Phase Converter Sizing Guide: How to Size for Any Application

Getting the right size phase converter is the single most important decision in your purchase. Undersizing leads to low voltage, failed motor starts, overheating, and premature equipment failure. Oversizing wastes money—though it won't hurt anything mechanically, it's money better spent elsewhere.

This guide walks you through exactly how to size a phase converter for any application, from a single motor in a home shop to a multi-machine industrial facility. We've sized over 50,000 converter installations at Phoenix Phase Converters, and these are the same steps we use on every customer call.

Why Sizing Matters

A phase converter that's too small for your load creates problems immediately:

• Voltage sag on startup: Your motor tries to start, pulls too much current from an undersized converter, and voltage drops below the level needed to develop starting torque. The motor stalls or trips the breaker.
• Chronic low voltage: Even if the motor starts, running on low voltage means higher current draw, excess heat, insulation breakdown, and a dramatically shortened motor life.
• Nuisance tripping: Overload protectors and breakers trip frequently because the system is constantly running near or above its limits.
• Poor power quality: Voltage imbalance between the three phases increases, causing motors to run rough and hot.

An oversized converter, on the other hand, simply runs at a lighter load. The idler motor spins happily, voltage balance is excellent, and you have plenty of headroom for starting surges. The only downside is the extra upfront cost and a slightly higher no-load electrical draw.

Rule of thumb: When in doubt, go one size up. You'll never regret having extra capacity.

The Basic Sizing Rule

Here's the fundamental rule that covers 80% of installations:

Phase Converter HP ≥ 2× the HP of Your Largest Single Motor

This means:

• 5HP motor → 10HP converter minimum
• 10HP motor → 20HP converter minimum
• 15HP motor → 30HP converter minimum
• 25HP motor → 50HP converter minimum

Why 2x? Because a three-phase motor draws 3-6 times its rated current during startup (called inrush current or locked rotor amps). The converter needs enough capacity to supply that surge without voltage collapse. The 2x rule provides sufficient headroom for most standard motor loads.

Real-World Example

You have a Haas VF-2 CNC mill. The spindle motor is 15HP. Following the 2x rule:

• 15HP × 2 = 30HP minimum converter
• Recommended: GP30PL (Pre-Linked, ready to install)
• The GP30PL easily handles the spindle starting surge plus the servo motors, coolant pump, and hydraulics running simultaneously

Hard-Starting Loads: When 2x Isn't Enough

Some equipment draws significantly higher inrush current than standard motors. For these loads, you need 2.5× to 3× the motor HP:

Reciprocating Compressors

Air compressors and refrigeration compressors start under load (against compressed air or refrigerant pressure). They can draw 6-8× rated current on startup. A 10HP compressor may need a 25-30HP converter, not just 20HP.

CNC Machines with Servo Rapid Traverse

During rapid positioning moves, CNC servo motors create sudden, high current demands. While the spindle is the largest single motor, the combined servo loads during rapids can spike the total draw significantly. Size for 2.5× the spindle HP for safety.

Scroll and Screw Compressors

HVAC scroll compressors often start against a pressure differential. These are excellent candidates for our DualZone series, which uses patented technology to independently energize each winding zone for maximum starting torque with minimal inrush current.

Large Pumps

Submersible pumps and centrifugal pumps that start against a head of water draw heavy starting current. Size at 2.5× minimum.

Equipment with Loaded Starts

Any machine that doesn't unload before stopping will restart under load. Examples: hydraulic presses, conveyors with material on them, mixers with product in the bowl. Size at 2.5-3× for these.

Sizing Chart by Application

Use this chart as a starting point. Always verify with your equipment's actual nameplate data.

Don't see your equipment? Call us at 800-417-6568—we've sized converters for everything from dental labs to oil rigs.

Step-by-Step Sizing Process

Follow these five steps and you'll get the right size every time:

Step 1: Read the Motor Nameplate

Every motor has a nameplate with critical information. You need:

• Horsepower (HP) — The rated output of the motor
• Voltage — 230V or 460V (this determines which converter voltage you need)
• Full Load Amps (FLA) — The current draw at rated load
• Phase — Confirm it says "3" or "3PH"
• Service Factor (SF) — Usually 1.0 or 1.15; a 1.15 SF means the motor can run at 115% of rated HP continuously

If the nameplate is missing or unreadable, look for the motor model number and search the manufacturer's catalog online, or call us with whatever information you can find.

