Field guide: installing a subpanel, code citations (edition 3)

Plan the load and the feeder before you pull a permit

Subpanel work fails inspection most often because the feeder was sized on vibes. Start with a load calc per NEC 220, Part III or IV. Write it down. If the subpanel feeds a detached structure, you are also bound by 225.30 through 225.39, which limits you to one feeder per building absent specific exceptions.

Pick the feeder conductor using 310.16 at the 75 degrees C column for most modern lugs, then verify the terminal rating on both the supply breaker and the subpanel main lug kit. Apply 310.15(C)(1) adjustment if you are running three or more current-carrying conductors in a raceway, and do not forget 215.2(A)(1) minimum feeder ampacity after continuous load multiplication.

Voltage drop is not a code requirement but 215.2(A)(1) Informational Note No. 2 recommends 3 percent on feeders, 5 percent combined. For a 100 A feeder at 150 ft on copper, you are already looking at #2 or #1 Cu to keep drop reasonable.

Separate neutrals and grounds, every time

This is the single most common red-tag on subpanel work. Per 250.24(A)(5) and 408.40, the grounded (neutral) conductor and the equipment grounding conductor must be isolated at any panel downstream of the service disconnect. Remove the main bonding jumper or bonding screw in the subpanel. Full stop.

Run a four-wire feeder: two hots, an insulated neutral, and a separate EGC sized per 250.122 based on the upstream overcurrent device. The neutral bar floats on its insulated standoffs. The ground bar bonds to the enclosure. If your subpanel only shipped with one bar, order an accessory ground bar kit from the manufacturer and land it on the can.

Old hand tip: before you energize, ohm from the neutral bar to the enclosure. You should read open. If you read continuity, you missed a bonding screw or a landed EGC is touching the neutral lug.

Detached structures and the 2020 cycle shift

If the subpanel lives in a detached garage, shop, or ADU, 250.32(B)(1) requires a separate EGC run with the feeder and prohibits re-bonding neutral to ground at the second structure. The old three-wire feeder allowance is gone for new installs. Drive ground rods at the detached building per 250.32(A) and bond them to the EGC bus, not the neutral.

Grounding electrode conductor sizing comes from 250.66 based on the largest ungrounded feeder conductor. For a pair of 5/8 inch rods in parallel, #6 Cu is the practical ceiling per 250.66(A) since rods are the sole electrode.

• Two rods, 6 ft apart minimum, per 250.53(A)(3) unless you can prove 25 ohms with one.
• Acorn clamps listed for direct burial, 250.70.
• GEC unspliced or irreversibly spliced, 250.64(C).
• Disconnect at the structure, 225.31, readily accessible per 225.32.

Working space, mounting, and physical protection

110.26(A) still catches people. You need 36 inches deep, 30 inches wide or the width of the equipment (whichever is greater), and 6.5 ft of headroom in front of the panel. No water heaters, shelving, or studs framing the dead-front shut.

Mount the subpanel so the highest breaker handle sits no more than 6 ft 7 in off the finished floor, per 240.24(A). Support the feeder raceway within 3 ft of the enclosure, 358.30(A) for EMT. Where the feeder passes through a framed wall or fire-rated assembly, firestop it, 300.21.

NM cable feeding a subpanel is legal but watch the derate. 334.80 requires you to use the 60 degrees C column for NM ampacity, which tanks your available current fast. Most pros run SER or individual THHN in conduit for anything 60 A and up.

Overcurrent protection and breaker selection

Feeder OCPD sits at the supply end, sized to protect the conductor per 240.4. The subpanel main lug kit does not provide overcurrent; the upstream breaker does. If the subpanel is at a detached structure, you also need a disconnecting means at or near the building, 225.31, either a main breaker panel or a fused disconnect ahead of the MLO panel.

Check series rating and AIC. Modern residential service equipment is often 22 kAIC at the main and 10 kAIC at branch breakers, relying on a listed series combination per 240.86(B). Mixing manufacturers voids that listing. Use the breaker brand the panel is listed for, every time.

If you are adding a subpanel to feed a kitchen remodel or a bathroom, remember 210.8 and 210.52 still apply to the new branch circuits, not just the feeder. GFCI and AFCI protection travels with the load, not the panel.

Final checks before you call for inspection

Torque every lug to the label. 110.14(D) has made calibrated torque tools a de facto requirement since the 2017 cycle. Re-check after 24 hours on aluminum feeders because they relax.

Label the subpanel per 408.4(A) with each circuit's clear, specific use, and per 408.4(B) identify the source of the feeder. Add arc-flash marking where 110.16 applies. Close the knockouts you did not use, 110.12(A).

1. Verify main bonding jumper removed.
2. Neutral bar isolated, ground bar bonded.
3. EGC sized to 250.122, landed on ground bar.
4. Feeder OCPD matches conductor ampacity after adjustments.
5. Working clearances clear, handle height under 6 ft 7 in.
6. Directory filled out, torque verified, covers on.