Field guide: installing a subpanel, for apprentices (edition 6)

Plan the load and location before you touch a tool

Subpanel work dies or lives at the planning stage. Confirm the calculated load per NEC 220, verify the feeder ampacity matches the panel rating, and check that the main bonding jumper is removed on the subpanel. That last one catches more apprentices than any other single mistake.

Location matters as much as sizing. NEC 110.26 gives you 36 inches of working depth, 30 inches of width, and 6.5 feet of headroom. Measure before you mount. A subpanel crammed behind a water heater or under a staircase will fail inspection and earn you a demo day.

Before you pull the permit, walk the path for the feeder. Know where you're boring, where you're strapping, and where the conductors transition from damp to dry locations. Changes cost time you don't have.

Pick the right feeder and conduit

Feeder sizing starts with the calculated load, not the panel label. A 100 amp subpanel doesn't mean 100 amp feeders if your load calc says 72 amps. Use NEC Table 310.16 for ampacity and apply terminal temperature limits per 110.14(C). Most residential lugs are rated 75 degrees C, so size from the 75 column even if you're running 90 degree insulation.

Don't forget the grounded (neutral) and equipment grounding conductors. On a subpanel in a separate structure or inside the same building, the EGC runs separately from the neutral. Four wire feeder, always, for any subpanel installed today under NEC 250.32 and 408.40.

• 60 amp subpanel: typically 6 AWG copper THHN/THWN-2, 10 AWG EGC
• 100 amp subpanel: typically 3 AWG copper or 1 AWG aluminum, 8 AWG EGC
• 125 amp subpanel: typically 2 AWG copper or 1/0 aluminum, 6 AWG EGC
• Verify every size against the load calc, voltage drop, and the OCPD rating

Voltage drop is not code, but it is reality. For runs over 100 feet, bump up a size. NEC 210.19(A) Informational Note 4 recommends 3 percent on branch circuits and 5 percent total. Your customer will feel the difference on a long garage run.

Mount, land, and separate neutrals from grounds

Square the panel to the framing and leave enough slack on the feeder to re-land conductors twice. Strip carefully, torque to the manufacturer's spec listed on the label, and document it. NEC 110.14(D) requires a calibrated torque tool. Inspectors are checking.

The cardinal rule on every subpanel: neutrals and grounds are separated. Remove the bonding screw or bonding strap that ties the neutral bar to the enclosure. Add a separate ground bar kit if the panel didn't ship with one. Every EGC lands on the ground bar, every grounded conductor lands on the isolated neutral bar.

Tip from a 22 year journeyman: "Before you close it up, take a photo of the neutral bar and ground bar with your phone. If the inspector questions it later, you've got a dated picture showing you did it right."

One conductor per terminal unless the lug is listed for multiples. Double lugged neutrals on a single hole is a classic rework item and a violation of 408.41.

Grounding and bonding at the subpanel

For a subpanel in the same structure as the service, you run an EGC with the feeder and that is your ground path. No ground rods required at the subpanel itself. Neutral stays isolated from the enclosure. Simple.

For a subpanel in a separate structure, NEC 250.32(B)(1) requires a grounding electrode at that structure, bonded to the EGC and the neutral bar (which remains isolated from the enclosure). The grounding electrode system follows 250.50: ground rods, Ufer, water pipe, whatever is present and qualifying. Two ground rods 6 feet apart if you can't prove 25 ohms or less on one.

• Same building subpanel: EGC only, no rods, neutrals isolated
• Separate structure subpanel: EGC plus grounding electrode system, neutrals still isolated
• Existing three wire feeder to a separate structure: legacy installs may remain per 250.32(B)(1) Exception, but any new work is four wire

Bond the enclosure with a listed bonding bushing where required by 250.92 and 250.97, particularly on feeders over 250 volts to ground or where concentric knockouts interrupt the metal path.

Breakers, labeling, and the final walk

Use breakers listed for the panel. A Cutler Hammer CH breaker does not belong in a Square D QO panel, regardless of what fits physically. NEC 110.3(B) requires listed and labeled equipment, installed per listing.

Fill the directory before you energize. Circuit numbers, not vague descriptions. "Kitchen SABC 1" beats "kitchen." NEC 408.4(A) requires the directory to be legibly marked, kept up to date, and specific. AFCI and GFCI protection follow the branch circuit rules in 210.8 and 210.12, not the subpanel itself, but that is where the breakers live so verify every circuit meets current code.

Tip from the field: "Torque, photo the directory, photo the bonding, then power up. If you energize before you document, you will forget something, and the callback is always on a Friday afternoon."

Before the cover goes back on, verify torque one more time on the feeder lugs, check that all knockouts are closed or filled, and make sure no conductor crosses the dead front opening. Megger the feeder if you're spec'd to. Then energize, test for proper voltage phase to phase and phase to neutral, and check that the neutral to ground bond reads open at the subpanel. If you get continuity there, stop and find the short before anything else gets powered.