⚡ CircuitSecrets
Motor Capacitor Calculator
Start, Run & CSR capacitor sizing for single-phase AC motors — with reactive power, voltage rating, starting current & standard value matching
Start Cap
Run Cap
CSR Motor
Reactive Power
Standard Values
Voltage Rating
Startup Torque
Start Capacitor
⚡ Start CapacitorProvides extra torque for startup only (1–3 sec). Electrolytic type, switched out at ~75% speed by centrifugal switch. High capacitance, lower voltage rating.
Motor Power
Supply
Application
Start Cap Result
Capacitor Sizing
Start
—
Recommended Capacitance
—
µF (nearest standard value)
► Formula
Start Cap Sizing
C = Factor × HP × (120/V)² × (60/f)
Factor: Normal=70, High=85, VHigh=100 µF/HP
V_cap = 1.5 × V_supply (min rating)
I_FLC = P / (V × η × PF)
Continuous Duty
Run Capacitor
▶ Run CapacitorRemains in circuit continuously. Oil-filled or film type for reliability. Improves PF, efficiency and torque. Lower capacitance, higher voltage rating (370–440V).
Motor Power
Supply & PF
Run Cap Result
Capacitor Sizing
Run
—
Recommended Capacitance
—
µF (nearest standard value)
► Formula
Run Cap — PF Correction
kVAr = P × (tan φ₁ − tan φ₂)
C = kVAr × 10⁶ / (2π × f × V²)
Rule of thumb: 3 µF/HP @ 220V 50Hz
V_cap ≥ 370V for 220V, ≥ 440V cont.
CSR Motor
Both Capacitors
⚪ CSR MotorCapacitor Start – Capacitor Run. Best starting torque + best running efficiency. HVAC compressors, industrial pumps. Both caps sized simultaneously.
Motor Power
Application
CSR Result
Complete Sizing
Start
—
Run
—
CSR Motor — Total kVAR
—
kVAR reactive power
► Formula
CSR — Both Capacitors
Start C = Factor × HP × (120/V)² × (60/f)
Run C = kVAr×10⁶ / (2π×f×V²)
Start V_cap = 1.5 × V_supply
Run V_cap ≥ 370V (continuous)
IEC Reference
Quick Reference Guide
Start Capacitors
• 70–800 µF typical
• 125–330V AC rating
• Electrolytic type
• Short duty only
• ~1–3 sec in circuit
• 125–330V AC rating
• Electrolytic type
• Short duty only
• ~1–3 sec in circuit
Run Capacitors
• 5–100 µF typical
• 370–440V AC rating
• Oil-filled / film
• Continuous duty
• Improves PF & η
• 370–440V AC rating
• Oil-filled / film
• Continuous duty
• Improves PF & η
Rule of Thumb
• Start: 70–100 µF/HP
• Run: 2–4 µF/HP
• V_cap ≥ 1.5× supply
• Match nameplate
• ±10% tolerance ok
• Run: 2–4 µF/HP
• V_cap ≥ 1.5× supply
• Match nameplate
• ±10% tolerance ok
Safety Tips
• Discharge before handling
• Match voltage rating
• Replace same type
• Check for swelling
• Verify with nameplate
• Match voltage rating
• Replace same type
• Check for swelling
• Verify with nameplate
⚠ Disclaimer: Values are estimates based on standard formulas and industry guidelines. Always verify against motor nameplate data. Consult a qualified electrician for critical or industrial installations.
