Phase Converters

Article 455 covers phase converters — static and rotary devices that let three-phase motors and equipment run where only single-phase utility power is available, common on farms and in small shops. Because the converter creates a manufactured or 'wild' leg, its output doesn't behave like true utility three-phase.

The article sets conductor sizing on both the single-phase input and three-phase output, overcurrent protection, disconnecting means, and marking to warn about the manufactured phase. It ensures the unusual electrical characteristics of a converter are accounted for so the downstream equipment and wiring are protected.

When it applies

This applies when a site only has single-phase service but needs to run three-phase equipment — a shop with one three-phase machine, an irrigation pump, a compressor. A phase converter, rotary or static, synthesizes the third leg. The article sets how you size, protect and mark that manufactured supply.

Common mistakes

• Treating the generated leg as a normal utility phase and loading it beyond what the converter can hold up.
• Sizing conductors and overcurrent off the motor alone, ignoring the converter's own requirements.
• Leaving the system unmarked so the next tech does not realize one leg is manufactured.

In the field

With a rotary converter I keep the single-phase-only loads off the manufactured leg and size everything to the converter's listed capacity, not just the machine nameplate. The synthesized phase behaves differently under load, and respecting that keeps motors from running hot and unbalanced.