #4 bare copper as the EGC if you're pulling individual conductors, per Table 300.5 and 250.119.

Box fill and raceway fill are separate calculations. Chapter 9 Table 1 for raceway fill, 314.16 for box fill at the panel knockouts if you're using a junction above. Torque every lug to the manufacturer's spec, 110.14(D) makes this a code requirement now, not a suggestion. Keep the torque screwdriver in the truck.

Strip length matters. Too short and the lug bites insulation, too long and you leave exposed copper past the lug. Follow the strip gauge molded into the panel or printed in the instructions.

Final checks before you energize

Before you throw the feeder breaker, walk the install cold. Megger the feeder if the run is long or the environment is wet. Verify continuity on the EGC from the subpanel enclosure back to the service grounding electrode system. Confirm neutrals and grounds are on separate bars and the bonding screw is out.

Take a phone photo of the inside of the panel with the dead front off, before you close it up. When you're back in six months for an add-on, that photo tells you what's where without pulling the cover in a live panel.

1. Verify torque on feeder lugs and every branch breaker
2. Confirm directory matches actual circuits, not the last guy's guess
3. Check AFCI and GFCI coverage per 210.8 and 210.12 for the branch circuits you're landing
4. Measure voltage L1-N, L2-N, L1-L2 under load after energizing
5. Tighten the dead front screws, don't leave them finger-tight

A clean subpanel install is boring to look at and that's the point. Straight conductors, labeled neutrals, breakers in logical order, and a directory the next electrician can actually read.