Field guide: installing a subpanel, commercial version (edition 4)

Sizing and feeder calculations

Commercial subpanel work starts with load calculation per NEC 220. Don't guess the feeder. Pull the one-line, verify the connected load, apply demand factors from 220.42 through 220.60, and size conductors against 310.16 at the correct termination temperature rating per 110.14(C).

For a 400A subpanel feeding a tenant space with continuous lighting load, the feeder OCPD must be sized at 125% of the continuous load plus 100% of non-continuous per 215.3. Neutral sizing follows 220.61, and on a 3-phase 4-wire wye with nonlinear loads (LED drivers, VFDs, switchmode supplies), treat the neutral as a current-carrying conductor per 310.15(E).

Check voltage drop on long runs. NEC 215.2(A)(1) Informational Note recommends 3% on feeders, 5% total. On a 300 ft feeder at 400A 480/277V, copper, you're pushing 500 kcmil minimum just to stay inside that window.

Working space and mounting

110.26 is where inspectors live. For commercial 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. Over 150V to ground with exposed live parts on one side, Condition 2 jumps you to 3.5 ft. Over 600V you're in 110.34 territory.

The 30 in working width doesn't have to be centered on the panel, but it cannot be obstructed. Storage in front of a panel is a violation every time. Dedicated equipment space above the panel extends 6 ft up or to the structural ceiling per 110.26(E)(1).

If the GC frames a wall in front of your panel and calls it a "chase access," it's still a violation. Get it in writing before the drywall goes up, or you're the one coming back to move the panel.

Grounding and bonding the subpanel

This is where the majority of failed inspections happen on commercial subpanels. A subpanel is not a service. The neutral and ground must be separated. Remove the main bonding jumper. The equipment grounding conductor lands on the ground bar, the grounded (neutral) conductor lands on an isolated neutral bar.

Size the equipment grounding conductor per 250.122 based on the feeder OCPD, not the feeder ampacity. For a 400A feeder, that's 3 AWG copper minimum. If you upsize the ungrounded conductors for voltage drop, you must upsize the EGC proportionally per 250.122(B).

• Feeder EGC: sized per 250.122, run with the feeder conductors
• Neutral bar: isolated from enclosure, no bonding screw, no bonding strap
• Ground bar: bonded to enclosure
• No supplemental grounding electrode unless the subpanel is in a separate structure (250.32)

Overcurrent protection and breaker coordination

The feeder OCPD sits at the supply end, typically in the upstream distribution panel or switchboard. Selective coordination is required for specific occupancies per 700.32, 701.32, and 708.54 (emergency, legally required standby, critical operations). For general commercial, coordination is a design call, but tenants notice when a branch fault takes out the whole floor.

Series-rated combinations are allowed per 240.86, but the markings must match exactly. Field-applied labels are required per 110.22(C) when series ratings are used. If the existing gear has an AIC rating below the available fault current, you need a short-circuit study before you energize.

1. Pull the available fault current from the utility
2. Calculate fault current at the subpanel location
3. Verify the subpanel and all downstream breakers meet or exceed that value
4. Label per 110.24 with the available fault current and date of calculation

Conductor installation and terminations

Pull the feeder with the correct conduit fill per Chapter 9 Table 1. For three 500 kcmil THHN plus an EGC, you're in 3 in EMT minimum. Use pulling lube, not soap. Set pulling tension limits before you put a truck on the rope.

Torque every lug with a calibrated torque wrench per 110.14(D). This became mandatory in the 2017 NEC and inspectors are checking. Manufacturer labels on the breaker and lugs give the value in inch-pounds or newton-meters. Write the torque values on the panel schedule and sign it.

A loose 400A lug will cook a panel in six months. A torque wrench costs less than one callback. Buy the wrench.

Labeling, documentation, and turnover

110.22(A) requires the disconnect to be legibly marked with its purpose. For a subpanel, that means identifying the source (upstream panel, circuit number) and the load it serves. Engraved phenolic, not a Sharpie.

Arc flash labeling per 110.16 is required on all equipment likely to require examination while energized. The label needs nominal voltage, arc flash boundary, and either incident energy or PPE category. An engineering study gives you real numbers; the PPE table method in NFPA 70E works if the equipment parameters match the table exactly.

Fill out the panel schedule completely. Circuit number, breaker size, conductor size, load description, phase. Leave it legible for the next electrician. The next electrician might be you in three years with no memory of what you did.