Field guide: installing a subpanel, code-compliant approach (edition 4)

Plan the load and feeder before you open a knockout

Subpanel jobs fail at the desk, not the wall. Size the feeder to the calculated load per NEC 220, not the sum of breaker handles. For a detached structure, run the optional method in 220.84 if it applies, otherwise 220.82 for dwellings. Write the calc down and staple it to the permit copy.

Confirm the main panel can give up the space. Check the bus rating against the main breaker plus the new feeder breaker under the 120% rule in 705.12(B)(3)(2) only if there is also a PV source. For a plain subpanel, the feeder breaker counts against the bus like any other load, so a 200A panel with a 200A main cannot host a 100A feeder if the existing load already pushes the bus.

Pick the conductor before the conduit. 4 AWG Cu at 75C lands 85A per Table 310.16, so a 100A feeder wants 3 AWG Cu or 1 AWG Al. Neutral sizes to the calculated unbalanced load per 220.61, never smaller than the EGC.

Grounding and bonding, the part that trips inspectors

At a subpanel, neutrals and grounds separate. Period. Remove the main bonding jumper or the green bonding screw on the neutral bar. NEC 250.24(A)(5) prohibits a neutral to ground connection on the load side of the service disconnect. If the subpanel sits in a detached building, 250.32(B)(1) requires a four wire feeder with an EGC, and the grounding electrode system at that building bonds to the EGC bar, not the neutral.

Size the EGC from Table 250.122 based on the feeder overcurrent device, not the conductor. A 100A feeder needs an 8 AWG Cu EGC minimum. If you upsized the ungrounded conductors for voltage drop, upsize the EGC proportionally per 250.122(B).

If you see a bonding screw still seated in a subpanel neutral bar, assume every metal box downstream is energized under a fault. Pull it before you energize, not after the megger surprises you.

Feeder routing, conduit fill, and derating

Pull the raceway before you worry about terminations. For EMT, 3/4 inch handles three 3 AWG THHN plus an 8 AWG EGC comfortably per Chapter 9 Table 4 and Table 5. Bump to 1 inch if you are pulling 1/0 Al or adding a second circuit.

Ambient and bundling derates are where 100A feeders quietly become 80A feeders. More than three current carrying conductors in a raceway over 24 inches triggers 310.15(C)(1). A shared neutral on a multiwire branch circuit counts, a grounded neutral on a balanced three phase does not, see 310.15(E). Attic runs above insulation need the 310.15(B) ambient adjustment, which for a 130F attic knocks 75C copper down by 0.58.

• Count current carrying conductors before you pick the raceway
• Check ambient at the hottest point of the run
• Apply derates in series, then compare against the feeder OCPD
• Log the calc on the as built so the next electrician does not guess

Overcurrent, disconnects, and the six handle rule

A subpanel fed from a breaker in the main panel is protected by that breaker, so it does not need a separate disconnect at the subpanel for service rules. But if the subpanel is in a separate building, 225.31 and 225.36 require a disconnect at or nearest the point of entry, and it must be suitable for service equipment. A standard main breaker panel qualifies, a main lug panel does not unless you add an upstream disconnect.

The six handle rule in 225.33 lets you use up to six disconnects to kill power to the separate building, but most AHJs prefer a single main. Draw it on the one line before you buy the panel. A main lug subpanel saves 40 dollars and costs a reinspection.

Terminations, torque, and labeling

Every lug has a torque spec, and 110.14(D) now requires a calibrated tool. Use an in lb driver on the EGC and neutral bars, a 1/2 inch drive click wrench on the feeder lugs. Mark each lug with a paint pen after torquing so the inspector can see you did it.

Aluminum feeders need antioxidant per the manufacturer and a wire brush on the strands. Strip length matters, too short and the set screw bites insulation, too long and you have exposed conductor past the lug. Match the strip gauge stamped on the panel.

Torque, mark, photograph. Three seconds at the panel saves an hour of argument with the inspector who shows up two weeks later.

Label per 408.4(A) with the circuit description, not just the room. 110.22 requires the feeder disconnect at the main panel to identify what it feeds, including the location of the subpanel. A label maker and a Sharpie both qualify, a blank directory does not.

Final checks before energizing

Megger the feeder conductor to conductor and conductor to ground before landing at either end. 500V insulation resistance above 100 megohm is a clean pull, anything under 1 megohm means a nick or a wet pull box. Ring out the EGC from the subpanel cabinet back to the main panel neutral to prove continuity.

1. Verify neutral isolation at the subpanel with an ohmmeter, neutral bar to cabinet should read open
2. Confirm EGC continuity, cabinet to main panel ground bar, under 1 ohm
3. Torque check every lug and mark
4. Label the feeder breaker and the subpanel directory
5. Energize with the feeder breaker off, close it last

Voltage check line to line, line to neutral, and line to ground at the subpanel bus. 240V, 120V, 120V on a single phase feed. If line to ground reads anything other than 120V on a 120/240 system, stop and find the bond problem before you load any circuits.