Step 2: Identify the Load Type

Is your equipment a standard motor load or a hard-starting load?

• Standard loads (2x rule): lathes, mills, drill presses, fans, centrifugal pumps, screw compressors, saws
• Hard-starting loads (2.5-3x rule): piston compressors, scroll compressors, loaded conveyors, submersible pumps, hydraulic presses, equipment that starts under load

Step 3: Check Starting Requirements

Does the equipment need to start under load? Does it have a soft-start mechanism? Across-the-line starting (most common) draws the highest inrush current. Soft starters or wye-delta starters reduce starting current, which can sometimes allow a slightly smaller converter.

Step 4: Size the Converter

Apply the appropriate multiplier to your largest motor:

• Standard load: Motor HP × 2 = Minimum converter HP
• Hard-starting load: Motor HP × 2.5 to 3 = Minimum converter HP
• Round up to the next available converter size

Step 5: Consider Future Expansion

Planning to add equipment in the next few years? Size for your future shop, not just today's. Going one size up now is far cheaper than replacing a converter later. A GP40PL instead of a GP30PL gives you room to grow and costs only marginally more.

Multiple Motor Installations

Most shops have more than one three-phase machine. Here's how to size for multiple motors:

The Key Principle

Size for the largest single motor's starting current, not the total of all motors.

Why? You typically don't start all motors simultaneously. The converter needs to handle the biggest single startup surge, plus whatever is already running.

Practical Approach

1. Identify your largest motor HP
2. Apply the 2x (or 2.5x) rule to that motor
3. Add up the HP of all other motors that might be running simultaneously (not starting, just running)
4. Running motors typically draw 50-80% of their rated HP from the converter
5. Verify the converter handles: (Largest motor × 2) + (Total running HP of other motors × 0.8)

Example: Machine Shop with Multiple Tools

Your shop has:

• Haas VF-2 (15HP spindle) — your largest motor
• Engine lathe (5HP)
• Surface grinder (3HP)
• Air compressor (5HP piston type)

Sizing calculation:

• Largest motor: 15HP × 2 = 30HP for CNC starting surge
• Other running loads: (5 + 3 + 5) × 0.8 = 10.4HP
• Total: 30 + 10.4 = 40.4HP
• Recommended: GP40PL

In practice, you probably won't run the grinder and lathe while the CNC is in a heavy cut, so a GP30PL might work. But a GP40PL gives you peace of mind and room to grow. This is a conversation worth having with us at 800-417-6568.

Staggered Starting

If you start your equipment one machine at a time (waiting 5-10 seconds between startups), the converter only needs to handle one starting surge at a time. This can sometimes allow a smaller converter than the pure math suggests. However, we always recommend sizing with margin—you don't want to remember a startup sequence every day.

Voltage Considerations

230V vs. 460V

Most residential and rural single-phase service is 240V (nominal 230V). Most phase converters are 230V units that produce 230V three-phase output.

If your equipment is 460V (common in larger industrial machines), you have two options:

1. Step-up transformer: Use a 230V converter with a step-up transformer to get 460V output. We offer transformers sized for this purpose.
2. Re-wire the motor: Many three-phase motors are dual-voltage (check the nameplate for "230/460V"). These can be rewired for 230V operation. This is usually the simplest and most cost-effective approach.

208V Equipment

Some commercial equipment (especially HVAC) is rated for 208V, which comes from a commercial 120/208V three-phase service. A standard 230V converter outputs about 230V, which is within the +/-10% tolerance most motors accept. However, verify with the equipment manufacturer for sensitive electronics.

Wire Sizing Guide

Proper wire sizing is critical for safety and performance. Undersized wire causes voltage drop, overheating, and fire risk. This table covers copper wire (most common for these distances):

Important notes:

• These are minimums based on NEC tables for 75°C copper conductors
• Longer wire runs require larger wire to compensate for voltage drop
• Aluminum wire requires upsizing by approximately 2 AWG sizes
• Always verify with NEC tables, local codes, and a qualified electrician
• Consult our Installation Manual for detailed wiring diagrams

Common Sizing Mistakes

After 50+ years and thousands of customer calls, these are the mistakes we see most often:

Mistake #1: Sizing for Total HP Instead of Largest Motor

"I have three 5HP motors, so I need a 30HP converter."

