Field guide: installing a subpanel, safety checklist (edition 3)

Before you pull a permit

A subpanel install lives or dies on the prep. Walk the job before quoting. Verify service capacity with a load calc per NEC 220, confirm the main bonding jumper location, and figure out how you are getting the feeder from point A to point B without opening six walls you did not bid.

Check the existing main panel for available breaker space and interrupting rating. If the service is 10kAIC and you are feeding a subpanel near a pad-mount transformer, you may need to recalculate available fault current per NEC 110.24. Document it. The AHJ will ask.

Confirm the subpanel location meets NEC 110.26 working space: 36 inches deep, 30 inches wide, 6.5 feet high, clear. No water heaters, no shelving, no stored paint cans above it.

Sizing the feeder and OCPD

Size the feeder conductors to the calculated load, not the panel rating. A 100A subpanel fed by a 60A breaker is legal if the load calc supports it. Apply NEC 215.2 for minimum ampacity and 310.16 for ampacity tables. Do not forget terminal temperature ratings per 110.14(C): most breakers under 100A are 60/75C rated, so you are pulling from the 75C column in practice.

For residential dwelling feeders, 310.12 allows the 83 percent rule where the feeder carries the entire load of the dwelling. That rule does not apply to subpanels fed from a main panel in the same structure. Size it the long way.

• Grounded (neutral) conductor sized per 220.61 after applying demand factors.
• Equipment grounding conductor per 250.122, sized to the upstream OCPD.
• Voltage drop check: aim for under 3 percent on the feeder, under 5 percent total.
• Conduit fill per Chapter 9, Table 1. Do not eyeball it.

Four wires, always

Since the 2008 NEC, subpanels in separate structures and all subpanels in the same structure require a four-wire feeder: two ungrounded, one grounded, one equipment grounding. The neutral bar is isolated from the enclosure. The ground bar is bonded to the enclosure. No bonding screw, no bonding strap between them.

This is where most failed inspections happen. Pull the bonding screw. Verify with a continuity tester between the neutral bar and the can. You should read open. If you read continuity, you have a parallel neutral path through the EGC, and that is a code violation and a shock hazard.

Pull the green bonding screw, tape it to the inside of the dead front with electrical tape, and label it. If someone ever converts that subpanel to a service later, the screw is right there.

Grounding electrode at detached structures

Feeding a subpanel in a detached garage, shop, or barn? NEC 250.32 requires a grounding electrode system at the separate structure. That usually means two ground rods 6 feet apart, or one rod if you can prove 25 ohms or less (you cannot, practically, so drive two).

The grounding electrode conductor connects to the ground bar in the subpanel, not the neutral. The neutral stays isolated. The EGC in the feeder carries fault current back to the main. The ground rods are for lightning and stray voltage reference, not fault clearing.

1. Drive rods per 250.53(A), minimum 8 feet, fully below grade.
2. Bond with an acorn clamp listed for direct burial.
3. GEC sized per 250.66, minimum #6 Cu to rods.
4. Protect the GEC from physical damage per 250.64(B).

Breaker install and labeling

Torque every lug. Every one. Use a calibrated torque screwdriver and match the manufacturer's spec, usually printed on the panel label or inside the breaker packaging. NEC 110.14(D) requires torque values be followed. Loose lugs cause 80 percent of the panel fires we see after the fact.

Label the subpanel directory legibly. NEC 408.4(A) requires every circuit be described with enough detail to identify it. "Lights" is not enough. "Kitchen counter receptacles east wall" is. Mark the feeder breaker in the main panel with the subpanel location per 408.4(B).

If the subpanel feeds more than one structure or the feeder runs more than 50 feet, add a permanent plaque at the main showing the subpanel location and feeder size. The next electrician will thank you. So will the fire marshal.

Pre-energize checklist

Before you close the main breaker, walk the job with this list. Do not rely on memory at the end of a 10-hour day.

• Bonding screw removed, neutral bar isolated, verified with meter.
• EGC landed on ground bar, ground bar bonded to enclosure.
• All lugs torqued to spec, witnessed or photographed.
• GEC landed at detached structure, rods driven and clamped.
• Feeder breaker sized to conductor, not panel rating.
• Working space clear, dead front on, cover screws in.
• Directory filled out, feeder labeled at main.
• AFCI/GFCI protection verified per 210.8 and 210.12 for branch circuits.

Megger the feeder before energizing if the run is long or was pulled through wet conduit. A 500V insulation resistance test catches nicked insulation before it becomes a ground fault at 2 a.m. the night after you leave.

Energize the feeder with branch breakers off. Verify voltage L-L and L-N at the subpanel bus. Then bring up branches one at a time. Log it. Sign it. Move on.