Field guide: installing a subpanel, for journeymen (edition 2)

Sizing the feeder before you cut a single wire

Subpanel work goes sideways when the feeder is undersized for the load you actually end up putting on it. Run a load calc per NEC 220 Part III before you pick conductor size. A 100A subpanel feeding a detached shop with a welder, dust collector, and lighting circuits eats more than the homeowner's "just a few outlets" pitch suggests.

Pick your conductor from NEC Table 310.16, using the 75 degree C column for terminations on most modern breakers and lugs (check the listing on both ends, the lower rating wins per 110.14(C)). Voltage drop matters on long runs. For a 100A feeder pushing 150 feet to a detached structure, 1/0 copper or 2/0 aluminum keeps you under 3 percent.

If the customer is talking about adding a hot tub or EV charger "someday," size the feeder for someday now. Pulling 4/0 aluminum once is cheaper than pulling it twice.

Grounding and bonding, the part everyone gets wrong

This is where inspectors fail subpanels more than anywhere else. In a subpanel, neutrals and grounds must be separated. The neutral bar floats, isolated from the enclosure. The equipment grounding conductor lands on its own bar bonded to the can. The bonding screw or strap that came in the panel from the factory comes out, full stop. NEC 250.24(A)(5) and 408.40 are the citations to know cold.

You need four wires to a subpanel: two hots, a neutral, and a separate equipment grounding conductor. Three-wire feeders to subpanels in the same building have been off the table since the 2008 NEC. If you're working on an existing three-wire feed, it's getting upgraded.

• Remove the main bonding jumper or screw in the subpanel
• Land neutrals on the isolated neutral bar only
• Land EGCs on the bonded ground bar
• Size the EGC per NEC Table 250.122 based on the upstream OCPD

Detached structure rules

Feeding a subpanel in a separate building changes the rules. NEC 250.32 governs this. You need a grounding electrode system at the detached structure, typically two ground rods 6 feet apart, or a Ufer if the slab is being poured. The EGC from the main building bonds to that local electrode system, but the neutral stays isolated from ground at the subpanel, just like an interior subpanel.

You also need a disconnecting means at the detached structure per NEC 225.31 and 225.32, located either outside or nearest the point of entry. The subpanel's main breaker can serve as that disconnect if it's rated as service equipment or you order it with a main breaker for that purpose.

Drive your ground rods before you backfill the trench. Coming back to drive rods through fresh sod after the inspector flags it is a bad afternoon.

Working space, mounting, and physical install

NEC 110.26 working clearance is non-negotiable: 36 inches deep, 30 inches wide, 6.5 feet high, clear in front of the panel. No water heaters, no shelving, no stored junk. If the panel ends up in a finished space later, you want that clearance documented in the rough-in photos.

Mount the panel so the highest breaker handle is no more than 6 feet 7 inches off the finished floor per 240.24(A). On masonry walls, use a plywood backer board, paint it if the AHJ requires it for fire rating. Torque every lug to the manufacturer's spec stamped inside the can. NEC 110.14(D) made torque values mandatory in the 2017 cycle, and inspectors are checking with calibrated wrenches now.

1. Verify panel rating matches feeder ampacity and available fault current
2. Mount level and plumb, fasteners into structure, not just drywall
3. Knockouts only where you'll use them, seal unused openings
4. Torque all connections to spec, mark with a torque seal stripe

Conductor management and circuit layout

Strip your feeder conductors only as much as needed to land in the lugs. No exposed copper above the lug. Bend radius matters: NEC 312.6 sets minimums based on conductor size, and crowding 4/0 into a flush-mount can without respecting those bends will cost you. Make your bends before you land the conductors, dress them flat against the can.

Balance your circuits across both phases as you populate breakers. Heavy 240V loads like dryers, ranges, and EV chargers should land near the top, close to the main lugs, to minimize bus heating. Label every breaker with the actual circuit, not "bedroom" when it's really "bedroom outlets and hall light." NEC 408.4(A) requires legible, specific identification.

Final checks before you call for inspection

Megger the feeder if you have any doubt about the run, especially on direct-burial or long conduit pulls where pulling tension might have nicked insulation. Verify continuity on the EGC end to end. Check that AFCI and GFCI requirements per NEC 210.8 and 210.12 are met for every applicable circuit, including the newer dwelling-unit expansions.

Walk the install with the same eyes the inspector will use. Cover plate on, deadfront secured, all knockouts filled, working clearance clear, panel directory filled out in pen, not pencil. If the AHJ has local amendments (Chicago conduit rules, California seismic bracing, Florida wind ratings), confirm them before the inspection, not during.

• Feeder conductors and OCPD coordinated and properly sized
• Neutral isolated, EGC bonded, grounding electrode system in place if detached
• All terminations torqued and marked
• Working clearance maintained and documented
• Panel directory complete and accurate