Field guide: installing a subpanel, code-compliant approach (edition 5)

Plan the load and feeder before you touch conduit

Start with a load calculation per NEC Article 220. Size the subpanel feeder to the calculated load, not to the main panel ampacity. A 100A subpanel fed from a 200A main still needs its own 100A overcurrent protection at the source, per NEC 408.36 and 215.3.

Pick your feeder conductors using Table 310.16 at the 75 degree C column for most lugs. Remember 310.15(B) adjustments if you are bundling, and voltage drop if the run is long. For a 100A subpanel at roughly 100 feet, jump to 1 AWG copper or 1/0 aluminum to keep drop under 3 percent on a 240V feed.

• Calculate load per 220.40, not guesswork.
• Verify ampacity at the correct terminal rating, 60 or 75 degree C.
• Check voltage drop on runs over 75 feet.
• Confirm the main panel has room for a two-pole breaker of the correct size.

Four wires, always, for a separate structure or interior sub

Since the 2008 NEC cycle, interior subpanels require four-wire feeders: two hots, one insulated neutral, and a separate equipment grounding conductor. The neutral and ground must be isolated at the subpanel. This is 250.32(B) for separate structures and 408.40 for panelboards in general.

Pull the bonding screw or strap out of the subpanel. Leave it in the main only. If the neutral bar is bonded to the enclosure at the sub, you create parallel paths on the equipment grounding conductor, which heats ground wires, nuisance-trips GFCIs, and violates 250.24(A)(5).

Field tip: the bonding screw usually lives in a small bag taped inside the panel door. Pull it before you set the can. If it is already threaded in, back it out and tape it to the inside of the cover so the next tech knows it was intentional.

Grounding electrode at a detached structure

If the subpanel feeds a separate building, such as a detached garage or shop, you need a grounding electrode system at that structure per 250.32(A). One ground rod is rarely enough. You need either two rods spaced 6 feet apart, or a single rod with a tested resistance of 25 ohms or less per 250.53(A)(2). Most inspectors will not accept the resistance test in the field, so drive two.

The grounding electrode conductor bonds to the subpanel enclosure and the isolated ground bar, not the neutral bar. Size the GEC per Table 250.66 based on the largest ungrounded feeder conductor.

1. Drive two 8-foot rods, minimum 6 feet apart.
2. Bond with a 6 AWG copper GEC, continuous or irreversibly spliced.
3. Land on the equipment ground bus in the subpanel.
4. Do not re-bond the neutral.

Feeder routing, raceway, and conductor protection

Underground feeders in PVC need a minimum 18 inch cover per Table 300.5 when installed under a 20A or less residential branch circuit protection, but feeders typically follow the 24 inch rule for direct burial or the raceway-specific depths in that table. Verify the column you are reading.

For interior runs, SER cable is common for dwelling subpanels. Where SER passes through thermal insulation in walls or ceilings, you must derate per 310.15(B)(3)(c), which dropped into 334.80 for NM and got messy for SER too. When in doubt, pull THHN in EMT and move on.

Field tip: if the AHJ is particular about SER in insulation, bid the job with EMT and THHN from the jump. You lose an hour on the bend, you save three on the re-inspection.

Breakers, lugs, and panel labeling

Torque every lug to spec. Manufacturer torque values are now an explicit code requirement under 110.14(D). Buy a calibrated torque screwdriver and a 1/4 inch drive torque wrench. Hand-tight plus a grunt is not compliance, and loose lugs are the number one cause of panel burn-ups.

Label the subpanel per 408.4(A) with a directory identifying each circuit's purpose. If the feeder originates in a different building or a different room, mark the location of the disconnecting means per 408.4(B). Use a label, not a Sharpie scribble, if the inspector is strict.

• Torque feeder lugs, branch breakers, and neutral/ground terminations.
• Verify breaker compatibility with the panel, listed combinations only per 110.3(B).
• Install AFCI and GFCI per 210.8 and 210.12 on the branch side, not at the feeder.
• Apply the arc flash and available fault current labels where required by 110.16 and 110.24.

Final checks before you energize

Meg the feeder conductors before you land them if the run was pulled through wet conduit or sat on the truck for a week. Ring out the neutral to confirm it is not bonded downstream. Verify breaker positions are off, then energize from the main, then close the subpanel main, then branch breakers one at a time.

Document the load calc, feeder size, and GEC method in the permit package. If the inspector asks why you used 1/0 aluminum on a 100A feeder, you want the voltage drop math on paper, not in your head.