What Size Phase Converter for a 10 HP Motor? 8 Makers' Answers
A 10 hp motor needs a phase converter labeled anywhere from 10 hp to 30 hp. Part of the spread is what the label counts. American Rotary's manual rates its AR 10 for 10 hp of total load, meaning all the motors running at once, but only a 5 hp largest motor. American Rotary's FAQ and Phoenix's sizing guide both say to double the motor. That puts a 10 hp lathe or mill on a 20 hp rotary converter. Phase Technologies and ARCO rate their 10 hp units to start a 10 hp motor.
How hard the machine starts moves the number too. A piston compressor starts with a load already on it, and a dust collector has a heavy fan to bring up to speed. Phoenix's sizing guide puts a 10 hp piston compressor at 25 hp and a 10 hp dust collector at 30 hp.
To compare makers, look past the label. Find the largest motor the unit starts for your kind of load. Most makers publish that figure in a table or a rule. When yours doesn't, ask the maker. At Built for Production, we read eight phase converter makers' sizing documents and put each one's answer for a 10 hp motor on this page, with the math and the document it comes from. Figures are from the makers' documents as read in October 2026.
Chapters: 0:00 What size converter for a 10 hp motor · 0:32 Why a 10 hp converter isn't always 10 hp · 1:12 Lathe or mill: each maker's answer · 1:55 Piston compressor and dust collector · 2:28 What your panel needs · 2:53 How to compare makers
Why doesn't a 10 hp phase converter always run a 10 hp motor?
The 10 hp on the label counts different things at different makers. At some it's the total load, and at others it's the largest motor the unit starts. Here's what a unit labeled 10 hp is rated for at five makers:
- American Rotary AR 10: largest load 5 hp, 10 hp total, per its AR user manual (R2.7).
- NAPCco PL-10: listed for 7.5 hp motor loads on an Easy start, 6 hp Medium, 5 hp Hard and 3 hp Very Hard, per its Need Help Sizing chart. NAPCco's tiers are explained below the main table.
- Phase-A-Matic R-10: largest motor 6 hp at 230V, per its Rotary Phase Converter Instruction Sheet.
- ARCO HD10: largest motor 10 hp, per its Heavy Duty series page.
- Phase Technologies PT010: a digital (solid state) converter, sold as Phase Perfect. Its manual says Phase Perfect units are "rated to across the line start motors up to the nameplate horsepower rating of the convertor except for 50 HP models and larger." Across the line means the motor is switched straight on at full voltage. So the PT010 starts a 10 hp motor.
A unit labeled 10 hp starts a 5 hp motor at American Rotary and a 10 hp motor at Phase Technologies.
If the 10 hp machine runs alongside other 3 phase machines, the math changes. Our phase converter sizing guide walks through American Rotary's, Phoenix's, NAPCco's and Phase Technologies' rules in full, including several machines at once.
What size phase converter does each maker name for a 10 hp motor?
For an ordinary load like a lathe or a mill, here's the unit each maker's own rule or table names. All are rotary converters except the Phase Perfect PT010, which is digital.
| Maker | Unit for a 10 hp lathe or mill | How the maker gets there | Document |
|---|---|---|---|
| American Rotary | AR 20 | 2x the motor: 10 x 2 = 20. Its manual lists the AR 20 for a 10 hp largest motor | Phase converter FAQ; AR user manual R2.7 |
| Phoenix Phase Converters | GP20NL (20 hp). Its other pages give 10 and 15 hp | 2x the largest motor: 10 x 2 = 20. The 20 HP page calls a 10 hp lathe a Hard load and gives the GP20NL a 10 hp Hard-load start rating, so that page covers a 10 hp lathe with the GP20NL | Phase Converter Sizing Guide; 20 HP Rotary Phase Converter page |
| North America Phase Converters (NAPCco) | PL-15 to PL-20, or SB-15 to SB-20 | By how hard the load starts (tiers below) | Need Help Sizing chart |
| Phase Technologies (Phase Perfect) | PT010 (digital) | Starts motors up to its 10 hp nameplate | Digital Phase Converter Operation & Installation Manual V4.7 |
| Kay Industries (Phasemaster) | MA-2 by its table. MA-3 by our math on its 2026 manual | Table: 10 hp largest motor. Manual: starting hp up to 75% of the rating | Sizing and Selection Tables; MA Series manual, revised 7-15-2026 |
| Phase-A-Matic | R-15 at minimum | Largest motor 10 hp at 230V. "50% larger or more": 10 x 1.5 = 15 | Rotary Phase Converter Instruction Sheet |
| Ronk Electrical | Type 2P at 20 kVA, or Type 2D-1 at 12 kVA | Largest motor 10 hp (the 2P on an easy start) | Types P & D-1 spec sheet |
| ARCO Electric Products | Model B (Standard Duty series) | Largest motor 10 hp | Standard Duty series page |
Phoenix gives three answers
- 20 hp from its sizing guide: "For most motors, your phase converter should be 2× the HP of your largest motor."
