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

Sizing the feeder and subpanel for industrial loads

Industrial subpanels rarely look like their residential cousins. You are feeding motor loads, welders, VFDs, and long conductor runs, so start with a load calculation under NEC Article 220 Part IV and size the feeder conductors per NEC 215.2. Do not guess. Pull the actual nameplates, note the largest motor for the 125% adder under NEC 430.24, and document continuous loads at 125% per NEC 215.3.

Voltage drop will bite you on long industrial runs. NEC 215.2(A)(1) Informational Note 2 recommends 3% on feeders and 5% combined. On a 400 ft run feeding a CNC cell at 480V, that informational note becomes a hard design rule if you want the drives to ride through sags.

Common sizing checkpoints before you cut anything:

• Feeder ampacity verified against NEC 310.16 with correct temperature column for the terminations (NEC 110.14(C)).
• Neutral sized for nonlinear loads per NEC 220.61 and 210.4 if VFDs or SMPS gear are downstream.
• Equipment grounding conductor sized from NEC Table 250.122, upsized proportionally if ungrounded conductors are upsized for voltage drop (NEC 250.122(B)).
• Short circuit current rating of the subpanel meets or exceeds the available fault current at its line terminals (NEC 110.10, 110.24).

Overcurrent protection and coordination

The feeder OCPD at the main goes in per NEC 215.3 and 240.4. In industrial facilities, selective coordination is not just good practice, it is often required for emergency and legally required standby systems under NEC 700.32 and 701.32. Even where it is not mandated, production managers notice when one fault drops the whole plant.

For motor feeder taps and subpanel feeds handling group motor loads, apply NEC 430.62 for the feeder OCPD rating. Do not confuse this with branch circuit protection. The feeder breaker protects conductors, the motor branch device protects the motor circuit.

Pull the time current curves from the manufacturer before you set adjustable trips. A factory default LSIG setting will almost never coordinate with the upstream 1600A main. Ten minutes with the curves saves a two hour outage hunt.

Grounding, bonding, and the neutral question

This is where residential habits get industrial electricians in trouble. A subpanel is a separately derived system only if it is fed from a transformer or generator with its own derived neutral. A subpanel fed from the main service is not separately derived, so the neutral and ground must be kept isolated at the subpanel per NEC 250.24(A)(5) and 408.40.

Remove the main bonding jumper. Install a separate equipment grounding bus. Land the EGC on the ground bus, the grounded conductor on the insulated neutral bar. If the subpanel is in a separate building or structure, apply NEC 250.32 and decide whether you are running a four wire feeder with isolated neutral, which is the only option for new installations.

• Neutral bar isolated from enclosure: verified with an ohmmeter before energizing.
• Equipment bonding jumper installed on any metallic raceway entering the enclosure per NEC 250.92.
• Supplemental grounding electrode at separate structures per NEC 250.32(A), bonded to the EGC, not the neutral.

Enclosure, working space, and installation details

Working space kills more jobs than ampacity. NEC 110.26(A) gives you the depth table, but industrial environments add complications: piping, conveyor legs, and forklift traffic. Get 110.26(A)(1) depth, 110.26(A)(2) width of 30 inches or the equipment width, and 110.26(A)(3) headroom of 6.5 ft or the equipment height. For gear over 1200A, NEC 110.26(C)(2) requires two entrances to the working space.

Environmental rating matters. A NEMA 1 panel in a wash down area is a callback waiting to happen. Match the enclosure type to NEC Table 110.28 for the location. Dust tight, rain tight, corrosion resistant, and classified locations all pull from different rows.

If the plant engineer says the area is "maybe Class II Division 2," stop and get a written area classification drawing. Guessing wrong here means tearing out the install.

Conductor installation and terminations

Industrial feeders are often parallel sets. NEC 310.10(G) requires parallel conductors to be the same length, same conductor material, same size, same insulation, and terminated in the same manner. Cut them on the same reel, from the same end, and label as you pull.

Terminations deserve a torque wrench, not a lineman's pair of pliers. NEC 110.14(D) now explicitly requires calibrated torque tools for listed terminations. Document the torque values. When the breaker manufacturer specifies 375 in lb on a 400A lug, that is what goes in the closeout package.

1. Verify conductor insulation rating matches the termination temperature rating (NEC 110.14(C)).
2. Apply antioxidant compound on aluminum terminations per the lug manufacturer's instructions.
3. Torque to spec with a calibrated tool, then mark the termination with a paint pen for retorque verification.
4. Phase rotation verified before connecting to any three phase load.

Commissioning and documentation

Before you energize, megger the feeders phase to phase, phase to neutral, and phase to ground per the spec section or at minimum 1000V DC for 600V class conductors. Insulation resistance below 100 megohms on new conductors means you have a problem, not a pass.

Update the panel schedule, the one line diagram, and the arc flash study. NEC 110.16(B) requires the arc flash label on service equipment 1200A and above to show nominal voltage, available fault current, clearing time, and label date. Industrial facilities under NFPA 70E need the subpanel labeled too, with the incident energy from the updated study.

Leave the jobsite with the as built drawings, torque log, megger results, and phase rotation confirmation in the panel pocket. The next electrician who opens that door will either curse you or thank you.