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

Plan the feeder before you cut a single knockout

Subpanel work lives or dies at the load calc. Before you touch the truck, know the subpanel's calculated load per NEC 220, the feeder OCPD at the main, the distance of the run, and whether the structure is separate or on the same premises. That tells you conductor size, conduit fill, and whether you're dealing with 250.32 (separate structure) or straight 250.142 (same building).

For a 100A subpanel off a 200A main, most residential jobs land on 3 AWG Cu or 1 AWG Al THHN/THWN-2, per Table 310.16 at 75°C. Longer runs, derate for voltage drop (aim under 3% on the feeder, 5% total). Don't upsize the EGC unless you upsize the ungrounded conductors, then scale per 250.122(B).

Confirm the panel directory at the main. If the main is already loaded past 80% continuous, you have a bigger conversation before you pull any wire.

Four wires, always, when it's a separate structure

Since the 2008 cycle, 250.32(B) has required a 4-wire feeder to separate buildings or structures with an equipment grounding conductor run with the feeder. No more bonding neutral to ground at the detached garage. That myth still shows up on job sites. Kill it.

Same premises, different room? Same rule applies under 250.142. The neutral is a current-carrying conductor only. The EGC is the fault path. Bond them once, at the service, and nowhere downstream.

• Remove the main-bonding jumper (green screw or strap) in the subpanel.
• Isolate the neutral bar from the enclosure.
• Land equipment grounds on a separate ground bar bonded to the can.
• Pull a dedicated EGC sized per 250.122 with the feeder.

If the neutral bar has a green bonding screw backed out in the bag taped to the door, leave it in the bag. I've seen inspectors red-tag a job because that screw walked off the bench and into the panel an hour later.

Feeder OCPD, tap rules, and the 83% trap

The feeder breaker at the main protects the feeder conductors, not the subpanel bus. Size the OCPD to the conductor ampacity after all adjustments per 240.4, then verify the subpanel's bus rating is equal to or greater than the OCPD. A 100A breaker feeding a panel with a 100A bus is fine. Feeding a 125A bus is also fine. Feeding a 70A bus is a violation waiting to burn.

Dwelling service and feeder conductors get the 83% allowance under 310.12 when they carry the entire load of the dwelling. That rule does not extend to every subpanel. If the subpanel is a secondary panel in the same dwelling and carries the full load, it qualifies. If it feeds a shop, a workshop, or a detached ADU with its own calc, run the numbers straight from 310.16.

Tap conductors off a feeder fall under 240.21(B). The 10-foot and 25-foot tap rules have strict conditions. Don't improvise them on a subpanel install unless the geometry truly forces it.

Panel mounting, working space, and conduit entry

110.26 is the article inspectors quote most often on rough-in failures. You need 3 feet of depth in front of the panel, 30 inches of width or the panel width (whichever is greater), and 6.5 feet of headroom. No storage, no water heaters sharing that envelope, no shelving creeping into the zone.

Mount height puts the highest breaker no more than 6 feet 7 inches above the floor, per 240.24(A). In a garage, keep the panel out of the splash zone and away from overhead door tracks. In a basement, watch for the condensate line from the HVAC above.

1. Dry-fit the panel, mark the knockouts before you set it.
2. Use listed connectors, bonded bushings on metallic conduit over 250V to ground per 250.97.
3. Keep feeder bends within 360 degrees total between pull points.
4. Torque lugs to the label. Every time. Use a calibrated driver.

Grounding electrode at a separate structure

250.32(A) requires a grounding electrode system at any separate building or structure supplied by a feeder. That's usually two 8-foot ground rods spaced at least 6 feet apart, bonded with a 6 AWG Cu grounding electrode conductor to the subpanel's ground bar.

One rod is only acceptable if you can prove 25 ohms or less with a clamp meter or fall-of-potential test. Nobody carries that gear to a residential punch list. Drive two rods and move on.

On a detached shop, I land the GEC directly on the ground bar, not on the neutral bar. Inspectors read the panel left to right. Make the separation obvious so the grounding conversation never starts.

Label, torque, and close it out

110.22 requires the disconnecting means to be legibly marked. That means the feeder breaker at the main gets a label identifying the subpanel it feeds, and the subpanel gets a directory that actually matches the circuits. "Lights" is not a directory entry. "Kitchen SABC 1, north wall receptacles" is.

Torque every connection to the manufacturer spec listed on the panel label, per 110.14(D). Document it if the AHJ asks. Then megger the feeder if the run is long or buried, verify phase rotation if it's three-phase, and energize with the main off, subpanel main off, then branch breakers off. Bring them up in order.

Walk the job with the homeowner or GC. Show them the panel, the directory, and the disconnect. The last five minutes of a subpanel install are where callbacks get prevented.