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

Sizing the feeder and panel for real industrial loads

Industrial subpanels don't behave like the 100A residential add-on in a detached garage. You are feeding motor loads, welders, VFDs, and sometimes continuous process equipment running 18 hours a day. Start with a documented load calculation per NEC 220, and separate continuous loads (operating 3 hours or more) from non-continuous before you pick the OCPD and conductor.

For motor-heavy panels, the feeder ampacity follows NEC 430.24: 125% of the largest motor FLC plus the sum of the other motor FLCs plus any other loads. Don't forget NEC 215.2(A)(1) requires the feeder to carry 125% of continuous load plus 100% of non-continuous. Round up conductor size after you apply ambient and bundling corrections from Table 310.15(B), not before.

Pick the panel bus rating with headroom. A 400A calculated load on a 400A bus leaves zero margin for the plant manager adding a 50HP air compressor next quarter.

If the facility has any future expansion plans on paper, size the feeder raceway for the next conductor size up. Pulling new wire is cheap. Trenching or coring a second run is not.

Grounding and bonding, the part people get wrong

This is where industrial subpanel jobs fail inspection more than any other item. A subpanel is a separately derived system only if it's fed from a transformer or generator with its own neutral derivation. A subpanel fed from the service or another panel is NOT separately derived, and the rules are different.

For a standard (non-separately-derived) subpanel, per NEC 250.32 and 408.40:

• Neutral bar must be isolated from the enclosure. Remove the bonding screw or strap.
• Equipment grounding conductor (EGC) runs with the feeder and lands on the ground bar, which IS bonded to the enclosure.
• No grounding electrode conductor connection to the neutral bar at the subpanel.
• EGC sized per NEC Table 250.122 based on the feeder OCPD, upsized proportionally if the ungrounded conductors are upsized for voltage drop (NEC 250.122(B)).

For a separately derived system (transformer-fed subpanel), you bond the X0 at the transformer or at the first disconnect, and run a grounding electrode conductor per NEC 250.30. Pick one bonding point and document it.

Feeder routing, raceway, and conductor protection

Industrial environments punish lazy raceway work. Know your area classification before you pick fittings. Class I Div 2 gets sealed fittings at boundaries per NEC 501.15. Wet locations need listed wet-location conductors and fittings with drain provisions.

For rigid metal conduit feeder runs, watch your fill (NEC Chapter 9 Table 1), derate for more than three current-carrying conductors in the raceway (310.15(C)(1)), and confirm the bend radius for the largest conductor. A 500 kcmil XHHW doesn't negotiate a standard 90 without stress on the insulation.

Support intervals matter. EMT every 10 feet and within 3 feet of each box (NEC 358.30). Strut-mounted runs across a plant ceiling need seismic bracing in many jurisdictions, check local amendments.

Working space, labeling, and arc flash

Working space per NEC 110.26 is non-negotiable. For voltages to ground up to 150V, 3 feet of clear depth. 151V to 600V, Condition 1 gets 3 feet, Condition 2 gets 3.5 feet, Condition 3 gets 4 feet. Width is the greater of 30 inches or the width of the equipment. Headroom is 6.5 feet minimum, or the height of the equipment.

Do not store pallets, barrels, or tooling within that envelope. Mark the floor with yellow striping if housekeeping is a recurring problem.

Before you energize, do an arc flash incident energy calculation or confirm the existing study covers the new panel. NEC 110.16 requires the arc flash warning label, and NFPA 70E drives the PPE category. A label with the actual cal/cm2 value saves the next tech from guessing.

Panelboard circuit directory must be legible, accurate, and specific (NEC 408.4(A)). "Spare" is fine. "Misc" is not.

Commissioning, testing, and turnover

Before you close the cover, do the electrical checks that catch the expensive mistakes. Torque every lug and terminal to the manufacturer's spec with a calibrated torque wrench or screwdriver (NEC 110.14(D)). Write it down.

1. Insulation resistance test the feeder, phase to phase and phase to ground, before terminating.
2. Verify phase rotation at the subpanel matches the upstream source if any three-phase motor loads are fed from it.
3. Measure bond impedance from the furthest device box back to the service ground. High resistance means a loose connection or a missed bond.
4. Energize, check voltage at the bus phase to phase and phase to neutral, log the readings.
5. Thermal scan after 30 minutes under load if the site has an IR camera.

Turn over a red-lined single-line, the load calc, the torque log, and the arc flash label photo. The facility engineer will thank you the next time something trips and they need to trace it without opening the panel blind.