Field guide: installing a subpanel, code citations (edition 4)

Plan the feeder before you open a knockout

Subpanel jobs go sideways when the feeder is an afterthought. Size the feeder to the calculated load per NEC 220, not to the panel's bus rating. A 100A subpanel fed with #4 copper THHN at 75°C terminations is fine for most detached garages and finished basements, but verify the termination rating on both breakers per NEC 110.14(C).

Voltage drop matters more than code minimums on long runs. NEC 210.19(A) Informational Note 4 recommends 3% on branch circuits and 5% total. On a 120 foot run to a workshop, #4 copper at 80A continuous gets you close to the edge. Bump to #2 if the customer plans to add a welder or EV charger later.

Measure the actual run with a wheel, not the blueprint. Framers move walls, and that extra 15 feet turns a #4 into a #2 every time.

Grounding and bonding, the part everyone fails inspection on

Four wires to a subpanel. Always. Two hots, a neutral, and a separate equipment grounding conductor. The neutral bar and ground bar must be isolated at the subpanel per NEC 250.24(A)(5) and 408.40. Remove the main bonding jumper or green bonding screw if the panel came with one installed from the factory.

For a detached structure, NEC 250.32(B)(1) requires the EGC to run with the feeder. No more separate grounding electrode at the sub without an EGC, that rule changed in the 2008 cycle and still trips up guys who learned the trade before then. You still need a grounding electrode system at the detached building per 250.32(A), typically two ground rods 6 feet apart bonded to the ground bar.

• Neutral bar: floating, isolated from the enclosure
• Ground bar: bonded to the enclosure
• EGC from main panel lands on the ground bar
• Detached building: add grounding electrodes bonded to ground bar
• Remove the factory bonding screw or strap

Feeder overcurrent protection and tap rules

The overcurrent device at the supply end protects the feeder. Match the breaker to the conductor ampacity per NEC 240.4, with the small conductor rules in 240.4(D) and the next-size-up allowance in 240.4(B). A 90A feeder on #3 copper is legal; a 100A breaker on #4 copper is legal when the load calc supports it and terminations are rated 75°C.

Tap conductors under NEC 240.21(B) come up on remodels when you are pulling from an existing feeder. The 10 foot and 25 foot tap rules have specific ampacity requirements, and the tap must terminate in a single overcurrent device. Do not improvise here, read the section before you cut anything.

Working space, mounting, and accessibility

NEC 110.26 is non-negotiable and inspectors love to flag it. 30 inches wide, 36 inches deep, 6.5 feet high, clear in front of the panel. No water heaters, no shelving, no furnace filters hanging in the zone. The 30 inch width is the minimum dimension of the working space, not the width of the panel.

Mounting height per NEC 240.24(A) puts the highest breaker handle at no more than 6 feet 7 inches above the floor or platform. In a garage or basement, mount the panel so the main breaker sits around 5 to 5.5 feet up. That keeps every handle reachable and leaves room for future circuits without cramping the bottom of the enclosure.

• 30" wide x 36" deep x 6.5' tall clear working space
• Highest breaker handle under 6'7" per 240.24(A)
• Dedicated equipment space above the panel per 110.26(E)
• Illumination required at the panel per 110.26(D)

Conductor fill, wire management, and terminations

Conduit fill catches people on compact subpanel jobs. Four current-carrying conductors in a raceway trigger the 80% adjustment factor in NEC 310.15(C)(1), and the neutral only counts as current-carrying on nonlinear loads per 310.15(E). A 4-wire feeder to a residential subpanel usually has three current-carrying conductors, so no derate applies.

Terminate on the correct lug. Aluminum feeders need antioxidant compound and a torque wrench set to the manufacturer's spec. NEC 110.14(D) now requires torquing per listing instructions, and inspectors in several jurisdictions are asking to see the torque screwdriver on site. Buy one, keep it in the truck.

Label the feeder breaker at the main panel with the subpanel location and the date. Future you, or the next electrician, will thank you.

Circuit directory, AFCI, and GFCI at the subpanel

NEC 408.4(A) requires a legible, specific circuit directory. "Lights" is not specific. "Kitchen south wall receptacles" is specific. Fill it out with a fine-tip marker before you leave, not on the tailgate after.

AFCI and GFCI requirements follow the branch circuit, not the panel. A subpanel in a finished basement feeds bedrooms, which need AFCI per 210.12(A), and unfinished basement receptacles, which need GFCI per 210.8(A)(5). Dual-function breakers solve most of this, but verify the panel is listed for them before you load it up.