- 10 hp from its 10 HP Rotary Phase Converter page, which says its 10 hp units are "sized to run any single three-phase motor up to 10 HP."
- 15 hp for a mill from its Sizing Wizard page, which uses 1.5x for mills and saws and 2x for lathes.
Ask Phoenix which one applies to your machine.
Kay gives two answers
- Its Sizing and Selection Tables list the 10 hp MA-2 for a 10 hp largest motor.
- Its 2026 MA Series manual says, "Total 3 phase motor starting HP must not exceed 75% of the rated HP of the converter." On the MA-2, that's 10 x 0.75 = 7.5 hp, so under the manual the MA-2 won't start a 10 hp motor. Kay doesn't name a replacement. By our arithmetic on the same line, the 15 hp MA-3 starts up to 15 x 0.75 = 11.25 hp, so it starts a 10 hp motor.
Ask Kay which document governs before you order.
NAPCco sizes by how hard the load starts
NAPCco's quick charts cover two of its rotary lines, the Pro-Line (PL) and the Smart-Boost (SB). NAPCco describes Smart-Boost as having a "current booster for those hard starting / high inertia loads." Its Motor Load Chart sorts machines into tiers: mills are Easy or Medium, lathes with a clutch are Medium or Hard, and lathes without a clutch are Hard. Its quick chart then lists the motor hp for each unit in each tier. A lathe or mill lands in the first three rows below, and the Very Hard row is for loads like a piston compressor. For a 10 hp motor, the smallest units listed at 10 hp or more are:
| Tier | Pro-Line | Smart-Boost |
|---|---|---|
| Easy | PL-15 | SB-15 |
| Medium | PL-20 | SB-15 |
| Hard | PL-20 | SB-20 |
| Very Hard | PL-25 | SB-20 |
Ronk rates in kVA
Ronk rates its converters in kVA (kilovolt-amperes, a measure of electrical capacity) instead of hp. Its FAQ says a Type P converter starts a largest motor up to half its kVA rating: 20 kVA x 0.5 = 10 hp. It says its Type D and D-1 start a single motor close to their kVA rating, which is why the 12 kVA Type 2D-1 lists a 10 hp motor.
ARCO has two 10 hp units
ARCO lists a 10 hp largest motor on two units in two series. The Model B is in its Standard Duty series. The HD10 is in its Heavy Duty series. The Heavy Duty page names the uses that series is for, including air compressors, refrigeration compressors and grain dryers.
What size phase converter for a 10 hp air compressor, dust collector or CNC?
On a hard-starting load, several makers name a bigger unit. For a 10 hp piston compressor, Phoenix's sizing guide says 25 hp and NAPCco's chart lists a PL-25 or an SB-20. For a 10 hp dust collector, Phoenix's sizing guide says 30 hp. Phoenix's 20 HP Rotary Phase Converter page lists dust collectors as a Medium load instead, so ask Phoenix which applies. Phase Technologies rates its units to start motors up to the converter's own nameplate hp, with no separate rule by load. By that rule, the 10 hp PT010 is still its answer for a 10 hp motor on these loads.
Phoenix groups piston compressors with "equipment that starts under load" and sizes them at 2.5x. A dust collector's big fan is a high-inertia load, heavy and slow to come up to speed, and Phoenix sizes those at 3x.
| Maker | Piston compressor | Dust collector or big fan | CNC |
|---|---|---|---|
| American Rotary | Add 25 to 50% capacity; base not stated | No separate rule | No separate rule |
| Phoenix | 2.5x: 25 hp (sizing guide) | 3x: 30 hp (sizing guide). 20 HP page: a Medium load | Roughly 2x spindle: 20 hp (CNC guide). 2.5x with servo rapid traverse: 25 hp (sizing guide) |
| NAPCco | Very Hard: PL-25 or SB-20 | Hard: PL-20 or SB-20 | 10 hp spindle: PL-20 or SB-20 |
| Phase Technologies | Same rule: nameplate hp | Same rule: nameplate hp | Same rule: nameplate hp |
| Phase-A-Matic | No separate rule | No separate rule | Rigid tapping: R-20 |
| Ronk Electrical | No separate rule | No separate rule | No separate rule |
| ARCO | Heavy Duty series lists air compressors; HD10 is its 10 hp unit | No separate rule | CNC series covers 1 to 50 hp; no hp per model |
"No separate rule" means the documents we read give no figure for that load. Kay publishes no figures by load type in the documents we read. If your load isn't covered, ask the maker.
