Field guide: installing a subpanel, commercial version (edition 3)

Plan the feeder before you touch a knockout

Commercial subpanels live or die on feeder math. Start with the calculated load per NEC 220, not the nameplate of the existing panel. Tenant fit-outs inherit oversized conductors all the time, and you do not want to discover the feeder is undersized after the drywall is closed.

Confirm the available fault current at the source and stamp it on the panel per 110.24. If the upstream service is 42 kAIC and your new panel is a 22 kAIC bolt-on, you have a problem before you start. Get the utility letter or run the point-to-point yourself.

• Voltage drop target: 3% feeder, 5% total per 210.19(A) Informational Note 4.
• Verify OCPD coordination with the upstream device, especially in healthcare or elevator circuits.
• Check 408.36 for panelboard OCPD requirements on the supply side.

Sizing conductors and the equipment grounding conductor

Use 75°C column ampacity from Table 310.16 unless the terminations are listed otherwise. Most commercial gear is 75°C, but field-installed lugs sometimes are not. Read the label, not the spec sheet.

Apply ambient and conduit fill adjustments before you pick the wire. A 400 A feeder in a rooftop EMT run through 110°F ambient is not the same calculation as the same feeder in a conditioned electrical room. Neutral sizing follows 220.61, and remember the 70% demand factor does not apply to the portion over 200 A on nonlinear loads.

Size the EGC per Table 250.122 based on the upstream OCPD, not the conductor. If you upsize phase conductors for voltage drop, upsize the EGC proportionally per 250.122(B). Inspectors catch this one often.

Label the feeder at both ends with the source panel, circuit number, and AIC rating. Future you, or the next electrician, will thank you when troubleshooting at 2 a.m.

Working space, mounting, and clearances

110.26 is not negotiable. 3 feet of depth for 0 to 150 volts to ground, 3.5 feet for 151 to 600 in Condition 2, and 30 inches of width or the width of the equipment, whichever is greater. The space extends from the floor to 6.5 feet or the top of the equipment, whichever is higher.

Dedicated equipment space per 110.26(E) runs from the floor to 6 feet above the panel, or to a structural ceiling if lower. No piping, ducts, or foreign systems in that zone. Sprinkler piping is allowed above the dedicated space if the panel has drip protection.

1. Verify the door swing clears 90 degrees minimum.
2. Mount the top breaker no higher than 6 feet 7 inches per 404.8(A).
3. Confirm illumination at the working space per 110.26(D).

Grounding and bonding the subpanel correctly

This is where commercial subpanels get violated most. The neutral and ground must be separated at any panel downstream of the service disconnect. Remove the main bonding jumper, the green screw, or the bonding strap, whatever the manufacturer calls it. 250.24(A)(5) and 408.40 are explicit.

Run a separate EGC with the feeder. Do not rely on the conduit alone unless every fitting is listed and the raceway is continuous, and even then, pulling a wire-type EGC is the defensible choice. For separately derived systems like a transformer feeding the subpanel, the bonding rules shift to 250.30, and you will need a system bonding jumper and a grounding electrode conductor at the transformer or first disconnect.

If the subpanel serves a separate building or structure, 250.32 applies. Since the 2008 NEC cycle, you must run an EGC with the feeder and keep the neutral isolated, period. No more second ground rod as a substitute bond.

Circuit breakers, labeling, and the directory

Series-rated combinations must be labeled on the panel per 110.22(C) and 240.86. Field-engineered series ratings are not permitted unless done under 240.86(A) by a licensed PE. Most of the time, stick with fully rated breakers and move on.

408.4(A) requires a legible circuit directory identifying the purpose and location of each circuit. "Spare" and "lights" do not cut it in commercial work. Use room numbers, equipment tags, or architectural grid references.

Fill the directory as you energize, not at the end of the day. Guessing from memory is how receptacles get miswired on as-builts and why the next trade trips the wrong breaker.

Final checks before energizing

Torque every lug to the manufacturer's spec with a calibrated torque wrench or screwdriver. 110.14(D) made this explicit in the 2017 cycle, and inspectors are asking for torque records. Mark each lug with a paint pen after torquing so you can prove the work.

Megger the feeder phase-to-phase, phase-to-ground, and phase-to-neutral before landing at the subpanel. Record the readings. Then verify phase rotation matches the upstream panel, especially if motors or VFDs are downstream.

• Arc flash label per 110.16, with incident energy if a study was done.
• Available fault current label per 110.24(A).
• Short-circuit study updated if the feeder changed the calculation.
• GFCI or GFPE where required by 210.8(B) or 230.95.

Close it up, energize under load if possible, and walk the circuits with a thermal camera after an hour. Loose terminations show up hot before they show up as a callback.