Field guide: installing a subpanel, code-compliant approach (edition 6)

Plan the feeder before you touch metal

Subpanel work lives or dies on the feeder calculation. Size the conductors for the calculated load per Article 220, not the panel's bus rating. A 100A subpanel does not require a 100A feeder if the load calc says 62A... but most inspectors will still want to see your math on the job folder.

Check the distance. Voltage drop past 100 feet on a 120/240V feeder starts eating into your 3% recommendation in NEC 210.19(A) Informational Note No. 4. Bump the conductor one size when the run exceeds roughly 75 feet on #4 copper at full load. Do the math, do not guess.

Confirm the supply side can carry it. A 200A service feeding a 125A subpanel plus existing loads needs a fresh load calculation under 220.83 if the house has been added to since the last service upgrade.

Four wires, always, on a separate structure or remodel

The four-wire feeder rule is where most callbacks happen. Since the 2008 cycle, NEC 250.32(B) requires an equipment grounding conductor run with the feeder to any separate building or structure. No more bonding the neutral at the remote panel. The 2011 code closed the last loophole for existing installations being extended.

Inside the same structure, a subpanel always gets four wires: two hots, a neutral, and an equipment grounding conductor. The neutral bar floats. The ground bar bonds to the enclosure. Pull the green bonding screw out of the neutral bar and bag it with the panel paperwork.

If you find a three-wire feeder on an existing detached garage subpanel, it is not grandfathered the moment you touch it for a remodel or addition. Repull or trench a ground rod system and a new EGC back to the source. Inspectors are reading 250.32 tight these days.

Grounding and bonding, by the book

At a separate structure, drive or install a grounding electrode system per 250.50 and bond it to the subpanel ground bar. Two 8-foot rods spaced at least six feet apart satisfies 250.53(A)(2) unless you can prove 25 ohms or less with a single rod, which nobody bothers to test. Just drive two.

Inside the same building, the subpanel does not get its own ground rods. The EGC from the main panel carries the fault path. Adding rods at an interior subpanel creates parallel paths and objectionable current under 250.6. Skip it.

• Neutral bar: isolated, floating, no bonding screw
• Ground bar: bonded to the enclosure, tied to the EGC from the source
• Separate structure: add GES (rods, Ufer, or water pipe if qualifying) bonded to ground bar
• Same structure: EGC only, no local electrodes

Conductor sizing and overcurrent protection

Size feeder conductors per 310.16 at the 75C column for most modern terminations, verified by the listing on the breaker and lugs. A 100A subpanel feeder is typically #3 copper THHN or #1 aluminum SER. For 125A, go #1 copper or 2/0 aluminum. Check the breaker listing, not just the table.

The feeder breaker lives at the source panel. It protects the conductors, not the subpanel bus. You can feed a 200A rated subpanel with a 60A breaker all day long. The subpanel's main lug or main breaker configuration is about disconnect location, not OCPD sizing.

For a detached structure, 225.36 requires a disconnect suitable for service equipment at the remote building, OR feed from a panel in the primary structure that has a clearly marked disconnect. Most inspectors prefer a main breaker subpanel at the detached structure. Saves the argument.

Physical install details that pass the first inspection

Torque every lug with a calibrated screwdriver or T-handle. NEC 110.14(D) now explicitly requires it. Write the torque value on the panel schedule sticker so the inspector sees you read the label.

Working clearance per 110.26: 36 inches deep, 30 inches wide or panel width (whichever is greater), 6.5 feet headroom. Do not install the panel behind the water heater or above a utility sink. The inspector will make you move it.

1. Mount the enclosure plumb, accessible, and within the 110.26 working space
2. Land the feeder with proper strain relief and listed connectors for the cable type
3. Separate the neutral and ground bars, verify the bonding screw is removed
4. Torque all lugs to the label spec and document it
5. Label the feeder breaker at the source as "Feeds [location] subpanel" per 408.4
6. Fill out the panel directory completely, no "spare" shortcuts on used circuits

Before you close the cover, run a megger or at minimum an insulation check between the isolated neutral bar and the enclosure. If you read continuity, you missed pulling the bonding screw. Catch it now, not at rough-in inspection.

Common failure points on final

The top three rejections on subpanel finals: bonded neutral at the subpanel, missing or undersized EGC on a separate structure feed, and inadequate working clearance. All three are 10-minute checks before you call for inspection.

Double-check AFCI and GFCI requirements on the branch circuits landing in the subpanel. Article 210.8 and 210.12 apply to the circuit's location, not the panel's location. A subpanel in the garage feeding bedroom circuits still needs AFCI protection on those bedroom breakers per 210.12(A).

Photograph the open panel before you close it. Feeder landings, bar separation, torque marks, and the panel schedule. When a callback comes six months later, that photo is the difference between a warranty visit and an argument.