Field guide: installing a subpanel, safety checklist (edition 1)

Plan the feeder before you cut a knockout

Load calc first, metal second. Run Article 220 on the subpanel's expected load, add continuous loads at 125%, and size the feeder conductors per 215.2 with ampacity from 310.16. If the subpanel is in a detached structure, you are under 225.30 and 225.31, not just 215, so the disconnect rules change.

Pick the feeder OCPD before you pick the wire. A 100A subpanel on a 60A feeder is legal and common; a 60A panel on a 100A feeder is not. The OCPD at the supply end protects the feeder, and the panel rating must be at least equal to the feeder OCPD per 408.30 and 408.36.

Confirm the path. Measure the actual run including vertical drops, derate for ambient and conduit fill per 310.15(B) and (C), and verify the voltage drop target before you commit. A 3% feeder VD budget saves callbacks on long runs to shops and barns.

Grounding and bonding, get this right or redo it

Same structure: four wires, isolated neutral, bonding screw removed. The subpanel neutral bar must float, and the equipment grounding conductor lands on its own bar bonded to the enclosure per 250.24(A)(5) and 408.40. If you leave the main bonding jumper in, you create parallel neutral paths on the EGC, and every piece of metal in that run becomes a current-carrying conductor.

Detached structure: run an EGC with the feeder and drive grounding electrodes at the second building per 250.32(B)(1). The 2008 and later code killed the old three-wire feeder to outbuildings except for existing installations meeting 250.32(B)(2). Do not reuse an old three-wire feeder on a new panel.

Field tip: before you energize, ohm from the neutral bar to the enclosure with the feeder disconnected. Any continuity means the bonding screw or a bonding strap is still in. Find it before the POCO does.

Working space, mounting, and clearances

110.26 is not negotiable. 36 inches of depth in front, 30 inches of width or the width of the equipment (whichever is greater), 6 foot 6 inch headroom, and a clear path out. No shelving, no water heater, no laundry sink encroaching. If the homeowner built a closet around the old main, you are moving it or rebuilding the space before inspection.

Mount the panel plumb to a solid surface, not sheetrock alone. In damp or wet locations use a Type 3R enclosure and drip-loop every conduit entry. Indoors in a garage, watch for 312.2 requirements if the wall is damp masonry, use spacers to keep a 1/4 inch air gap.

• Verify 110.26(A) working clearances before setting the can.
• Confirm panel is listed for the location (dry, damp, wet) per 110.3(B).
• Label the panel with feeder source, max OCPD, and available fault current per 110.24.
• Torque every lug to the listed spec, log it if the AHJ requires it.

Pre-energization safety checklist

Walk this before you pull the main. Missing one of these is how people get hurt or fail inspection.

1. Feeder conductors sized per 215.2 and load calc, OCPD at source verified.
2. Neutral bar isolated, bonding screw removed and bagged inside the can.
3. EGC landed on grounding bar, grounding bar bonded to enclosure.
4. For detached structure: grounding electrode system installed per 250.50 and 250.52, bonded per 250.32.
5. All knockouts filled or closed, no open bushings, AFCI and GFCI breakers where required by 210.8 and 210.12.
6. Working clearance per 110.26 verified and clear.
7. Circuit directory filled out per 408.4, legible, specific to room and load.
8. Torque all line, load, neutral, and ground terminations to spec.
9. Arc-flash and shock hazard labels applied per 110.16 where required.
10. Lockout/tagout in place, voltage tested dead before touching any bus.

Energizing and first-on checks

Kill the feeder at the source, verify dead, then land feeder conductors on the subpanel main lugs or main breaker. Torque, photograph, and move on. Close the dead front before you energize. Stand to the side of the panel, off the hinge side, when you flip the feeder breaker.

Meter each bus leg to ground and to neutral. You should see nominal line-to-ground and zero or near-zero neutral-to-ground. If neutral-to-ground reads line voltage, kill it and find your missing or swapped conductor. Check breaker seating and bus temperature after 10 to 15 minutes under load.

Field tip: put a clamp meter on the EGC at the subpanel with everything energized and loads running. Any current on the EGC means you have a neutral-to-ground bond you missed, or a neutral fault downstream. It should read zero.

Documentation and handoff

Finish the directory. Room, load, and circuit number, not "lights" or "misc." Inspectors read this, so do the next electrician and the homeowner at 2 a.m. during an outage. 408.4(A) requires it, and the time you spend here saves an hour on every future service call.

Leave the load calc, torque log, and a photo of the interior with the dead front off in the job folder. If available fault current was calculated, mark it on the panel per 110.24(A). For commercial, update the single-line diagram and the arc-flash study if the feeder change affected incident energy.

Walk the customer through the main disconnect location and how to kill the subpanel from the source. Two minutes of handoff prevents a panicked phone call during the first fault. Then close out the permit and move on to the next one.