100 Amp Phase Converter: Can You Size a Converter by Amps?
You can size a phase converter by amps once you turn the amps into motor horsepower. A 3 phase motor that draws about 100 A at 230V is close to a 40 hp motor. For that motor, the rotary phase converter makers' own sizing rules call for a converter between 40 hp and 120 hp.
Which size in that range you need depends on how hard the machine is to start. Phoenix Phase Converters and American Rotary's FAQ set the converter's hp as a multiple of the motor's hp. Phoenix's sizing guide uses 2 times the motor's hp for standard loads like a lathe or a saw, 2.5 times for hard-starting compressors, and 3 times for a dust collector. Phoenix also publishes a second rule, in its article on why it doesn't size by amps alone: a converter the same hp as the motor for the easiest loads. On a 40 hp motor, that rule gives the 40 hp low end of the range.
At Built for Production, we put each maker's published rule side by side and show the math. Before you use any of it, check which 100 A you have. "100 amp phase converter" can mean the amps your machine draws, the amps the converter pulls from your single phase panel, or your panel's rating. A converter pulls more amps from the panel than it sends to the machine, so 100 A going in runs a smaller machine than 100 A coming out. Your panel's rating also limits which converter you can run, because the converter draws its power from the panel. American Rotary lists a minimum service, meaning the power supply feeding your panel, for its converter sizes.
Can you size a phase converter by amps?
A machine's running amps alone can't size a rotary phase converter. You need the hp those amps point to and how hard the machine starts. Phoenix's article on why it doesn't size by amps alone puts it this way: "The amperage on a motor or equipment nameplate usually shows the running load, not the starting load."
A motor pulls far more current while it starts. ABB's motor protection course gives the starting current as "typically 6 times the Full Load Current." On a 100 A motor, ABB's 6 times is about 600 A for the moment it starts. The same Phoenix article says a phase converter "must be large enough to handle that start-up demand".
Digital phase converters and VFDs with single phase input are the exception. The section "When do you size a converter by amps?" below shows the makers that size them from amps.
What size phase converter do you need for 100 amps?
Turn 100 A into motor hp
Find your amps in a 3 phase motor table and read across to hp. Yaskawa's application note AN.AFD.15 prints the standard full load amps (FLA) for 230V motors from the NEC (National Electrical Code) motor table. Your motor's nameplate wins over any table.
| Motor hp | Full load amps, 3 phase 230V |
|---|---|
| 5 | 15.2 |
| 7.5 | 22.0 |
| 10 | 28.0 |
| 15 | 42.0 |
| 20 | 54.0 |
| 25 | 68.0 |
| 30 | 80.0 |
| 40 | 104.0 |
100 A falls between the 30 hp row (80 A) and the 40 hp row (104 A), so the answers below use a 40 hp motor. If your nameplate shows hp, use that instead. For a 10 hp motor at 28 A, our 10 hp phase converter post works through every maker's answer. For 208V or 460V machines, see our post on 3 phase voltage.
Apply each maker's rule
On most rows, the converter size depends on how hard your machine is to start, so find your machine's load type in the rule column. American Rotary's 100 amp page gives one answer whatever the machine.
| Maker's document | The rule | Converter for a 40 hp, 100 A motor |
|---|---|---|
| American Rotary, 100 Amp Phase Converter page | About 100 A of 3 phase goes on a 75 hp unit, rated for loads up to 37.5 hp or 105 A | 75 hp |
| American Rotary, FAQ | At least 2 times the load's hp; add 25 to 50% capacity for hard-starting loads like compressors and punch presses | 80 hp; 100 to 120 hp for hard starts (our math: 25 to 50% added to 80 hp) |
| Phoenix, sizing guide | By equipment and load type: 2 times the motor hp for standard loads like lathes, mills and saws; 2.5 times for hard-starting compressors; 3 times for dust collectors and large fans | 80, 100 or 120 hp |
| Phoenix, article on why it doesn't size by amps alone | Four load types, easiest to hardest: the same hp as the motor, then 1.5, 2 and 2.5 times | 40, 60, 80 or 100 hp |
| NAPCco (North America Phase Converters), sizing charts | Lists the largest motor each model starts, for Easy, Med, Hard and Very Hard load types. The Pro-Line chart labels its models in hp; the Smart-Boost chart doesn't | Easy: Smart-Boost SB-50. Med: Smart-Boost SB-60. Hard: Pro-Line PL-80 (80 hp). Very Hard: Pro-Line PL-100 (100 hp) |
Two of these makers publish two answers each:
- American Rotary. Its 100 amp page puts loads up to 105 A on its 75 hp unit, so a machine drawing 100 A fits it. Its FAQ calls for a converter at least twice the load's hp, so a 40 hp motor gets an 80 hp converter. The FAQ also adds 25 to 50% capacity for hard-starting loads like compressors and punch presses. Adding 25 to 50% to the 80 hp gives 100 to 120 hp (our math). The 75 hp and 80 hp answers differ because the 100 amp page treats 105 A as a 37.5 hp load, and 2 times 37.5 is 75. The NEC table above lists 104 A as a 40 hp motor, and 2 times 40 is 80. Which one applies to your machine is American Rotary's call.
- Phoenix. Its sizing guide starts at 2 times the motor hp. Its article on why it doesn't size by amps alone starts at the same hp as the motor for the easiest loads. If you're looking at a Phoenix unit, ask Phoenix which page applies to your machine.
