How to Size a Starting and Running Capacitor for Motors
Introduction
In the electrical engineering and industrial applications, capacitors are important components when it comes to motor starting and running. Selecting the correct size for both starting capacitors and running (run) capacitors is critical to ensure the motor operates smoothly, avoids overheating, and extends its operational life.
From plant installations to HVAC, pumps, and automation systems experience —I’ve encountered many of capacitor sizing issues. This article guides you through how to properly size capacitors for single-phase motors, complete with real-world examples, formulas, and a color-coded infographic-style rating table.
What Are Motor Capacitors?
Starting Capacitor
A starting capacitor provides a quick torque boost to start the motor. It’s only in the circuit for a few seconds during motor startup.
Running Capacitor
A running capacitor improves the efficiency and power factor during continuous operation.
Both are typically used in single-phase induction motors, such as compressors, fans, pumps, and HVAC systems.
Why Correct Sizing Matters
Improper sizing can result in:
- Hard starting or failure to start
- Excessive current draw
- Overheating and premature motor failure
- Reduced torque and performance
Basic Sizing Principles
Formula for Running Capacitor Sizing
For standard 50/60Hz motors:
C (µF) = (746 × P × 1000) / (V² × 2π × f × η × PF)Where:
- P = Motor power in HP
- V = Voltage
- f = Frequency (Hz)
- η = Efficiency (assume 0.9 if unknown)
- PF = Power Factor (assume 0.9 if unknown)
Rule of Thumb for Starting Capacitor
- Starting capacitors typically range from 70 to 100 µF per HP.
- Voltage rating must be 1.5x the supply voltage (e.g., for 230V supply, use at least 330V capacitor).
Example:
- 1 HP motor → 70–100 µF starting capacitor
Recommended Capacitor Sizes by Motor HP
| Motor HP | Start Capacitor (µF) | Run Capacitor (µF) | Voltage Rating (min) |
|---|---|---|---|
| 0.25 | 35–50 | 5–7.5 | 250V–330V |
| 0.5 | 60–80 | 10–15 | 250V–370V |
| 1.0 | 70–100 | 25–30 | 330V–440V |
| 1.5 | 120–160 | 35–40 | 370V–450V |
| 2.0 | 180–220 | 45–60 | 400V–450V |
Infographic: Capacitor Sizing Table with Star Ratings
| Brand | Starting Range (µF) | Running Range (µF) | Reliability | Cost | Availability | Rating |
| Danfoss | 70–220 | 10–60 | 🟩🟩🟩🟩🟩 | $$$ | High | 🟩🟩🟩🟩🟩 |
| CDE (Cornell Dubilier) | 80–200 | 10–50 | 🟩🟩🟩🟩⬜ | $$$ | Medium | 🟩🟩🟩🟩⬜ |
| EPCOS | 60–180 | 5–40 | 🟩🟩🟩🟩⬜ | $$ | High | 🟩🟩🟩🟩⬜ |
| TEMCo | 100–220 | 10–50 | 🟩🟩🟩⬜⬜ | $ | Medium | 🟩🟩🟩⬜⬜ |
| Titan Pro | 70–200 | 15–55 | 🟩🟩🟩🟩⬜ | $$ | High | 🟩🟩🟩🟩⬜ |
Common Mistakes to Avoid
- ❌ Using a capacitor with a lower voltage rating than supply voltage
- ❌ Using electrolytic start capacitors continuously (they’re for short durations only!)
- ❌ Not matching capacitor value to motor HP
- ❌ Ignoring capacitor polarity in non-polarized installations
Maintenance Tips
- 🔧 Inspect for bulging, oil leaks, or cracks
- 🔋 Periodically measure capacitance with a multimeter
- 💡 Always discharge capacitors before testing
Final Verdict
Choosing the right starting and running capacitor isn’t just about specs—it’s about understanding your motor’s application. For general-purpose motors up to 2 HP, use the rule-of-thumb table. For special motors (capacitor-start capacitor-run types), consult the OEM datasheet.
When in doubt, use slightly higher voltage ratings and keep a few common values on hand in your maintenance toolbox.