Not necessarily. If your largest single motor is 5HP, a 10HP converter handles the starting surge. The question is whether you run all three simultaneously—if so, 10HP + (10HP × 0.8) = 18HP, so a 20HP converter works. You saved $800+ by sizing correctly.

Mistake #2: Ignoring Starting Current

"My compressor is 10HP, so a 10HP converter should work."

A 10HP piston compressor starting against tank pressure draws 60-80 amps—far more than a 10HP converter can supply. You need at least a 25HP converter, possibly 30HP. This is the single most common sizing error we see.

Mistake #3: Forgetting About Ancillary Motors

"My CNC has a 20HP spindle, so 40HP converter."

That's the right starting point, but don't forget the hydraulic unit (2-5HP), coolant pump (1-2HP), chip conveyor (0.5-1HP), and servo motors. These all run simultaneously with the spindle. A 40HP might be tight—a 50HP gives you breathing room.

Mistake #4: Using Nameplate HP Instead of Actual Equipment Requirements

Some equipment lists total HP differently than individual motor HP. A machine might say "15HP" but actually have a 10HP main motor and a 5HP auxiliary. Size for the 10HP motor (20HP converter), not 15HP × 2 = 30HP.

Mistake #5: Not Planning for Growth

"I only have one machine right now."

Right now. But shops grow. That 20HP converter you buy today for your lathe won't handle the CNC mill you buy next year. Spend the extra $400-800 now to get a larger converter, and you won't need to replace it when you add equipment.

When to Call an Expert

While the rules above cover most installations, some situations benefit from a conversation with someone who's done this a few thousand times:

• Multiple large motors (3+ motors over 10HP each)
• Mixed hard-starting and standard loads on the same converter
• 460V equipment requiring transformer sizing
• Very long wire runs (over 100 feet from panel to converter or converter to equipment)
• Sensitive electronic equipment (CNC controls, medical equipment, precision test equipment)
• Unusual power situations (low incoming voltage, generator backup, solar/battery systems)
• Industrial facilities with complex load profiles

We've sized converters for every application imaginable—from one-man garage shops to industrial plants. A five-minute phone call can save you from buying the wrong size and the hassle of exchanging it.

Frequently Asked Questions

What happens if my converter is too small?

The motor may not start at all (the converter trips its breaker), or it may start slowly with a voltage sag that stresses the motor windings. Running consistently on an undersized converter shortens motor life dramatically and can damage sensitive electronics like CNC controls.

What happens if my converter is too big?

Nothing bad. An oversized converter runs at a lighter load, produces excellent voltage balance, and has plenty of headroom for starting surges. The only downsides are higher upfront cost and a slightly higher no-load electrical draw (a few extra dollars per month).

Can I run a static converter on a CNC machine?

We strongly advise against it. Static converters produce unbalanced power that can damage CNC control boards, cause erratic machine behavior, and void equipment warranties. Always use a rotary phase converter for CNC machines.

Do I need a separate converter for each machine?

No—one properly sized converter can power your entire shop. Size it for the largest single motor starting surge plus the running load of everything else. Most shops run everything from a single converter feeding a three-phase sub-panel.

How do I know if my motor is hard-starting?

If it's a reciprocating (piston) compressor, a scroll compressor, a submersible pump, or any machine that starts under load, it's hard-starting. Check the nameplate for "Code Letter"—letters toward the end of the alphabet (K through V) indicate higher starting current per HP.

I have a dual-voltage motor (230/460V). Which should I use?

230V whenever possible. Standard phase converters output 230V, so connecting at 230V eliminates the need (and cost) for a step-up transformer. An electrician can re-wire most dual-voltage motors in under an hour.

What's the DualZone technology, and do I need it?

Our DualZone series uses patented technology that independently energizes each motor winding zone during starting. This delivers maximum starting torque with minimal inrush current—perfect for hard-starting loads like compressors and HVAC systems. If you have hard-starting equipment, DualZone can allow proper starting with a smaller converter size.

Get Your Phase Converter Sized Right—the First Time

Proper sizing saves you money, protects your equipment, and eliminates frustration. Follow the steps in this guide, use the sizing chart as a starting point, and when in doubt, go one size up.

For personalized sizing help, visit our Sizing Help page or call us directly at 800-417-6568. We'll ask about your equipment, your application, and your plans—and recommend exactly the right converter.

Ready to buy? Browse our full line of American-made phase converters—every one backed by a lifetime warranty and 90-day money-back guarantee.