Piston compressor
American Rotary's FAQ says, "For hard-starting loads (compressors, punch presses), add 25-50% capacity." It doesn't say what to add it to. We added it to the 20 hp from its 2x rule, which gives 25 to 30 hp. Confirm that base with American Rotary before you order.
Phoenix's 20 HP Rotary Phase Converter page says, "The GP20NL is rated for a maximum 7.5 HP compressor." So on that page, a 10 hp compressor needs more than Phoenix's 20 hp unit.
Ronk publishes no rotary size for a compressor. Its General Usage Guide matches a cycling piston compressor, one that starts and stops as the tank fills, to ADD-A-PHASE, its static line. Its spec sheet has a hard start column, where the 30 kVA Type 2P is the first to list 10 hp, but Ronk doesn't say a compressor belongs in that column. Ask Ronk before you order.
ARCO doesn't say the HD10 is for a 10 hp compressor, so confirm with ARCO.
CNC
Phoenix's CNC guide says roughly 2x the spindle hp, or 20 hp. Its sizing guide says 2.5x, or 25 hp, for CNC machines with servo rapid traverse. Your CNC's manual or spec sheet tells you whether yours has it. Phase-A-Matic's sizing guidelines say instant reversing, as in rigid tapping, takes "twice the HP rating of the motor." For a 10 hp spindle, that's 10 x 2 = 20 hp, which is its R-20. ARCO's Standard Duty and Heavy Duty pages say to consult the factory for sizing on instant-reversing applications.
A CNC also has a limit on voltage imbalance, which is how far apart the voltages are on the three wires of the 3 phase supply. A lower figure means a tighter match. Phase Technologies' manual lists "Output Voltage Imbalance ≤2%" for Phase Perfect units. ARCO's CNC series page says "Voltage Balance Within 2% or Less Under All Loading Conditions." Your CNC maker's manual sets how much imbalance its control accepts, so compare these figures to that.
How many amps does a 10 hp phase converter setup need?
Your panel feeds the converter's single phase side, and makers publish two kinds of figure for it. American Rotary gives a minimum single phase service size for each unit: 60 A for its 20 hp converter, 80 A for 25 hp and 100 A for 30 hp. Compare that to your main breaker. Phase Technologies and Ronk give input current, the amps the unit draws from your panel. Your electrician adds that to everything else the panel already carries.
Each maker publishes these figures for its own units:
| Unit | Figure | What it measures | Document |
|---|---|---|---|
| American Rotary 20 hp | 60 A | Minimum single phase service | Phase converter FAQ, "Amperage Requirements (240V)" |
| American Rotary 25 hp | 80 A | Minimum single phase service | Same |
| American Rotary 30 hp | 100 A | Minimum single phase service | Same |
| Phase Perfect PT010 | 62 A at 187 to 260 VAC | Maximum AC input current | Operation & Installation Manual V4.7 |
| Ronk Type 2P, 20 kVA | 96 A at 240V | Single phase input current | Types P & D-1 spec sheet |
| Ronk Type 2P, 30 kVA | 135 A at 240V | Single phase input current | Same |
| Ronk Type 2D-1, 12 kVA | 54 A at 240V | Single phase input current | Same |
A licensed electrician sizes every install to the NEC and local code.
If your house or barn has a 100 A panel, American Rotary's 100 A minimum for its 30 hp unit equals the whole panel. Talk to your electrician before you order, and ask:
- Does my service have room for this unit's input amps with the rest of the house or shop running?
- What does the maker's manual call for on breaker and wire for this exact model, and does local code change that?
- Do I need a permit or an inspection here?
On the motor side, a 10 hp 3 phase motor at 230V draws roughly 24 to 28 A at full load, going by three Baldor catalog motors: 24 A (CEM3714T-G), 26 A (EM2238) and 28.2 A (VEM2332T). Your own nameplate has the real number, and you'll need it for the VFD math below.
Can a VFD run a 10 hp motor on single phase power?
A VFD (variable frequency drive) can run a 10 hp motor from single phase power. It's the one-machine option. For several 3 phase machines, look at a converter. There are two ways to do it with a VFD.
A drive built for single phase input
Few drives take single phase in at this size. Lenze's application note on derating VFDs for single phase input says, "Most manufacturers of VFDs only offer products up to 3 Horsepower (HP) for single phase input."
