Field guide: installing a subpanel, rough-in phase (edition 3)

Plan the feeder before you cut a hole

Size the feeder to the calculated load, not the panel rating. A 100A subpanel fed from a dwelling service only needs conductors rated for the actual demand under Article 220, but most inspectors and GCs expect the feeder to match the panel's main breaker. Pick your lane early and document it.

Run the math for voltage drop before you pull wire. NEC 210.19(A) Informational Note No. 4 recommends 3% on branch circuits, 5% total. On a 150 foot run at 100A, #2 copper lands around 2.8% at 240V. Aluminum 1/0 gets you similar numbers for less money and lighter pulls.

If the feeder path crosses a future remodel zone, add a junction box with slack now. Fishing a 1/0 AL through a finished soffit later will ruin your week.

Grounding and bonding: get this right once

A subpanel in a separate structure or downstream of the service disconnect keeps grounds and neutrals separated. Remove the main bonding jumper. This is NEC 250.24(A)(5) and 250.32(B). The neutral bar floats, the ground bar bonds to the enclosure.

For a detached structure fed by a single branch circuit or feeder, NEC 250.32(A) requires a grounding electrode system at the second building unless an equipment grounding conductor is run with the feeder and there are no other metallic paths. Drive two rods 6 feet apart if you cannot prove 25 ohms on one, per 250.53(A)(2).

• Neutral bar: isolated, floating, bonded only at the service
• Ground bar: bonded to can, lands EGCs and GECs
• Main bonding jumper: removed or never installed at the subpanel
• Separate structure: GES required at the second building

Conductor sizing and the 83% rule

For a dwelling main power feeder, NEC 310.12 lets you size conductors at 83% of the service or feeder rating when the feeder is the main power feeder to a dwelling unit. A 100A feeder in this case can use #4 copper or #2 aluminum THHN/THWN-2. This does not apply to commercial or accessory structures without a dwelling occupancy.

For every other case, use the 75°C column of Table 310.16. Terminations on standard breakers and panels are rated 75°C up to 100A per 110.14(C)(1). Do not chase the 90°C column unless you are doing ampacity adjustment or correction, and then derate back to the termination rating.

Run an EGC sized per Table 250.122. A 100A feeder gets a #8 copper or #6 aluminum EGC. The neutral is a current carrying conductor and sized per the calculated unbalanced load, but in practice match it to the ungrounded conductors on residential work.

Panel location, working space, and knockouts

NEC 110.26(A) governs working space. 30 inches wide, 36 inches deep at 120/240V, headroom of 6.5 feet or the height of the equipment, whichever is greater. The door must open 90 degrees. This is the single most common rough-in fail on inspection.

Do not mount a panel in a bathroom (240.24(E)), clothes closet (240.24(D)), or over steps. Garages and unfinished basements are fair game. Keep the panel face flush with the finished wall surface, within 1/4 inch per 312.3 for combustible surfaces and 1/4 inch setback for noncombustible.

• Dedicated space: 6 feet above the panel, full width and depth, no piping or ducts, per 110.26(E)
• Working space: 30 x 36 x 78 minimum, illuminated
• Conductor bending space at terminals: Table 312.6(B)
• Two egress paths for equipment rated 1200A or more (not a concern here but commit it to memory)

Cable and conduit rough-in

Secure NM cable within 12 inches of the panel and every 4.5 feet thereafter, per 334.30. When stacking multiple NMs into the top of a can, use listed cable connectors rated for the number of cables, and do not exceed the connector's fill. Staple neatly to the framing, not the cable itself.

If you are running EMT or PVC, calculate conduit fill per Chapter 9 Table 1. Three current carrying conductors or fewer is the easy path. Four or more triggers ambient adjustment under 310.15(C)(1). For a 100A feeder in 1.25 inch EMT, three #2 THHN plus a #8 EGC is well under 40% fill.

Label every home run at the panel before sheetrock goes up. A piece of tape with the room and circuit intent saves two hours of ringing out later.

Inspection prep checklist

Rough-in inspection is about mechanical work, not trim. The inspector wants to see secure cables, proper box fill, correct staple spacing, grounded boxes, and a panel that is plumb, level, and in a code compliant location. Open the can, show the feeder terminations if they are made, and have your load calc on hand.

1. Feeder conductors sized and terminated per 110.14 and 310.12 or 310.16
2. Neutral floated, ground bar added and bonded
3. EGC landed, GES installed for detached structures
4. Working space and dedicated space clear
5. All knockouts filled or closed, connectors tight
6. Cables stapled per 334.30, box fill within 314.16 limits

Clean rough-ins pass the first time. Sloppy ones eat your schedule and your margin. Take the extra twenty minutes before you call it in.