Field guide: installing a subpanel, for journeymen (edition 4)

Sizing the feeder before you touch anything

Start with the load calc. Article 220 Part III gets you there for a dwelling, Part IV for optional. Do not guess the subpanel amperage from the panel schedule on the wall, run the numbers cold and compare against the existing service. If the service is 200A and already carries 150A of calculated load, a 100A subpanel feeder is a paper exercise only.

Pick the feeder conductor per 310.16 at the correct termination temp column, usually 75C for breakers rated 100A and up per 110.14(C)(1)(b). Then check 215.2(A)(1) for minimum feeder ampacity and 215.3 for overcurrent. The grounded conductor on a feeder to a separate building or a subpanel inside the same structure is sized per 250.122 if it is acting as the equipment grounding conductor, otherwise 215.2 for the neutral.

Voltage drop is advisory in 210.19 Informational Note No. 4, but on a detached garage run of 140 feet you will feel it. Bump one size if the run is long, the customer wants a welder, or both.

Four wires, always, on a new install

Since the 2008 code cycle, 250.32(B)(1) requires four wires to a separate structure. Hot, hot, neutral, and a separate equipment grounding conductor. The old three-wire with bonded neutral trick is gone on new work. If you are replacing an existing three-wire feeder to an outbuilding, 250.32(B)(1) Exception is narrow and will not save you on a remodel that touches the feeder.

Inside the same building, a subpanel is always four-wire. The neutral floats, the ground bonds to the enclosure. This is the number one callback I see on homeowner work.

If you pull the main bonding jumper out of a subpanel and the lights go out, the neutral was carrying current through the ground path. That is a fire waiting for a dry day. Fix the bond, do not reinstall the screw.

Bonding, grounding, and the green screw

The main bonding jumper, that green screw or strap shipped with the panel, only lives in service equipment or in the first disconnect after the service. See 250.24(A) and 250.28. In a subpanel it comes out and goes in the parts bag taped to the door. Write the date on the bag so the next guy knows you were deliberate.

Separate the bars. Neutrals on the isolated bar, grounds on the bar bonded to the can. Many load centers ship with both bars bonded to the enclosure through a metal strap, you remove the strap on the neutral side. Read the label inside the door, not the one on the box.

• Service or first disconnect: main bonding jumper installed, neutral bonded to enclosure, GEC landed per 250.64.
• Subpanel same structure: bonding jumper removed, neutrals isolated, EGC from feeder lands on ground bar.
• Subpanel separate structure: same as above, plus a grounding electrode system at that structure per 250.32(A).

Breaker selection and the 42 circuit myth

The 42 circuit limit in 408.54 went away in the 2008 NEC. Panel capacity is now whatever the listing says, so read the label. That said, keep spare spaces. A panel with zero spare breakers is a panel the homeowner will call you back about in six months.

Handle ties versus two-pole breakers: 210.4(B) requires simultaneous disconnect of all ungrounded conductors of a multiwire branch circuit. An identified handle tie meets this, a nail through two breakers does not. For 240V loads, use a proper two-pole breaker with a common internal trip.

AFCI and GFCI follow the load, not the panel. 210.8 and 210.12 tell you where the protection lands. If you are feeding a detached garage subpanel, 210.8(A)(2) still applies at the receptacles, and 210.8(F) catches outdoor outlets on the feeder side of things depending on what is out there.

Working space, wire management, and the inspector

110.26 is where most rough inspections fail. Thirty inches of width, three feet of depth for 120/240 residential, and headroom of six and a half feet or the height of the equipment, whichever is greater. No storage in that zone, ever. A water heater installed eight inches in front of the subpanel is a red tag regardless of who was there first.

Inside the can, leave six inches of free conductor at each termination per 300.14, measured from where the conductor enters the box. On a retrofit where the existing wires are short, add a junction box above the panel rather than splicing inside the gutter.

If the inspector cannot stand square in front of the panel with the door open 90 degrees and still have a foot of air behind him, you are not getting a sticker. Move the shelving before you call.

Labeling, torquing, and closing out

408.4(A) requires every circuit to be legibly identified as to its clear, evident, and specific purpose. "Bedroom" is not enough if there are three bedrooms. "SE bedroom, ceiling fan and receptacles" is. Use the directory, not a Sharpie on the dead front.

Torque every lug and every breaker to the value printed on the label per 110.14(D). A calibrated screwdriver, not feel. This became explicit in the 2017 cycle and inspectors are enforcing it. Keep the tool in the truck and the calibration card with it.

1. Verify load calc against existing service capacity.
2. Pull four-wire feeder, size per 215 and 250.
3. Remove main bonding jumper at subpanel, isolate neutrals.
4. Land EGC on ground bar, establish electrode at separate structures.
5. Torque to label, fill directory, check 110.26 clearances.