Invertek lists one at 10 hp. Its Optidrive E3 datasheet for the ODE-3-420300-104A shows 200 to 240V single phase in and 3 phase out, with 30 A output. Its continuous supply current, what it draws from your panel, is 66 A. Invertek lists it only in an IP66 enclosure, which it labels for outdoor use. Invertek's own E3 specification page still caps 230V single phase input at 5 hp, so confirm the model with Invertek before you order.
A 3 phase drive run below its rating
The other way is to buy a drive built for 3 phase input, rated well above the motor, and feed it single phase. Lenze's note gives the rule as "VFD Input Current > Motor Current Rating * 1.73." Its example 10 hp motor draws about 27 A at 230V. 27 x 1.73 = 46.71, so the drive's input current has to be more than 46.71 A, and Lenze recommends a drive rated at or above 47 A. TECO-Westinghouse takes a stricter line in its E510 instruction manual: "Please consult the factory when applying single phase input for all models rated 230 & 460V 5HP and above."
Our guide to rotary vs digital phase converters vs VFDs compares the three types.
Will a static phase converter run a 10 hp motor?
By Phase-A-Matic's figures, a static converter runs a 10 hp motor at reduced power. Its UL Series brochure lists the UL-1200HD static converter for 8 to 10 hp motors. It says the converter runs wye-wound motors at 2/3 of rated hp and delta-wound motors at 50%. On a 10 hp motor, that's about 6.7 hp or 5 hp. Ask the motor's maker or your electrician which winding yours has. NAPCco's Motor Load Chart marks piston air compressors "Static Converter not recommended." Our static phase converter guide covers the hp loss in more detail.
What should you have ready before you order?
Whether you size it yourself, call a maker or call us, have these numbers ready:
- Motor hp from the nameplate, for every motor on the machine, such as a coolant pump or a power feed. Each one adds to the load the converter carries.
- Voltage: 230V, 460V, or dual voltage like 230/460
- FLA (full load amps) at the voltage you'll run
- What the motor drives and whether it starts under load: lathe, mill, piston or screw compressor, dust collector, CNC spindle
- For a CNC: spindle hp, whether it does rigid tapping, and the voltage tolerance in the CNC maker's manual
- Other 3 phase machines that run at the same time
- Your single phase service in amps, from the main breaker
Our phase converter sizing calculator takes part of that list: the machine type, the motor hp or its FLA at 208 to 240V, the total hp running at once, the machine voltage and your service amps. It applies American Rotary's, Phoenix's, NAPCco's and Phase Technologies' published rules and names the document for each. It also shows a static converter result from Phase-A-Matic's figures, for comparison. Keep the CNC details for a call to us or the maker. The calculator doesn't apply Ronk's rules. For a Ronk unit, ask Ronk: its General Usage Guide says, "All converter quotes must come from the factory to ensure proper sizing and application."
Documents named in this guide
- American Rotary: Phase converter FAQ; User Manual AR Low Voltage 208-240V, R2.7
- Phoenix Phase Converters: Phase Converter Sizing Guide (2026-02-25); The Complete Guide to Phase Converters for CNC Machines 2026; 10 HP and 20 HP Rotary Phase Converter pages; Sizing Wizard
- North America Phase Converters (NAPCco): Need Help Sizing, quick chart and Motor Load Chart
- Phase Technologies: Digital Phase Converter Operation & Installation Manual, V4.7
- Kay Industries: Sizing and Selection Tables; Installation & Operation Instructions, Phasemaster MA Series, 240v (revised 7-15-2026)
- Phase-A-Matic: Rotary Phase Converter Instruction Sheet; RTN: Guidelines for Sizing Phase-A-Matic Rotary Phase Converters; Static Phase Converter UL Series brochure
- Ronk Electrical: Types P & D-1 Rotary Phase Converters spec sheet; FAQs; Phase Converter General Usage Guide
- ARCO Electric Products: Standard Duty, Heavy Duty and CNC series pages
- Invertek Drives: Optidrive E3 datasheets and specification page
- Lenze Americas: application note on derating VFDs for single phase input
- TECO-Westinghouse: E510 Inverter Instruction Manual
- Baldor: catalog pages CEM3714T-G, EM2238 and VEM2332T
Want the math run on your nameplate?
Call us at (855) 923-2610 or email contact@builtforproduction.com with a photo of your motor nameplate. We'll run it through the makers' published rules and show you the arithmetic. Anything about your panel or wiring goes to a licensed electrician and your local inspector. You can also reach us through our contact page.