Field guide: installing a subpanel, during the job (edition 3)

Set the Panel Before You Pull Conductors

Mount the can plumb and at a height that gives you working space per NEC 110.26. Thirty inches wide, three feet deep, six and a half feet headroom. If you cheat the location to dodge a stud, you will pay for it the rest of the job. Verify the panel is listed as suitable for use as service equipment only if you are bonding it as such, otherwise treat it as a subpanel from the jump.

Knockouts get punched before any wire enters the room. Use a Greenlee slug buster, not a hole saw. A clean knockout takes a connector that actually seals. Ragged metal cuts THHN insulation when you feed it, and you will not see the nick until the megger tells you about it.

Confirm the bonding screw or strap is removed. This is the single most common subpanel failure on inspection. The neutral bar floats, the ground bar bonds to the can. If the panel shipped with the green screw installed, back it out and bag it to the door.

Feeder Sizing and the 83 Percent Rule

For a dwelling unit feeder supplying the entire load, NEC 310.12 lets you size conductors at 83 percent of the service rating. A 100 amp subpanel feeding a detached garage as the only feeder still has to carry the calculated load per Article 220, but the conductor ampacity table you land on is the relaxed one. Read the article, do not assume.

For non-dwelling feeders or subpanels not carrying the full load, go back to NEC 310.16 at the 75 degree column for terminations rated 75C, which covers most modern breakers and lugs. Do not size off the 90 degree column unless every termination in the path is rated for it. They are not.

• 100A feeder, dwelling, full load: 4 AWG copper or 2 AWG aluminum per 310.12
• 100A feeder, non-dwelling or partial: 3 AWG copper or 1 AWG aluminum per 310.16
• Always run a separate equipment grounding conductor sized per 250.122
• Neutral sized for the maximum unbalanced load per 220.61, never smaller than the EGC

Four Wire Feeders, Always

If the subpanel is in a separate building or anywhere downstream of the service disconnect, you are running four wires. Two hots, an insulated neutral, and an equipment grounding conductor. The 2008 cycle killed the three wire feeder for separate structures, and the 2011 cycle put the nail in. NEC 250.32(B) is the section to cite when the GC asks why you are pulling an extra conductor.

Old timer tip: if the existing feeder to a detached shop is three wire, you do not get to extend it. New feeder, new ground rods, new bonding. The grandfather clause covers existing, not your add.

Ground rods or a concrete encased electrode at the separate structure are required by 250.32(A). Two rods six feet apart unless you can prove 25 ohms or less with a single rod, which nobody actually tests for, so drive two and move on.

Neutral and Ground Separation

Inside the subpanel, the neutral bus and the ground bus must be electrically isolated. The neutral bar sits on its insulating standoffs. The ground bar bonds directly to the enclosure, usually with a green screw or a factory strap. Add a supplemental ground bar if the factory bar is full, screwed direct to the can with the paint scraped where it lands.

Every neutral lands on the neutral bar, one conductor per hole. Every equipment grounding conductor lands on the ground bar. Mixing them creates parallel paths on the EGC, which means current on metal that should be at zero potential. That is how you energize a dryer chassis.

Breaker Selection and AFCI/GFCI

Match the breaker to the panel. A Square D breaker in a Cutler Hammer panel is not listed for that use, even if it physically clicks in. NEC 110.3(B) requires installation per the listing, and listings are panel specific with a short list of cross compatible exceptions documented in the UL classified breaker guide.

For dwelling branch circuits, 210.12 covers AFCI requirements and 210.8 covers GFCI. The 2023 cycle expanded GFCI to cover basically every 125V through 250V receptacle in a dwelling up to 250 amps in the listed locations. If the subpanel feeds a kitchen, laundry, garage, basement, or outdoor area, plan on dual function breakers and budget the cost up front.

• Kitchen small appliance circuits: GFCI per 210.8(A)(6)
• Laundry: GFCI per 210.8(A)(10)
• Garage and accessory buildings: GFCI per 210.8(A)(2)
• Bedrooms, living rooms, hallways: AFCI per 210.12(A)
• Dishwasher and ranges in dwellings: GFCI per 210.8(D) and (F)

Torque, Label, Close It Up

Torque every lug and every breaker terminal to the manufacturer spec stamped inside the can or printed on the breaker. NEC 110.14(D) made calibrated torque tools mandatory in the 2017 cycle. A click wrench or a screwdriver style torque driver is not optional anymore, and inspectors are checking. Mark each terminal with a paint pen as you go so you do not double torque or miss one.

Field tip: torque cold, then come back after the panel has carried load for a heating cycle and re-torque. Aluminum especially will creep. The NEC does not require it, but warranty calls do.

Fill out the directory legibly with circuit numbers matched to room and device, not just "lights." Label the feeder breaker at the upstream panel with the destination per 408.4(A). Close the dead front, install all unused knockout seals, and verify the cover screws are all driven before you call for inspection. Missing a single filler plate is a failed inspection and a second trip.