Field guide: installing a subpanel, safety checklist (edition 4)

Before the panel comes off the truck

A subpanel job lives or dies on what you settle before you cut a single knockout. Load calc, feeder sizing, conductor fill, and grounding scheme all need to be locked in on paper. If the homeowner or GC changes scope mid-install, stop and redo the math rather than eyeballing a bigger breaker.

Pull the existing service rating and check available fault current at the main. NEC 110.24 requires the service equipment to be marked with the maximum available fault current, and your new subpanel's interrupting rating must meet or exceed what shows up at its terminals after feeder impedance. A 10 kAIC panel on a short feeder from a 22 kAIC service is a failed inspection waiting to happen.

Confirm the feeder path, bend radius, and support spacing before ordering conduit. Measuring twice here beats crawling an attic with a 2 inch EMT that won't make the offset.

Sizing the feeder and OCPD

Feeder ampacity follows NEC 215.2 and Table 310.16. For a dwelling, you can often apply NEC 310.12 for the service and feeder serving the main load, but a subpanel feeder that doesn't carry the entire dwelling load does not qualify. Run the calc per Article 220 and size accordingly.

Neutral sizing under NEC 220.61 allows reduction for the unbalanced load, but don't shrink it below the equipment grounding conductor requirement for the enclosure bonding path on the line side. The EGC itself is sized from NEC Table 250.122 based on the feeder OCPD, not the conductor.

• Verify conductor ampacity at the terminal temperature rating, usually 75 degrees C for anything over 100 A (NEC 110.14(C)).
• Derate for ambient and conduit fill per NEC 310.15(B) and (C) before locking in the wire size.
• Confirm the OCPD at the supply end protects the feeder, not the panel bus rating alone.

Grounding and bonding, the part that fails inspections

This is where most subpanel jobs get red-tagged. A subpanel in a separate structure or downstream of the service disconnect must have the neutral isolated from the equipment ground. NEC 250.32 and 408.40 are clear. Remove the main bonding jumper, pull the bonding screw, and make sure the neutral bar floats.

Run a dedicated equipment grounding conductor with the feeder. For a separate building, NEC 250.32(B) requires the EGC plus a grounding electrode system at the second structure, bonded to the panel's ground bar, not the neutral.

Rule of thumb from a 30 year journeyman: if you can measure continuity between the neutral bar and the enclosure with the main off, you've got a bond that shouldn't be there. Find it before the inspector does.

Physical install and working space

NEC 110.26 working space is non-negotiable. 30 inches wide, 36 inches deep for most residential installs at 150 V to ground or less, and 6.5 feet of headroom. Panels buried behind shelving, water heaters, or a washer get flagged every time. Measure from the face of the enclosure, not the wall.

Mount the panel plumb and at a height where the highest breaker handle is no more than 6 feet 7 inches above the finished floor (NEC 240.24(A)). In unfinished basements, think about where drywall and flooring will land later if the space gets finished.

1. Dry-fit the panel with feeder and branch conduits stubbed before final mounting.
2. Torque all lugs to the manufacturer's spec using a calibrated wrench, NEC 110.14(D) requires it.
3. Label every circuit legibly per NEC 408.4, including spares and spaces.

Safety checklist before energizing

Walk the install once with fresh eyes. Lockout-tagout on the upstream OCPD, verify zero energy at the feeder lugs with a meter you just tested on a known source, and only then start landing conductors. The two minutes it takes to re-test has saved more than one apprentice.

After terminations, before closing the cover, run through the short list below. If any item is a no, stop and fix it.

• Neutral bar isolated from enclosure, bonding screw removed.
• EGC landed on the ground bar, ground bar bonded to the enclosure.
• Feeder conductors torqued to spec, markings visible.
• AFCI and GFCI protection per NEC 210.8 and 210.12 applied to the right branch circuits.
• Panel directory filled out, not left blank.
• Cover screws all present, no missing knockouts, filler plates in every unused opening (NEC 408.7).

If the breaker kicks the moment you energize, don't just reset it. Kill power, meg the feeder, and find the fault. Resetting into a short is how panels get destroyed and people get hurt.

Final walkthrough with the inspector

Have the load calc, torque records, and AFCI/GFCI test documentation ready. Inspectors appreciate an electrician who hands them paperwork instead of making them ask. If your AHJ has local amendments to the NEC, know them cold before the appointment.

Test every GFCI and AFCI with the built-in test button and an external tester. Breakers ship defective often enough that catching one bad device before the inspector does is worth the ten minutes. Close it up, sign the sticker, and move on.