For CNC machines and heating elements, NAPCco's charts size by total amps instead of motor hp. Its PL-80 row lists 100 total amps. Each maker's full rules are in our phase converter sizing guide.
What does "100 amp phase converter" mean?
| If your 100 A is... | Where you read it | Converter it points to |
|---|---|---|
| The amps your 3 phase machine draws | Motor nameplate | 40 to 120 hp, by load type (table above) |
| The amps the converter pulls from your panel | Converter nameplate, input side | About 20 hp: Phoenix lists 88 to 97 A input for its 20 hp unit |
| Your panel's rating | Main breaker | 30 hp: American Rotary lists 100 A as the smallest service its 30 hp unit needs |
A licensed electrician sizes every install to the NEC and local code.
How many amps does a phase converter draw from a single phase panel?
A phase converter pulls about 1.73 to 2 times as many amps from your single phase panel as it delivers to your 3 phase machine, depending on whose figure you use. The 1.73 comes from the 3 phase power formula: at the same voltage, single phase needs 1.732 times the amps to carry the same power, as EC&M's motor calculations article by Mike Holt shows.
| Maker's document | Input ratio | Single phase amps drawn for 100 A of 3 phase load |
|---|---|---|
| Phase Technologies, sizing reference (digital) | 1.73 | 173 A |
| Phoenix, amperage guide (rotary) | About 1.73 | 173 A |
| Phoenix, article on how much power converters draw | About 2 | 200 A |
The right-hand column is our math on each maker's ratio. Phoenix's amperage guide also lists running input at 240V for its own units: 45 to 52 A for 10 hp, 88 to 97 A for 20 hp, and 128 to 140 A for 30 hp.
The number your electrician works from is the single phase input amps on the converter's nameplate. In EC&M's summary of NEC Article 455, the code sizes the converter's supply wiring from the "phase converter nameplate single-phase input full-load amperes." Take that figure, or the maker's spec sheet, to your electrician.
Can a 100 amp panel run a phase converter?
A 100 A panel can't feed a converter carrying 100 A of 3 phase load, on these makers' figures. That load pulls 173 to 200 A of single phase from the panel.
Here is what two makers publish near 100 A, measured two different ways:
- Phoenix, running input: its amperage guide lists 88 to 97 A of running input for its 20 hp converter, close to all of a 100 A panel. On the 2 times rule in Phoenix's sizing guide for standard loads, a 20 hp converter runs a 10 hp motor (our math: 20 / 2).
- American Rotary, minimum service: its FAQ lists 100 A as the minimum service for its 30 hp converter. On the FAQ's 2 times rule, that converter runs a 15 hp motor (our math: 30 / 2).
For hard-starting loads, both makers call for more converter per motor hp, so the largest motor a given converter can run is smaller.
Here are more rows from that American Rotary FAQ list, under "Amperage Requirements (240V)":
| American Rotary converter | Minimum single phase service |
|---|---|
| 20 hp | 60 A |
| 25 hp | 80 A |
| 30 hp | 100 A |
| 40 hp | 125 A |
| 75 hp | 200 A |
American Rotary publishes this table for its own units. A licensed electrician sizes every install to the NEC and local code.
At 30 hp, American Rotary's listed minimum equals the whole 100 A service, before the rest of the shop or house is counted. Its 75 hp unit, the one its 100 amp page names for 100 A of load, lists a 200 A minimum.
Phoenix's amperage guide says a 200 A service handles most shop applications with converters up to 25 to 30 hp. For larger converters, it says you may need a 320 A or 400 A service.
Whether your service can carry a converter is a licensed electrician's call, and sometimes the utility's.
When do you size a converter by amps?
A rotary converter makes the third leg with a spinning idler motor. Its makers size it by motor hp. A digital phase converter makes 3 phase electronically. Phase Technologies sizes its Phase Perfect digital converters by output amps. A VFD (variable frequency drive) with single phase input runs one 3 phase motor and controls its speed. Lenze and Yaskawa size it from that motor's amps.
| Equipment and document | Maker's rule | Answer for 100 A of load |
|---|---|---|
| Digital: Phase Technologies, sizing reference (rule) and Phase Perfect Gen4 manual (ratings) | Size by output amps: the model's max output must cover the load | Phase Perfect PT040: 130 A max steady output, 225 A max input. The Phase Perfect PT030's max steady output, 95 A, is short of 100 A |
| VFD: Lenze's application note on derating VFDs for single phase input | Drive input rating above motor amps x 1.73 | A drive rated above 173 A input (100 x 1.73) |
| VFD: Yaskawa AN.AFD.15, Table 1 | Drives matched to 230V motors on 240V single phase input | 2075 drive for a 40 hp, 104 A motor: 117.4 A out, 236.1 A in |
Derating here means picking a bigger drive because it runs on single phase input. Yaskawa's table shows how much bigger: a drive rated for 117.4 A of output for a 104 A motor.
The PT040's 225 A max input and the Yaskawa drive's 236.1 A are both more than a 200 A service. If your shop has a 200 A panel, have a licensed electrician check the service before you buy either one. For how the three types compare beyond sizing, see our guide to rotary vs digital vs VFD.
Size yours from the nameplate
Our phase converter sizing calculator takes your largest motor's full load amps at 208 to 240V, or its kW or hp. It turns the amps into hp with American Rotary's published figures, then shows each maker's rule with the math. You can add your panel's service size too.
If you'd rather talk it through, call us at (855) 923-2610 with your machine's nameplate numbers. We'll run each maker's rule against your numbers. You can also reach us through our contact page.