Grounding, Bonding, and the NEC
Phase converter installations get failed inspections most often because of grounding errors. Here's what the NEC requires and how to do it right.
The Key Distinction: Grounding vs Bonding
Grounding connects the system to earth — it's a voltage reference and a fault-current return path to trip the breaker. Bonding connects metal enclosures and equipment frames together so they're at the same potential, preventing shock if a live wire contacts a chassis.
Both are required. Missing either one creates a shock hazard or prevents breakers from tripping on a fault.
Phase Converter Grounding
A rotary phase converter is a piece of utilization equipment, not a separately derived system. This means:
- The converter enclosure must be bonded to the equipment grounding conductor (EGC) from the panel
- There is no new neutral at the converter output — the neutral from the panel passes through or is not used on the three-phase side
- The three-phase output conductors are L1, L2, and L3 — no neutral
- The EGC (green wire or bare copper) runs from the panel to the converter and to all connected three-phase equipment
NEC References
| NEC Section | Requirement |
|---|---|
| 250.110 | Equipment grounding — metal enclosures must be grounded |
| 250.118 | Types of equipment grounding conductors |
| 250.122 | EGC sizing based on overcurrent device |
| 455.8 | Phase converter installation — grounding requirements specific to converters |
| 455.10 | Disconnecting means for phase converters |
Common Grounding Errors
- No EGC to converter enclosure — most common inspection failure
- Grounding to water pipe only — not acceptable as sole grounding electrode
- Bootleg ground — connecting neutral to ground at converter — creates circulating currents, code violation
- Undersized EGC — EGC must be sized to the overcurrent device per NEC 250.122 table
- Missing bond on motor frames — three-phase motors connected to converter output must have EGC to each motor frame
Inspector Checklist
- ☐ Converter enclosure bonded to EGC
- ☐ EGC sized per NEC 250.122
- ☐ EGC continuous from panel to converter to all motors
- ☐ No neutral-to-ground bond at converter (only at service entrance)
- ☐ Disconnect within sight of converter per NEC 455.10
- ☐ All motor frames bonded
Questions?
If your electrician or inspector has a specific question about the installation, call us. We work with electricians and AHJs regularly. (602) 962-8859