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. After sizing over 50,000 converter installations at Phoenix Phase Converters, these are the same steps we use on every customer call.

Looking for specific equipment recommendations? See our Complete Sizing Chart by Equipment Type for quick picks on CNC machines, compressors, HVAC, irrigation, and more.

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 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.

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:

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

Why 2×? 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 2× rule provides sufficient headroom for most standard motor loads.

Real-World Example

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

15 HP × 2 = 30 HP 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 2× 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 (Piston) Compressors — Use 2.5×

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

Dust Collectors & Large Fans — Use 3×

Industrial dust collectors and large centrifugal fans have high-inertia impellers that take significant time and current to reach operating speed. A 5 HP dust collector often needs a 15 HP converter to start cleanly without blowing caps or tripping breakers. This is the #1 undersizing mistake we see in woodworking shops.

CNC Machines with Servo Rapid Traverse — Use 2.5×

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 — Use 2.5×

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, Centrifugal) — Use 2.5×

Pumps that start against a head of water draw heavy starting current. Size at 2.5× minimum. For deep well pumps specifically, see our guide on DualZone for deep well pumping.

Equipment with Loaded Starts — Use 2.5–3×

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, elevators. Size at 2.5–3× for these.

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 (2× rule): lathes, mills, drill presses, fans, centrifugal pumps, screw compressors, saws
• Hard-starting loads (2.5× rule): piston compressors, scroll compressors, loaded conveyors, submersible pumps, hydraulic presses, equipment that starts under load
• High-inertia loads (3× rule): dust collectors, large industrial fans, flywheel-driven equipment

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 = Minimum converter HP
• High-inertia load: Motor HP × 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.

Sizing for Multiple Motors

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 2× (or 2.5×, or 3×) rule to that motor
3. Add up the HP of all other motors that may run simultaneously
4. The converter rating needs to equal or exceed: (Largest motor × multiplier) + (sum of other running motors)

Example — Small Machine Shop

Example — Farm with Grain Dryer

Example — Woodworking Shop with Dust Collector

Common Sizing Mistakes

Mistake #1: Undersizing (The #1 Killer)

An undersized converter can't deliver enough starting torque. Your equipment may not start, may run hot, may trip breakers, or may damage expensive motors. This is the most expensive mistake you can make. When in doubt, go up a size.

Mistake #2: Ignoring Dust Collectors and Fans

People underestimate the starting current of high-inertia loads. A 5 HP dust collector feels small, but its impeller wheel takes 3–5 seconds to spin up — drawing 5–7× rated current the entire time. You need the 3× rule here. Always.

Mistake #3: Not Accounting for Loaded Starts

Nameplate HP tells you running load, not starting load. A piston compressor starting against tank pressure, an elevator starting under passenger load, or a conveyor restarting with material on it — these all draw 3–6× rated current at startup. Size at 2.5× minimum.

Mistake #4: Matching HP Exactly (1:1 Rule is WRONG)

Some online guides and competitor websites still recommend "match converter HP to motor HP" as a rule. This is incorrect. A 10 HP converter cannot reliably start a 10 HP motor — the inrush exceeds the converter's instantaneous capacity. Always use 2× minimum.

Mistake #5: Extreme Oversizing (2x Is Enough — Don't Go 5x)

A little headroom is good. Massive oversizing (running a 5 HP load on a 50 HP converter) can cause unbalanced voltage at light load. This is where our DualZone series excels — it handles mixed loads that would challenge a standard converter. But in general, 2× is plenty. Don't go crazy.

Frequently Asked Questions

What if I am not sure what HP my motor is?

Check the motor nameplate — look for "HP" or "kW". If you only find kW, multiply by 1.34 to get HP. If the nameplate is unreadable, take a photo of whatever information is visible and text or email it to us. We can usually identify the motor from partial information.

What if I have multiple motors but they never all run at the same time?

Size for the largest single motor using its appropriate multiplier (2×, 2.5×, or 3×). You don't need to account for equipment that won't be running simultaneously. Our engineers can verify this with you on a phone call.

Is it ever wrong to oversize a phase converter?

Not mechanically — a converter running under its rated load is happy. But extreme oversizing (5× or more) can cause voltage imbalance at very light loads. In those cases, a DualZone converter is a better solution than a single massive unit.

My equipment has a nameplate with kVA instead of HP — how do I size?

kVA and HP aren't directly convertible (kVA includes both real power and reactive power), but a reasonable approximation: kVA × 1.0 = approximate HP for motor loads. For more precision, call us with the voltage, kVA, and full-load amps.

Can I use the same converter for both 230V and 460V equipment?

Not directly. A phase converter outputs the same voltage as its input (230V in → 230V three-phase out). For 460V three-phase equipment from a 230V single-phase source, you need an NLT transformer package that combines a phase converter with a step-up transformer.

How do I know if my equipment is a "hard-starting load"?

Three questions:

• Does it start under load (not unloaded)?
• Does it have a heavy flywheel, impeller, or compressor piston?
• Does it require high torque at zero RPM (like hydraulic pumps or elevators)?

If yes to any, it's a hard-starting load. Use 2.5× or 3× accordingly.

Should I size based on nameplate amps or HP?

Use HP as your primary sizing reference. Nameplate amps are a secondary check — if the amps look unusually high for the HP (possible for old motors or special-purpose motors), size up one increment.

Further Reading

• Complete Sizing Chart by Equipment Type — Quick-reference chart for common equipment
• Phoenix Knowledge Hub — Sizing & Selection — In-depth technical resources
• Three-Phase Motor Wiring Diagrams — After you pick your converter, wire it right
• How Phase Converters Work — Understanding the technology