Phase Converter Sizing: Get It Right the First Time

Choosing the right size phase converter is the single most important decision you'll make when converting single-phase power to three-phase. An undersized converter won't start your equipment. An oversized one wastes money. This step-by-step phase converter sizing guide will help you determine exactly what size phase converter you need — whether you're running a CNC machine, air compressor, HVAC system, or an entire shop.

With over 50 years of experience, Phoenix Phase Converters has helped thousands of customers size their systems correctly. Here's how we do it.

Step 1: Identify Your Largest Motor

Start with the nameplate horsepower (HP) of your largest single motor. This is your baseline. Every phase converter must be rated to start and run your biggest load.

Example: You have a 10 HP CNC mill, a 5 HP lathe, and a 3 HP dust collector. Your starting point is 10 HP.

Step 2: Account for Multiple Machines

If you're running multiple machines simultaneously, add up the horsepower of everything that runs at the same time, then add 25–50% as a buffer.

The Rule of Thumb:

  • Match or exceed your largest motor's HP rating
  • Add 25–50% for shops running 2+ machines simultaneously
  • Round up to the next available converter size

Example: Running the 10 HP CNC and 5 HP lathe at the same time? That's 15 HP combined. Add 30% and you're looking at roughly 20 HP. A 20 HP rotary phase converter handles this with ease.

Step 3: Adjust for Hard-Starting Loads

Some equipment draws significantly more power at startup than during normal operation. These hard-starting loads include:

  • Air compressors — especially reciprocating/piston types
  • Grain dryers and agricultural equipment
  • Large pumps and hydraulic presses
  • Refrigeration compressors

For hard-starting loads, size your converter at 1.5x to 2x the motor's rated HP. A 10 HP compressor? Plan on a 15–20 HP phase converter.

Step 4: Consider Future Expansion

Adding equipment later? It's far cheaper to buy a slightly larger converter now than to replace one later. If you're building out a shop, consider sizing for where you'll be in 2–3 years.

Real-World Sizing Examples

Small Machine Shop

  • 10 HP CNC milling machine
  • 5 HP lathe
  • 2 HP surface grinder
  • Running 2 machines at once max

Recommended: 20 HP Rotary Phase Converter

Farm with Grain Dryer

  • 15 HP grain dryer (hard-starting)
  • 5 HP auger motor

Recommended: 30 HP Rotary Phase Converter (2x for the hard-starting dryer)

HVAC Contractor Shop

  • 7.5 HP compressor (hard-starting)
  • 3 HP exhaust fan
  • 5 HP package unit for testing

Recommended: 15 HP Rotary Phase Converter

Mixed-HP Workshop (Wide Range)

  • 25 HP primary machine
  • Several small 1–3 HP tools

Recommended: Our DualZone phase converter is purpose-built for shops with a wide HP range. It delivers clean, balanced power to both large and small motors without the voltage issues that plague standard converters running small loads.

Common Sizing Mistakes

Mistake #1: Undersizing

An undersized converter can't deliver enough starting torque. Your equipment may not start, may run hot, or may trip breakers. Worst case, you damage expensive motors.

Mistake #2: Extreme Oversizing

While a little headroom is good, a massively oversized converter running tiny loads can produce unbalanced voltage. This is where the DualZone excels — it handles mixed loads that would challenge a standard rotary converter.

Mistake #3: Ignoring Starting Current

Nameplate HP tells you running load, not starting load. Compressors and other hard-starting equipment can draw 3–6x their rated current at startup. Always account for this.

Still Not Sure What Size You Need?

Every application is different. That's why we offer free, expert sizing consultations. Call us at 800-417-6568 and tell us what you're running — we'll recommend exactly what you need. No guesswork, no overselling.

All Phoenix Phase Converters are Made in the USA and backed by a lifetime warranty. Browse our full line of rotary phase converters or read our Ultimate Guide to Phase Converters for more in-depth information.