Field guide: installing a subpanel, commercial version (edition 2)

Scope and load calc first

Before you pull a permit or price gear, run the load calc per NEC 220. Commercial feeders live or die on demand factors, not nameplate sums. Pull lighting at 125% of continuous per 210.19(A)(1), motor loads per 430.24, and receptacle loads at 180 VA each per 220.14(I) with the demand factors in Table 220.44 if you exceed 10 kVA.

Confirm the existing service has spare capacity. Article 220.87 lets you use 12 months of recorded demand plus the new load at 125% if the facility has metering history. No history, no shortcut, you calc from scratch.

Document the calc on paper and leave a copy in the panel. The AHJ will ask, and the next electrician on this job will thank you.

Sizing the feeder and OCPD

Size the feeder conductors to the calculated load, not the subpanel bus rating. A 200A bus fed by a 125A feeder is legal if the calc supports it, per 408.36. The OCPD at the supply end protects the feeder per 215.3.

Voltage drop is not mandatory in the NEC body, but 210.19 Informational Note 4 recommends 3% on feeders and 5% total. On long commercial runs, upsize one or two trade sizes. Cheaper than a callback when the VFD throws undervoltage faults.

• Feeder ampacity per 310.16, applying ambient and conduit-fill adjustments from 310.15(B) and (C).
• Neutral sized per 220.61, accounting for nonlinear loads (harmonics) where present.
• EGC per Table 250.122, upsized proportionally if the ungrounded conductors were upsized for VD per 250.122(B).
• Terminations rated 75°C for most commercial gear, verify on the label before you size off the 90°C column.

Grounding, bonding, and the neutral

This is where subpanel jobs go sideways. A subpanel is a separately derived... no, it is not. It is a feeder-fed panel. The neutral must be isolated from the equipment ground per 250.24(A)(5) and 408.40. Remove the factory bonding screw or strap. Every time.

Pull a separate EGC with the feeder. Do not rely on the conduit as the sole ground path on commercial work, even where 250.118 permits it. Redundancy is cheap insurance, and inspectors in most jurisdictions expect a wire-type EGC in PVC or flex runs.

Tip from a 30-year commercial hand: before you energize, meg the feeder and ohm from the neutral bar to the enclosure. If you read continuity, you left the bond in. Fix it before you close the door.

Working space, mounting, and labeling

110.26 is non-negotiable. 3 feet of clear depth for 0 to 150V to ground, 3.5 feet for 151 to 600V with exposed live parts on one side. Width is 30 inches or the width of the equipment, whichever is greater. Headroom is 6.5 feet per 110.26(E). No storage in that envelope, ever.

Mount the panel so the highest breaker handle is no more than 6'7" above the floor per 404.8(A). On commercial tenant work this is the one dimension that trips field verification the most. Measure from finished floor, not the slab.

1. Label the subpanel with source, feeder size, and AIC rating per 408.4(B) and 110.24.
2. Apply the arc flash warning per 110.16. Use a field-marked label with incident energy if an analysis exists.
3. Circuit directory must be legible and specific per 408.4(A). "Lights" is not specific. "Lights, north warehouse bay" is.
4. Identify the disconnect for the feeder at the supply end per 408.4(B).

Series ratings and available fault current

Commercial subpanels often end up with breakers that cannot stand alone against the available fault current. Series ratings per 240.86 are legal, but only if the combination is tested, marked on the equipment, and the installer applies the required field label per 110.22(C).

Get the available fault current from the POCO or calculate it from the transformer impedance. Document it on the service equipment per 110.24(A). When the service changes, the label must be updated per 110.24(B). Most shops miss this one on remodels.

If the panel schedule shows 10 kAIC breakers and the calculated fault current at the subpanel is 14 kA, you have a violation, not a subpanel. Swap gear or add current-limiting fuses upstream.

Final checks before energizing

Torque every lug to the manufacturer's spec per 110.14(D). Use a calibrated torque wrench or screwdriver. "Tight" is not a spec, and loose commercial terminations are the number one cause of thermal failures in the first year.

Walk the job with the inspector's checklist in hand: clearances, labels, bonding, EGC continuity, breaker AIC, directory. Meg the feeder at 1000V to ground and between phases. Record the readings. Energize one breaker at a time and verify rotation if any three-phase motor loads are on the panel.

• Neutral isolated from ground: verified.
• EGC bonded to enclosure and to the ground bar: verified.
• Feeder OCPD matches conductor ampacity and calc: verified.
• Working clearances per 110.26: verified and photographed.
• Labels, directory, arc flash, AIC: installed and legible.

Close the cover, sign the permit, and leave the calc sheet inside the dead front pocket. The next person in this panel is probably you in five years. Make their day easier.