Field guide: installing a subpanel, commercial version (edition 5)

Plan the feeder before you open a knockout

Commercial subpanel installs live or die on the feeder calc. Before you touch the building, you need the load on paper, the conductor sized, the OCPD selected, and the grounding scheme understood. NEC 215.2(A)(1) requires the feeder ampacity to be no less than the non-continuous load plus 125% of the continuous load. Miss that and everything downstream is wrong.

Pull the panel schedule and verify it against the prints. Commercial work almost always involves continuous loads (lighting, signage, HVAC fans running over 3 hours), so apply 215.3 for the OCPD sizing the same way. Check the termination temperature rating on both ends: most breakers and lugs are 75C rated, but if either side is stamped 60C you size off the 60C column in 310.16.

Verify the available fault current at the point of supply. The subpanel SCCR has to equal or exceed it, per 110.9 and 110.10. On a 480/277V system fed from a 1000 kVA transformer, you are often looking at 25kA or more. Stock panels rated 10kA will not cut it.

Grounding and bonding, the part that gets written up

The single most common red tag on a commercial subpanel is a bonded neutral in a downstream panel. 250.24(A)(5) and 408.40 are explicit: the neutral is bonded only at the service or at a separately derived system. At a subpanel, the bonding screw or strap comes out, and the equipment grounding conductor lands on its own bar.

Pull the EGC with the feeder. 250.32(B) ended the old three-wire feeder to a separate building for new work, and within the same building you are running a four-wire feeder every time. Size the EGC from Table 250.122 against the feeder OCPD, not the conductor. If you upsize the ungrounded conductors for voltage drop, you upsize the EGC proportionally per 250.122(B).

If the panel ships with the neutral bonded from the factory, back that screw out and bag it to the panel door with a zip tie. Next guy opens the cover, sees the bag, knows it was done on purpose.

• Neutral bar: isolated, floating on the plastic standoffs.
• Ground bar: bonded to the enclosure, feeder EGC and all branch EGCs land here.
• Bonding screw or strap: removed and stored, not reinstalled.
• Feeder neutral: full sized per 220.61 unless reduced for the calculated unbalanced load.

Working space and mounting

110.26 is non negotiable and inspectors measure it. For systems 600V and below with exposed live parts on one side, you need 3 feet of depth minimum at 0 to 150V to ground, or more depending on voltage and what is across from the panel. The workspace has to be 30 inches wide or the width of the equipment, whichever is greater, and 6.5 feet high.

The top of the highest breaker handle cannot exceed 6 feet 7 inches from the floor or working platform per 240.24(A). In a mechanical room with a raised housekeeping pad, measure from the pad, not the slab. Dedicated electrical space above the panel runs to the structural ceiling or 6 feet, with no foreign piping, ducts, or equipment in that zone, per 110.26(E).

Mount plumb. Use unistrut standoffs on block or tilt-up walls so the conduit entries sit flush and the panel is not racked. A panel that is 1/4 inch out of plumb will fight you when you dress conductors and will telegraph through the finished trim.

Conduit, conductors, and fill

Commercial feeders are almost always in EMT, IMC, RMC, or rigid PVC. Size the raceway from Chapter 9, Table 1 for fill and Table 4 for the actual raceway dimensions. Three current-carrying conductors or fewer, no derate. Four to six, derate to 80%. More than that and you are in 310.15(C)(1) territory fast, especially if you share a pipe with another feeder.

Ambient temperature matters in commercial buildings. A feeder running above a dropped ceiling in a mechanical penthouse can see 40C or 45C ambient in summer. Apply the correction factor from 310.15(B) and verify you still have the ampacity after all adjustments. Do that math before the conduit is in the air, not after.

1. Calculate load, select conductor size at 75C ampacity.
2. Apply ambient correction and conductor count adjustment.
3. Verify final ampacity equals or exceeds the OCPD.
4. Size the raceway from Chapter 9 tables.
5. Confirm bending space at terminals per 312.6.

Torque, label, and document

110.14(D) now requires terminations to be torqued to the manufacturer's spec with a calibrated tool. Bring the torque screwdriver and the torque wrench, and bring the label off the panel that lists the values. Mark each lug with a paint pen after you hit the spec. An inspector who sees paint marks on every termination usually skips the spot check.

Label the subpanel per 408.4(A) with the circuit directory filled out in permanent marker or typed. 408.4(B) requires the source identification: where the feeder comes from, which panel, which breaker. Arc flash labeling per 110.16 goes on the door for any panel likely to be worked on energized.

Photograph the inside of the panel with the dead front off before you button it up. Conductor routing, torque marks, neutral and ground separation. That photo has saved more callbacks than any punch list.

Meg the feeder before you energize. Phase to phase, phase to neutral, phase to ground, neutral to ground. Anything below 1 megohm, find it before you close the upstream breaker. Energize with the subpanel main off, verify voltage at the lugs, then close the main and check for balanced voltage under no load.