Motor CapacitorSizing Calculator
Find the right start and run capacitor for any single-phase motor. Select your motor HP, voltage, and type to get recommended capacitor sizes instantly.
Capacitor Sizing
Select motor parameters below
Quick Reference
Sizing Tips
Always check the nameplate first. The motor nameplate is the definitive source for capacitor specifications. This calculator provides approximate starting points only.
Pro Tip
When replacing HVAC capacitors, you can safely use a 440V-rated capacitor where a 370V is specified. The higher voltage rating provides an extra safety margin in systems with voltage fluctuations.
Safety Warning
- •Always disconnect power before testing or replacing motor capacitors.
- •Discharge capacitors before handling. Even disconnected capacitors can hold a dangerous charge.
- •Never exceed the motor nameplate capacitor specifications. Oversizing can damage windings.
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Motor Capacitor Sizing Chart
Reference table for single-phase capacitor-start motors at 230V. Values are approximate — always verify with motor nameplate data.
| HP | Voltage | Start Cap (µF) | Run Cap (µF) | Voltage Rating |
|---|---|---|---|---|
| 1/4 | 230V | 75 – 100 | 4 – 6 | 370V |
| 1/3 | 230V | 100 – 130 | 5 – 7.5 | 370V |
| 1/2 | 230V | 130 – 170 | 7.5 – 10 | 370V |
| 3/4 | 230V | 170 – 220 | 10 – 15 | 370V |
| 1 | 230V | 200 – 260 | 15 – 20 | 370V |
| 1.5 | 230V | 260 – 340 | 20 – 25 | 370V |
| 2 | 230V | 340 – 400 | 25 – 30 | 370V |
| 3 | 230V | 400 – 480 | 30 – 40 | 370V |
| 5 | 230V | 480 – 580 | 35 – 50 | 370V |
| 7.5 | 230V | 580 – 710 | 45 – 60 | 370V |
| 10 | 230V | 710 – 850 | 50 – 70 | 370V |
How to Size a Motor Capacitor
Selecting the correct motor capacitor requires matching the capacitance (µF), voltage rating, and type to your specific motor. Follow these steps:
- 1.Check the motor nameplate. Look for the capacitor specification printed on the motor data plate. It will list the required microfarad (µF) value and minimum voltage rating.
- 2.Identify the motor type. Determine if your motor is a PSC (Permanent Split Capacitor), capacitor-start, or capacitor-start-capacitor-run type. This determines whether you need a start capacitor, run capacitor, or both.
- 3.Note the supply voltage. Common single-phase motor voltages are 115V and 230V. The capacitor voltage rating must meet or exceed the motor's operating voltage — typically 370V or 440V for run capacitors.
- 4.Use a sizing chart as a starting point. If the nameplate is damaged or unreadable, refer to the motor capacitor sizing chart above. These values are approximate and based on standard NEMA tables for common motor sizes.
- 5.Test the existing capacitor. If the old capacitor is available, use a multimeter with capacitance measurement to check if it has drifted from its rated value. A capacitor that has lost more than 5% of its rated capacitance should be replaced.
Motor Start vs Motor Run Capacitors
Motor start and motor run capacitors serve fundamentally different roles in a single-phase motor circuit, and they are not interchangeable.
Start Capacitors
- • High capacitance (75 – 850+ µF)
- • Short duty cycle (a few seconds)
- • Electrolytic construction
- • Provides high starting torque
- • Disconnected by centrifugal switch after start
Run Capacitors
- • Lower capacitance (4 – 70 µF)
- • Continuous duty rated
- • Oil-filled film construction
- • Improves efficiency and power factor
- • Stays connected during operation
For a deeper comparison including wiring diagrams and failure modes, see our guide: Motor Start vs Motor Run Capacitors.
Common HVAC Capacitor Sizes
Most residential HVAC systems use dual run capacitors that combine the compressor and fan motor capacitors into a single unit with three terminals (C, FAN, HERM). The first number is the compressor capacitance and the second is the fan motor capacitance. These are typically rated at 370V or 440V.
| Dual Run Cap Size | Typical Application |
|---|---|
| 25/5 µF | Small fan motors, 1/4 HP compressors |
| 30/5 µF | 1/3 HP compressors, condenser fan motors |
| 35/5 µF | 1.5 – 2 ton AC units |
| 40/5 µF | 2 – 2.5 ton AC units |
| 45/5 µF | 2.5 – 3 ton AC units |
| 50/5 µF | 3 – 3.5 ton AC units |
| 55/5 µF | 3.5 – 4 ton AC units |
| 60/5 µF | 4 – 5 ton AC units |
For more on voltage rating selection, see our comparison: 370V vs 440V Capacitors: Which Do You Need?
When to Replace a Motor Capacitor
Motor capacitors degrade over time, especially in high-temperature environments like HVAC outdoor condensing units. Recognizing the signs of a failing capacitor can prevent costly motor damage.
Motor hums but won't start
A weak or failed start capacitor cannot provide the phase shift needed to generate starting torque. The motor may buzz or trip the overload protector.
Slow or sluggish starting
If the motor takes noticeably longer to reach full speed, the start capacitor may have lost significant capacitance.
Motor runs hot or draws excess current
A degraded run capacitor reduces motor efficiency, causing higher current draw and increased winding temperatures.
Visible damage
A swollen, bulging, cracked, or leaking capacitor should be replaced immediately. Oil leaking from a run capacitor indicates internal failure.
Capacitance out of tolerance
When measured with a capacitance meter, a good capacitor should read within 5% of its rated value. Readings more than 5 – 10% low indicate the capacitor should be replaced.
For step-by-step diagnosis and replacement instructions, see our motor capacitor troubleshooting guide. If you need replacement motor capacitors, browse our motor capacitor catalog or explore HVAC capacitor solutions.
Frequently Asked Questions
How do I determine what size capacitor my motor needs?
The most reliable method is to check the motor nameplate, which lists the required capacitance in microfarads (µF) and voltage rating. If the nameplate is unreadable, you can use a motor capacitor sizing chart based on the motor HP, voltage, and type (PSC, capacitor-start, or split-phase). Our calculator above provides approximate starting points based on standard NEMA motor tables.
What happens if I use the wrong size capacitor on a motor?
Using a capacitor that is too small will reduce starting torque and may prevent the motor from starting under load. Using one that is too large can cause excessive current draw, overheating, and premature winding failure. For run capacitors, an incorrect value can also reduce motor efficiency and increase power consumption. Always match the capacitor to the motor nameplate specifications.
Can I use a 440V capacitor in place of a 370V capacitor?
Yes. A 440V capacitor can safely replace a 370V capacitor of the same microfarad rating. The voltage rating indicates the maximum voltage the capacitor can handle, so a higher-rated capacitor provides an additional safety margin. However, you should never use a lower voltage rating than specified — never replace a 440V capacitor with a 370V one.
How often should motor capacitors be replaced?
Motor capacitors typically last 10 – 20 years under normal operating conditions. However, factors like high ambient temperatures, voltage spikes, and frequent motor cycling can shorten lifespan. Signs of a failing capacitor include the motor humming but not starting, slow starting, overheating, or a visibly swollen or leaking capacitor. HVAC technicians often recommend replacing capacitors proactively during annual maintenance if capacitance has dropped more than 5% from the rated value.