Quick Answer: The 2× Rule

Quick Answer: The 2× Rule

For most applications, choose a phase converter rated at LEAST 2 times your motor horsepower.

  • 5 HP motor → 10 HP phase converter minimum
  • 10 HP motor → 20 HP phase converter
  • 25 HP motor → 50 HP phase converter

Why 2×?

Three-phase motors draw significant starting current (locked rotor amperage) — typically 6-8× their running current. A properly sized rotary phase converter needs capacity to handle this startup surge.

Sizing by Application Type

Standard Applications (2× motor HP)

✅ Good for:

  • Air compressors (non-reciprocating)
  • Fans and blowers
  • Conveyor systems
  • Pumps (centrifugal)
  • Most machine tools under normal load

Example: 7.5 HP air compressor → 15 HP NL Rotary Phase Converter

Hard-Starting Loads (2.5-3× motor HP)

⚠️ Requires larger converter:

  • CNC machines with simultaneous axis movement
  • Reciprocating compressors
  • Punch presses
  • Large wood planers
  • Heavily loaded lathes/mills at startup

Example: 10 HP CNC mill → 30 HP NL Rotary Phase Converter (3× for safety margin)

Multiple Motors: Two Sizing Methods

Method 1: Largest Motor Rule

  1. Size converter to 2× your largest motor
  2. All other motors must be ≤ 50% of converter rating

Example:

  • Largest motor: 15 HP
  • Other motors: 5 HP, 3 HP, 2 HP (total 10 HP)
  • Converter needed: 30 HP NL (2× 15 HP)
  • Check: 10 HP others < 15 HP (50% of 30 HP) ✅

Method 2: Total Load Calculation

  1. Add nameplate amps of ALL motors
  2. Multiply by 1.25 safety factor
  3. Convert back to HP using phase converter amp ratings

When to use this: Running multiple motors simultaneously at high load.

Non-Standard Motors: Check Nameplate Amps

Some motors draw higher-than-normal current for their HP rating:

  • High-efficiency motors (NEMA Premium)
  • Variable torque motors
  • Older motors (60s-70s vintage)

How to check:

  1. Find nameplate full-load amps (FLA)
  2. Compare to standard motor amp chart
  3. If 10%+ higher, size up one converter size

Example:

  • 10 HP motor should draw ~28A at 230V
  • Your motor shows 32A on nameplate
  • Size up: 25 HP converter instead of 20 HP

Voltage Considerations

208-230V Motors (Most Common)

Standard sizing applies. All Phoenix converters output 220-240V three-phase.

460V Motors

Option 1: Use a 460V-input rotary converter
Option 2: Add a step-up transformer

575V Motors

Requires buck-boost transformer in addition to phase converter.

Start Type: Across-the-Line vs Reduced Voltage

Across-the-Line (Standard)

Full voltage applied instantly at startup. Requires standard 2× sizing.

Soft Start / VFD Starter

Reduces inrush current by ramping voltage gradually.

Benefit: Can use smaller phase converter (1.5× motor HP instead of 2×)

Example:

Real-World Sizing Examples

Example 1: Bridgeport Mill (2 HP)

  • Motor: 2 HP, standard tooling
  • Application: Moderate to light cuts
  • Recommended: 5 HP NL Rotary Converter (2.5× for safety)

Example 2: Air Compressor (10 HP Rotary Screw)

  • Motor: 10 HP, 28A at 230V
  • Application: Smooth rotary compressor (not reciprocating)
  • Recommended: 20 HP NL Rotary Converter (2×)

Example 3: CNC Lathe (15 HP Spindle + 5 HP Servos)

  • Largest motor: 15 HP spindle
  • Other motors: 5 HP (servos), run simultaneously under heavy load
  • Application: Hard-starting, simultaneous motion
  • Recommended: 40 HP NL Rotary Converter (2.5× safety margin)

Example 4: Woodshop (Multiple Tools)

  • Table saw: 5 HP
  • Dust collector: 3 HP
  • Planer: 3 HP
  • Jointer: 2 HP
  • Only ONE tool runs at a time
  • Recommended: 10 HP NL Rotary Converter (2× largest motor)

Phoenix vs Economy Phase Converters

Why Size Matters More with Cheap Converters

Phoenix (Made in Arizona):

  • 2× motor HP = reliable starting
  • Lifetime warranty covers undersizing errors
  • Over-built idler motors and capacitors

Economy imports:

  • Often need 3× motor HP to start reliably
  • Fail under sustained load
  • No support if you size wrong

Bottom line: Our 2× rule works because we build converters right the first time.

Still Not Sure? We'll Size It For You.

Free sizing consultation:

What we need:

  1. Motor nameplate info (HP, voltage, amps)
  2. What equipment it runs
  3. How you'll use it (heavy cutting? intermittent?)

We'll recommend the exact model — no guessing, no overselling.

Related Buying Guides

Quick Reference: Sizing Chart

Motor HP Standard Load Hard-Starting Multiple Motors*
2 HP 5 HP NL 7.5 HP NL 7.5 HP NL
3 HP 7.5 HP NL 10 HP NL 10 HP NL
5 HP 10 HP NL 15 HP NL 15 HP NL
7.5 HP 15 HP NL 20 HP NL 20 HP NL
10 HP 20 HP NL 30 HP NL 30 HP NL
15 HP 30 HP NL 40 HP NL 40 HP NL
20 HP 40 HP NL 50 HP NL 50 HP NL
25 HP 50 HP NL 60 HP NL 60 HP NL

*For multiple motors, verify smallest motor ≤ 50% of converter rating

Why Phoenix Phase Converters?

✅ Made in Phoenix, Arizona — not imported
✅ Lifetime warranty — we stand behind our sizing
✅ 50+ years engineering — founder Daniel Floreancig pioneered rotary converter design
✅ Free support — call us anytime for sizing help

Ready to order? → Shop Rotary Phase Converters