I wanted to explain why we do not size a phase converter by amperage alone.
Amps matter, but they are only one piece of the picture. A phase converter has to do more than supply the running amps shown on a nameplate. It also has to start the motor, handle the type of load, maintain voltage balance, and support any additional motors, compressors, pumps, electronics, or heating elements connected to the equipment.
That is why two machines with similar amp ratings can require very different size phase converters.
Why Amps Alone Can Be Misleading
The amperage on a motor or equipment nameplate usually shows the running load, not the starting load.
When a motor starts, it can briefly pull several times more than its normal running amperage. This is called inrush current or locked rotor amps. A phase converter must be large enough to handle that start-up demand, not just the running amperage after the machine is already moving.
For example:
- A 10 HP fan may be a relatively easy load to start.
- A 10 HP air compressor may be a much harder load because it starts under pressure.
- A 10 HP hydraulic pump may also need a larger converter because it has high starting torque.
- A CNC machine may require extra capacity because of spindle load, servo drives, electronics, and the need for clean, balanced power.
So even if the amperage looks close, the correct converter size may be very different.
The Main Things We Look At When Sizing
When sizing a phase converter, we look at:
- Motor horsepower
- Voltage (230V or 460V three-phase)
- Equipment type
- Starting load
- Running load
- Number of motors
- Whether motors start at the same time or separately
- Whether there are heating elements or resistive loads
- Whether the machine has electronics, controls, CNC drives, or compressors
- Whether the load is easy, medium, hard, or very hard to start
This gives us a much more accurate size than amps alone.
Simple Phase Converter Sizing Calculator
Here is a simplified way to estimate phase converter size:
Step 1: Start With the Largest Motor HP
Use the horsepower of the largest motor as the base.
Example:
Largest motor: 10 HP
Base size: 10 HP
Step 2: Apply the Load Type Multiplier
Different equipment types need different safety factors:
- Easy load: same HP as the motor
- Medium load: 1.5× motor HP
- Hard load: 2× motor HP
- Very hard load: 2.5× motor HP
- Air compressors: usually 2.5× motor HP
- HVAC compressors: usually 2.5× active compressor HP
- CNC machines: usually 2× the spindle HP
- Pumps: usually 2× the motor HP
Step 3: Add Other Motors That Run at the Same Time
If multiple motors run together, we size for the largest motor starting first, then add the remaining running motors.
Example:
Main motor: 10 HP
Second motor: 5 HP
Third motor: 3 HP
10 HP × 2 = 20 HP starting requirement
Add 5 HP + 3 HP = 8 HP
Estimated requirement: 28 HP
Recommended converter: 30 HP
Step 4: Add Heating Elements or Resistive Load
Heating elements do not have starting inrush like a motor, but they add a steady electrical load.
- At 230V, every 1 amp of resistive load ≈ 1 HP of converter capacity
- At 480V, every 1 amp of resistive load ≈ 2 HP of converter capacity
Example:
10 HP motor (medium/hard load) + 12 amps of heating elements at 230V
Motor: 10 HP × 2 = 20 HP
Heating: 12 amps = 12 HP
Total: 20 + 12 = 32 HP
Recommended converter: 40 HP
Sizing Examples
Example 1: 10 HP Lathe
A basic lathe with a 10 HP motor is usually a medium load.
10 HP × 1.5 = 15 HP → Recommended converter: 15 HP
Example 2: 10 HP Air Compressor
An air compressor is a hard-starting load.
10 HP × 2.5 = 25 HP → Recommended converter: 25 HP
Even if the running amps look similar to another 10 HP machine, the compressor needs more capacity because of the starting load.
Example 3: 15 HP CNC Machine
For CNC equipment, we usually double the spindle horsepower.
15 HP × 2 = 30 HP → Recommended converter: 30 HP
The extra capacity helps with spindle load, electronics, drives, and voltage stability.
Example 4: Two Motors Running Together
Machine has a 15 HP motor and a 5 HP motor, both running at the same time.
15 HP × 2 = 30 HP (largest motor)
30 HP + 5 HP = 35 HP → Recommended converter: 40 HP
Example 5: HVAC Compressor
HVAC compressors have high starting demand and need extra capacity.
10 HP active compressor load × 2.5 = 25 HP → Recommended converter: 25 HP
For HVAC, we recommend an NLA model with AutoLink so the converter starts first, then brings in the compressor after a delay.
Why Correct Sizing Matters
An undersized phase converter can cause:
- Hard starting
- Low voltage
- Poor voltage balance
- Overheating
- Contactor chatter
- Nuisance breaker trips
- Motor strain
- Compressor problems
- CNC control faults
- Shortened equipment life
A properly sized converter gives the machine enough power to start correctly and run more reliably.
Bottom Line
Amps are important, but they do not tell the full story.
The correct phase converter size depends on horsepower, starting load, equipment type, voltage, motor quantity, heating elements, and how the machine operates.
That is why we ask for the machine nameplate, motor HP, voltage, amp ratings, and application type before making a final recommendation. It lets us size the converter correctly instead of guessing from one number.
Have questions about sizing for your specific application? Contact us or call 602-487-2744 — we are happy to help.