Field guide: installing a subpanel, for journeymen (edition 5)

Plan the feeder before you touch a knockout

Subpanel work lives or dies on the feeder calc. Before you pull a single wire, know the load, the distance, and the breaker feeding it. Oversizing the feeder "just in case" wastes copper. Undersizing it gets you a callback or worse.

Size the feeder per NEC 215.2 and 220 Part III. For most detached structures or remodel adds, a 60A or 100A feeder covers the load, but run the math. Voltage drop matters more than people admit: if the run is over 100 ft, bump the conductor up one size. NEC 210.19(A) Informational Note 4 pegs 3% as the recommended max for branch circuits, 5% total with the feeder.

Check these before cutting anything:

• Calculated load vs. feeder ampacity (NEC 310.16 ampacity table, 75C column for most terminations)
• Overcurrent at the source panel, sized to protect the feeder conductors (NEC 240.4)
• Distance and voltage drop, especially for detached garages and shops
• Available fault current at the subpanel location, for breaker AIC rating

Grounding and bonding, the part everyone gets wrong

This is where inspectors fail jobs. A subpanel is not a service. The neutral and ground must be separated. Remove the main bonding jumper or bonding screw in the subpanel enclosure. NEC 250.24(A)(5) prohibits a neutral-to-ground connection on the load side of the service disconnect.

Run four wires to the subpanel: two hots, one insulated neutral, one equipment grounding conductor. The EGC bonds to the can. The neutral lands on an isolated neutral bar. If the existing subpanel only has a neutral bar, add a separate ground bar and bond it to the enclosure per NEC 408.40.

If you see a three-wire feeder to an existing subpanel in an older house, do not assume it is legal and leave it. That allowance was removed from the NEC in 2008 for separate buildings. Fix it or document it, but do not walk past it.

For detached structures, a grounding electrode is required at the second building per NEC 250.32(A). Two ground rods 6 ft apart, or a single rod if you can prove 25 ohms or less, plus any other present electrode (metal water pipe, Ufer, etc.).

Conductor selection and conduit fill

For a 100A feeder, #3 copper or #1 aluminum on the 75C column handles it. #4 copper is rated 85A at 75C and is only good for a 100A feeder under the 310.12 dwelling rule, which applies to services and main power feeders to dwelling units, not every subpanel. Read 310.12 carefully before you default to #4.

EGC size comes from NEC 250.122 based on the feeder overcurrent device, not the conductor size. 100A feeder gets a #8 copper EGC. Upsize the EGC proportionally if you upsize the ungrounded conductors for voltage drop (NEC 250.122(B)).

• Neutral: sized per calculated unbalanced load, NEC 220.61, minimum per 250.24(C) if it doubles as the grounded conductor feed
• Conduit fill per Chapter 9 Table 1, typically 40% for three or more conductors
• For PVC underground, 18 in minimum cover under a residential driveway, 24 in under open ground, per NEC Table 300.5

Mounting and working clearance

NEC 110.26 is not negotiable. 36 in deep working space in front of the panel, 30 in wide or the width of the equipment (whichever is greater), 6.5 ft high. No storage in that space. No water heater, no shelving, no washer.

Mount the panel so the highest breaker handle is no more than 6 ft 7 in above the floor (NEC 240.24(A)). In a garage or unfinished basement, screw to blocking or studs, not drywall anchors. Level matters: a crooked panel is a tell that the rest of the work is sloppy.

Terminations and torque

Torque is now explicitly required. NEC 110.14(D) mandates you use a calibrated torque tool at terminations where the manufacturer specifies a value. That means the breaker lug sticker, the neutral bar, the ground bar, the main lug. A click wrench or a torque screwdriver, not your wrist.

Strip lengths matter. Too short and the lug bites insulation. Too long and you have exposed copper outside the lug. Follow the strip gauge on the breaker or the listed length on the lug body.

Torque every lug, then go back an hour later and re-torque the aluminum. AL compresses. If you do not come back to it, you are leaving a loose connection for the homeowner to find at 2 am in July.

Final checks before the inspector

Label every breaker, not "lights" but the actual room or circuit. NEC 408.4(A) requires legible, specific identification. Arc-fault and GFCI protection follows the branch circuits, not the panel, but verify 210.8 and 210.12 still apply to anything you extended off the new subpanel.

Walk the job with a meter before you close it up:

1. Verify voltage L1-L2, L1-N, L2-N, L1-G, L2-G
2. Confirm zero voltage N-G at the subpanel (proves neutral isolation)
3. Check breaker tightness one more time
4. Photograph the panel directory and interior before the cover goes on

Clean the can. Vacuum the sheetrock dust. An inspector who opens a clean panel with torqued lugs and a legible directory is an inspector who signs and leaves.