Field guide: installing a subpanel, commercial version (edition 6)

Plan the feeder before you touch a knockout

Commercial subpanels live or die by the feeder calculation. Before ordering gear, pull the one-line, confirm the upstream OCPD, and run the load calc per NEC 220 Part III or IV. Demand factors in 220.42 through 220.60 will often shrink the conductor one trade size, which matters when you are fishing through a packed pipe rack.

Verify available fault current at the source and stamp it on the new panel per NEC 110.24(A). If the AIC rating of the subpanel bus or breakers is below the calculated value, you are not installing it, you are creating a bomb. Series ratings per 240.86 are allowed but require the exact breaker combos listed by the manufacturer, nothing else.

Coordinate with the GC on the housekeeping pad, clearances, and whether this panel feeds any elevator, fire pump, or life safety load. Those branches change everything downstream.

Working space and mounting

NEC 110.26 is not negotiable on a commercial job. For panels up to 150V to ground, you need 3 ft depth, 30 in width (or panel width, whichever is greater), and 6.5 ft headroom. Condition 2 bumps depth to 3.5 ft if you have grounded parts on the opposite wall. Over 150V to ground in Condition 2, you are at 4 ft.

Dedicated equipment space per 110.26(E) runs from the floor to 6 ft above the panel or to the structural ceiling, whichever is lower. No sprinkler piping, no HVAC ducts, no data trays passing through that column. Inspectors check this with a plumb bob more often than you think.

Before you set anchors, tape out the 110.26 envelope on the floor and walls. If the plumber shows up and wants to run a 4 inch waste line through your dedicated space, you have the tape to point at.

Feeder conductors and raceway

Size feeders to 215.2 for the calculated load, then check 215.3 to confirm the OCPD protects the conductor ampacity after adjustment and correction factors in 310.15(B) and (C). On a rooftop run, the 310.15(B)(1) rooftop adder is gone as of the 2017 cycle, but ambient temperature still kills ampacity fast.

Pull the EGC separately, sized to 250.122 based on the upstream OCPD, not the feeder ampacity. If you upsized the phase conductors for voltage drop, upsize the EGC proportionally per 250.122(B). Do not bond the neutral at the subpanel. The neutral and ground stay separated downstream of the service disconnect per 250.24(A)(5) and 408.40.

• Verify conduit fill per Chapter 9, Table 1 (40 percent for three or more conductors).
• Derate for more than three CCCs in a raceway per 310.15(C)(1).
• Use listed anti-short bushings on MC and reducing washers only where listed.
• Confirm the raceway is identified for the environment: wet, corrosive, or plenum.

Terminations, torque, and labeling

Torque matters. NEC 110.14(D) requires a calibrated torque tool for every termination with a manufacturer-specified value, and the 2023 cycle removed the ambiguity that let sparkies eyeball it. Log the torque values if the spec calls for commissioning documentation. A loose lug on a 400 A feeder will cook the bus in a year.

Match conductor temperature rating to the lug rating per 110.14(C). Most commercial panels terminate at 75C, so sizing from the 90C column is an error even if THHN is rated 90C dry. Check the label on the can.

Label per 408.4(A) with the circuit description, and per 408.4(B) with the source panel and circuit feeding this subpanel. Arc flash labels per 110.16 go on every panel likely to be worked on energized, and on service equipment 110.16(B) requires the expanded label with nominal voltage, clearing time, and arc flash boundary.

Grounding, bonding, and GFCI

On a separately derived system, follow 250.30. On a standard subpanel fed from the same service, you are running an EGC with the feeder, and the equipment ground bar bonds to the can. Remove the main bonding jumper or neutral-to-ground screw that ships installed from the factory. This is the single most common red-tag on a commercial subpanel.

GFCI and AFCI requirements do not stop at residential. NEC 210.8(B) covers commercial GFCI in kitchens, rooftops, indoor wet locations, locker rooms, garages, and within 6 ft of sinks. 210.8(F) added outdoor dwelling outlets but watch the mixed-use buildings closely. If the subpanel feeds any of these, plan the GFCI breakers at the panel, not downstream, to simplify troubleshooting.

On mixed-use commercial with ground floor retail, assume every receptacle within 6 ft of a mop sink, coffee station, or break room sink is GFCI. Inspectors have gotten aggressive on 210.8(B)(6).

Commissioning and turnover

Meg the feeder before energizing, phase to phase and phase to ground. Anything below 1 megohm and you are chasing a problem before the GC signs off. Verify phase rotation on 3-phase panels with a rotation meter, not by guessing from color code.

Fill the circuit directory legibly per 408.4(A), update the arc flash study if the available fault current changed, and photograph the inside of the panel before the cover goes on. The closeout package needs it, and six months from now when something trips, you will be glad.

1. Verify torque on every lug, phase, neutral, and ground.
2. Confirm neutral is isolated from ground at the subpanel.
3. Meg test, phase rotation check, and voltage verification under load.
4. Complete panel schedule, arc flash label, and source identification label.
5. Photograph the interior and file with the closeout documents.