Grounding, Bonding, and NEC Code for Phase Converters

NEC Article 455: Phase Converters

The National Electrical Code dedicates Article 455 specifically to phase converters. This article governs how phase converters must be installed, grounded, and protected. Key requirements from Article 455 include:

• 455.4 — Marking: Phase converters must be marked with rated input and output power, voltage, frequency, and current
• 455.6 — Conductors: Single-phase input conductors must be sized per the converter's rated single-phase input current
• 455.7 — Overcurrent Protection: A disconnect and overcurrent protection device must be installed within sight of the converter
• 455.8 — Grounding: Phase converter equipment must be grounded per Article 250
• 455.20 — Disconnecting Means: A disconnect must be provided within sight of the converter — and within sight of the equipment it serves

Equipment Grounding (Article 250)

What Is Equipment Grounding?

Equipment grounding provides a fault current return path that trips a breaker or blows a fuse when a conductor accidentally contacts a metal enclosure. Without it, a fault makes the equipment enclosure energized at line voltage — a lethal hazard.

An equipment grounding conductor (EGC) must run from your main panel's ground bus:

• Through the conduit to the phase converter's enclosure ground terminal
• From the converter's output ground terminal through the output conduit
• To each piece of three-phase equipment's ground terminal

Equipment Grounding Conductor Sizing

Size the EGC based on the ampacity of the overcurrent device (breaker) protecting the circuit, per NEC Table 250.122:

These are minimums — some jurisdictions require a larger EGC. If you're using aluminum conductors for the main circuit, use copper for the EGC (one size up from the copper equivalent).

Bonding

What Is Bonding?

Bonding connects metal parts of the electrical system together to ensure they're at the same potential. This prevents voltage differences between conductive surfaces — which can cause arcing, stray currents, and shock hazards.

What Must Be Bonded

• The converter enclosure (NEMA 4 steel enclosure)
• All metallic conduit connected to the converter
• Each piece of three-phase equipment powered by the converter
• Metal machine frames and guard structures on equipment
• Any metallic cable tray used to route wiring

Bonding Jumpers

Where metallic conduit is used as the grounding path, bonding bushings or bonding jumpers must be installed at each end of the conduit run — at the panel knockout and at the converter enclosure. Locknuts alone are not considered reliable bonding connections in many jurisdictions.

The Generated Leg (T3) and Grounding

One frequently misunderstood aspect of phase converter grounding involves the generated leg (T3). T3 is the synthetic third phase created by the converter. NEC 455.23 requires that the generated leg be identified at each junction and termination point — typically with an orange conductor or orange tape on the T3 terminal.

The generated leg must never be used as the grounded (neutral) conductor. It's a phase conductor only. The generated leg does NOT carry different grounding treatment — it connects to the machine's T3 motor terminal, just like T1 and T2.

Disconnecting Means Requirements

NEC 455.20 requires two disconnects:

1. A disconnect for the converter itself — must be within sight of the converter. This is typically the two-pole breaker in your main panel, or a separate disconnect switch mounted near the converter.
2. A disconnect at each motor-driven machine — must be within sight of the machine. This is the motor disconnect (fusible switch or circuit breaker) mounted at the machine.

"Within sight" means the disconnect must be visible and not more than 50 feet from the equipment it serves, per NEC 430.102.

Inspector-Ready Installation Checklist

Use this checklist before calling for inspection:

Common Grounding Violations

These are the most common grounding issues found during inspection:

• Missing EGC — running only the phase conductors, forgetting the ground wire
• Undersized EGC — using leftover small wire for ground instead of properly sized conductor
• No bonding bushing — locknuts alone at conduit entry points
• Generated leg not identified — T3 must be orange or labeled at every terminal
• No disconnect at machine — the converter disconnect is not a substitute for the machine disconnect
• Ground pigtailed to neutral — in subpanel installations, neutral and ground must be kept separate at any location other than the service entrance