Field guide: installing a subpanel, commercial version (edition 1)

Plan the feeder before you pull a permit

Commercial subpanels live or die on feeder sizing. Start with the connected load, apply the demand factors in NEC 220 Part III or IV, and pick a feeder that holds up under continuous duty at 125 percent per NEC 215.2(A)(1). If the panel feeds a single motor or motor group, layer in NEC 430.24 before you size conductors.

Voltage drop is not a code rule but it is a callback waiting to happen. Keep feeders under 3 percent to the subpanel and under 5 percent total to the farthest outlet, per the Informational Note in 215.2(A)(1). On long runs in big-box retail or warehouse work, upsize one or two trade sizes early and save the rework.

Before you order gear, confirm three numbers with the EC or engineer of record:

• Available fault current at the source and the SCCR of the subpanel (NEC 110.10, 110.24).
• Calculated load vs. panel bus rating, including any future spare capacity the owner wants.
• Overcurrent device at the feeder origin and whether it is 100 percent rated or standard 80 percent.

Locate the panel for code and for the trade

NEC 110.26 working space is non-negotiable: 3 ft depth for most 120/208 or 277/480 gear, 30 in. width or the panel width whichever is greater, and 6.5 ft headroom. Dedicated electrical space above the panel is 6 ft or to the structural ceiling, per NEC 110.26(E)(1). No plumbing, no ductwork, no sprinkler branch lines in that envelope.

Commercial tenant spaces love to bury panels behind shelving or inside storage closets. Push back early. If the panel lands in a storage room, get written acknowledgement that the working space stays clear, and mark the floor.

If the GC tells you the panel "can go behind that rack, we'll keep it clear," assume the rack will be full of inventory the day after final. Put it somewhere the 110.26 envelope is physically defended.

Grounding and bonding, done once

This is where commercial subpanels get hacked up. A subpanel is a separately derived system only if it is fed from a transformer or generator with its own neutral to ground bond. A subpanel fed from the service or another panel is not separately derived, so the neutral bar is isolated and the equipment ground bar is bonded to the enclosure, per NEC 250.24(A)(5) and 250.142(B).

Pull a full size equipment grounding conductor with the feeder, sized per NEC 250.122 from the feeder OCPD. Do not rely on the raceway alone on long runs or where flexible connections are used. If the feeder upsizes for voltage drop, the EGC upsizes proportionally per 250.122(B).

1. Remove the factory main bonding jumper or green screw in the subpanel.
2. Land the neutrals on the isolated neutral bar only.
3. Land the EGCs on a ground bar bonded to the can.
4. Verify continuity from the subpanel can back to the service grounding electrode system.

Feeder conductors, raceway, and terminations

Size conductors from NEC Table 310.16 at the termination temperature rating, almost always 75 degrees C for 100 A and larger terminations, per NEC 110.14(C)(1)(b). Check the label on the breaker and lugs, not the wire jacket. Derating for ambient temperature and conduit fill comes off the 90 degree C column if the conductor has a 90 C insulation, then the final ampacity must still meet the 75 C termination limit.

For parallel sets, all phases and neutrals must be the same length, same material, same insulation, and same raceway treatment per NEC 310.10(G). Auditors and inspectors do pull one set to check length. Cut them together.

Torque every lug to the manufacturer spec with a calibrated torque wrench or screwdriver, per NEC 110.14(D). This is a 2017 cycle requirement that inspectors now enforce. Keep the torque tool cert in your truck.

Circuit directory, labels, and arc flash

NEC 408.4(A) requires every circuit to be legibly identified as to its clear, evident, and specific purpose. "Lights" is not specific. "Lights, north half, sales floor" is. Update the directory at final, not from the rough-in tags.

NEC 110.21(B) and 110.16 drive the field marking: the panel source and available fault current on the gear, and an arc flash warning label. Many AHJs want the calculation date and the engineer on the label. Get the numbers from the EC before the walk.

Photograph the inside of the dead front with the directory visible before you close it. When the owner calls in six months asking which breaker feeds the cooler, you will have it.

Close out clean

Before you call for inspection, walk the panel cold: neutrals and grounds separated, torque marks on every lug, no double-lugged breakers unless the breaker is listed for it per NEC 408.41, knockouts filled, and the dead front screws all present. Meg the feeder phase to phase and phase to ground if the spec calls for it.

Leave the owner a one-page closeout: panel schedule, feeder size and length, OCPD at source, available fault current, and the torque spec sheet. It protects you on the next service call and it is what separates a commercial hand from a resi guy moonlighting on a strip mall.