How do I calculate motor capacitor size?

For a run capacitor: C ≈ P/(2π×f×V²×η×cosφ) × 10⁶ μF. A rough rule of thumb is 30-50 μF per HP for run capacitors and 100-150 μF per HP for start capacitors.

What is the difference between start and run capacitors?

Start capacitors are electrolytic, high-value (70-400 μF), used only during startup then disconnected by a centrifugal switch. Run capacitors are film type, lower value (5-60 μF), and remain in circuit continuously.

What happens if the capacitor is too small?

Too small: motor may not start (low starting torque), may overheat, reduced efficiency. Too large: excessive current in auxiliary winding, overheating, possible winding damage.

Can I use a higher voltage capacitor?

Yes, using a capacitor with a higher voltage rating is safe. However, never use a lower voltage rating than required — typically 1.5× the supply voltage for safety margin.

What are PSC and CSIR motors?

PSC (Permanent Split Capacitor) uses a run capacitor only — simple but lower starting torque. CSIR (Capacitor Start Induction Run) adds a start capacitor with centrifugal switch for higher starting torque, typical for compressors and pumps.

Free Engineering Tool

Motor Capacitor Calculator

Calculate start and run capacitor values for single-phase motors. C = I / (2πfV). Estimates based on motor power, voltage, and frequency.

Start CapacitorRun CapacitorSingle-Phase

Results

Run Capacitor (estimated)

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Start Capacitor (estimated)

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Motor Current

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Min Capacitor Voltage Rating

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Capacitor Formula

Start capacitors are typically 3–5× the run capacitor value. Run capacitors use film dielectric; start capacitors use electrolytic.

Motor Capacitor Calculator | Single-Phase Motor C = I/(2πfV) | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Capacitor Formula