Rotary Phase Converter — Single Phase to 3 Phase Size Calculator, Input Amps, Wiring Diagram & vs VFD Guide
Rotary Phase Converter
Rotary phase converter size calculator
Size a single-phase to 3-phase rotary phase converter for your shop — HP recommendation, single-phase input amps, breaker, and wire guidance.
Load profile
Idler ≈ 1.5× largest motor HP (common shop rule)
Recommended rotary phase converter
15 HP
Calculated need ≈ 15.0 HP → next standard size
75 A
at 240 V single-phase
26 A
converter spinning unloaded
100 A
~125% of FLA, standard size
1 AWG
copper guidance — verify NEC
- Sized for a 10 HP largest motor with 5 HP of other motors that may run at the same time.
- Load profile “General machine tools” uses a 1.5× factor on the largest motor.
- At 240 V, expect about 75 A single-phase input FLA for a 15 HP rotary phase converter.
- Always follow the converter manufacturer’s wiring diagram, grounding, and local electrical code. Hire a licensed electrician for installation.
- Multiple motors: verify the largest motor can start while others are already running — that is usually the hardest case for a rotary phase converter.
Single-phase input amps by HP
Quick lookup for common sizes (5 HP, 10 HP, 20 HP, 30 HP, 60 HP rotary phase converter single phase input amps, etc.) at 240 V.
≈ 150 A single-phase input FLA
| RPC HP | 1φ input FLA ≈ | Breaker ≈ | Feeder ≈ |
|---|---|---|---|
| 3 HP | 15 A | 20 A | 12 AWG |
| 5 HP | 25 A | 35 A | 8 AWG |
| 7.5 HP | 38 A | 50 A | 6 AWG |
| 10 HP | 50 A | 70 A | 4 AWG |
| 15 HP | 75 A | 100 A | 1 AWG |
| 20 HP | 100 A | 125 A | 1/0 AWG |
| 25 HP | 125 A | 175 A | 4/0 AWG |
| 30 HP | 150 A | 200 A | 250 kcmil |
| 40 HP | 200 A | 250 A | 250 kcmil |
| 50 HP | 250 A | 350 A | 400 kcmil+ (verify with electrician / NEC) |
| 60 HP | 300 A | 400 A | 400 kcmil+ (verify with electrician / NEC) |
| 75 HP | 375 A | 469 A | 400 kcmil+ (verify with electrician / NEC) |
| 100 HP | 500 A | 625 A | 400 kcmil+ (verify with electrician / NEC) |
Rotary phase converter wiring diagram (overview)
Educational overview of how a rotary 3-phase converter sits between a single-phase panel and a three-phase load. Always use the manufacturer's wiring diagram and a licensed electrician.
High voltage can kill. This is not a DIY build plan or installation manual. Improper rotary phase converter wiring is a fire and shock hazard. Follow NEC / local code and the converter maker's instructions (American Rotary, Phase-A-Matic, Pro-Line style units, etc.).
Rotary phase converter vs VFD
A VFD is usually better for a single motor that needs soft-start and speed control. A rotary phase converter is better when you need true 3-phase for multiple machines, CNC controls, or loads that dislike VFDs.
Static vs rotary
Static converters are cheaper but typically only run motors at reduced capacity and are a poor fit for CNC. Rotary phase converters generate a manufactured third leg with an idler motor for full shop use.
Digital / smart RPC
Digital / smart rotary phase converters (e.g. microprocessor-balanced designs) improve voltage balance and starting — useful for CNC and precision equipment compared with a basic DIY rotary.
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Rotary Phase Converter — Size, Amps & Wiring Guide
Free tool to size a rotary phase converter (single phase to 3 phase). Get HP recommendations for 10 HP, 20 HP, 30 HP and larger shops, estimate single phase input amps, review a rotary phase converter wiring diagram overview, and compare rotary phase converter vs VFD.
Frequently asked questions
- What is a rotary phase converter?
- A rotary phase converter (RPC) turns single-phase utility power into three-phase power using an idler motor and controls. It lets you run 3-phase machines — mills, lathes, CNC, compressors — from a single-phase service.
- How does a rotary phase converter work?
- Single-phase power starts and runs an idler (generator) motor. Capacitors and controls help create a manufactured third leg (T3). The idler plus the two utility legs supply three-phase voltage to your load.
- How do I size a rotary phase converter?
- Start with your largest motor HP and load type (soft, general, hard start, or CNC). Many shops size the converter at 1.5×–2× the largest motor, and also account for other motors that run at the same time. Use this calculator for a recommended HP, single-phase amps, breaker, and wire size.
- Rotary phase converter vs VFD — which should I buy?
- Choose a VFD for one motor that needs soft-start and speed control. Choose a rotary phase converter when you need shop-wide 3-phase for multiple machines or equipment that prefers utility-like three-phase power.
- Where do I find a rotary phase converter wiring diagram?
- Use the overview diagram on this page for concepts, then follow the exact wiring diagram from your converter manufacturer (panel, ground, idler, manufactured leg). Installation should be done by a licensed electrician to code.
Researching American rotary phase converters, Pro-Line PL-series sizes, Phase-A-Matic, Phoenix, or Baileigh-style rotary phase converters for sale? Use the calculator above to estimate HP and amps first — then compare manufacturer charts for price, autostart panels, and CNC-rated digital models.
Curious how to build a rotary phase converter? A DIY idler can work for hobby loads, but CNC and modern controls usually need a quality commercial or digital RPC with proper balance, protection, and a code-compliant install.