Field guide: installing a subpanel, code citations (edition 2)

Plan the subpanel before you pull a single wire

Size the feeder to the calculated load, not the panel rating. Run Article 220 calcs for the subpanel's branch circuits, then add 25% for the largest continuous load per NEC 215.2(A)(1). Round up to the next standard OCPD size allowed by 240.4(B) if you land between sizes.

Confirm the physical location meets NEC 240.24: readily accessible, not in bathrooms, not in clothes closets, and with the working space from 110.26 (30 inches wide, 36 inches deep, 6.5 feet high). A subpanel crammed behind a water heater fails inspection every time.

Before cutting drywall, verify the feeder path and whether you're feeding from the main service or another subpanel. Chained subpanels are legal but the grounding rules shift fast.

Feeder conductors and OCPD sizing

Pick conductors from Table 310.16 at the 75C column for most modern terminations, unless the equipment is listed for 60C only (common on lugs rated under 100A per 110.14(C)(1)(a)). A 100A subpanel feeder in copper is typically #3 THHN at 75C, or #1 in aluminum. Derate for ambient and conduit fill before you commit.

The feeder OCPD lives at the supply end, per 240.21. You cannot protect a 100A panel with a 200A breaker unless the panel itself is rated for series or the feeder taps qualify under 240.21(B). Match the breaker to the conductor ampacity and the panel's maximum.

• 100A subpanel: #3 Cu or #1 Al THHN, 100A breaker at the source
• 60A subpanel (detached shop, small addition): #6 Cu or #4 Al, 60A breaker
• Always verify lug ratings stamped in the panel, not the cover sheet

The four-wire rule and separated neutrals

This is where retrofits go sideways. Since the 2008 NEC, feeders to subpanels in the same structure must be four-wire: two hots, an insulated neutral, and a separate equipment grounding conductor (250.32 and 408.40). The neutral bar in the subpanel must be isolated from the enclosure. The ground bar bonds to the enclosure.

Pull the bonding screw or strap out of the subpanel. Leave it in and you create parallel neutral paths on the EGC, which will light up on a clamp meter and fail inspection under 250.24(A)(5).

If you're replacing a 1990s three-wire feeder to a detached garage, you now need a four-wire feeder or you need to drive ground rods and treat it as a separately derived system under the old rules... which no longer applies to new installs. Pull the fourth wire.

Grounding electrodes at detached structures

A subpanel in a detached building needs its own grounding electrode system per 250.32(A). Two ground rods 6 feet apart, or a Ufer if accessible, bonded to the ground bar with a #6 Cu GEC sized from Table 250.66. The neutral still stays isolated because the feeder carries an EGC.

The only exception: a single branch circuit to a detached structure with an EGC and no other metallic paths, per 250.32(A) Exception. Anything with a panel needs the electrode system.

• Two 8-foot rods, 6 feet apart minimum, driven flush or below grade
• #6 Cu GEC, irreversible crimp or listed acorn clamp at each rod
• Bond to the EGC bar in the subpanel, never the isolated neutral

Breaker selection, AFCI, GFCI, and listing

Use breakers listed for the panel. UL classified breakers are a gray area with some inspectors and most manufacturers void their listing if you mix brands. 110.3(B) is the hook they'll use.

Branch circuit protection follows the usual rules regardless of where the panel sits. AFCI per 210.12 for most dwelling living spaces, GFCI per 210.8 for kitchens, baths, garages, outdoors, unfinished basements, laundry, and within 6 feet of a sink. A subpanel in a garage feeding garage receptacles still needs GFCI on those circuits.

Dual-function breakers cost more but save headaches on island receptacles and laundry circuits where you'd otherwise need an AFCI breaker plus a GFCI device downstream.

Labeling, torque, and the final walk

Torque every lug and breaker to the manufacturer's spec printed inside the panel door, required by 110.14(D) since the 2017 cycle. Use a calibrated torque screwdriver, not feel. Inspectors increasingly ask to see the tool.

Fill out the circuit directory legibly per 408.4(A). "Kitchen" is not enough, "Kitchen SA 1, south wall" is. Mark the subpanel with the feeder source location per 408.4(B) so the next electrician knows where the upstream disconnect lives.

1. Verify neutral isolated, EGC bonded, bonding screw removed
2. Torque all terminations, mark with a paint pen
3. Megger or at minimum insulation test the feeder before energizing
4. Check voltage L-L and L-N, then verify balanced loading across both legs
5. Update the main panel directory to reference the new subpanel

The subpanel itself is straightforward. The mistakes that cost money are the bonding screw, the shared neutral bar, and a feeder sized off the breaker instead of the load. Get those three right and the inspection is a formality.