Motor Run Capacitor Calculator
Enter motor power, voltage and frequency to get the recommended run capacitor in µF and its working voltage.
Step-by-step
- Enter valid values to begin.
C = P × η / (2π f V²). This sizes a run capacitor for a typical fractional-to-small-horsepower single-phase motor; the nameplate or manufacturer value always overrides it. The start-capacitor range for CSCR is a rule-of-thumb 3.0–5.0× (nominal 3.5×) the run value. For three-phase motors use the power factor correction calculator instead.
Motor Run Capacitor Sizing: Phase Shift for the Auxiliary Winding
Core Engineering Principles
A single-phase induction motor has no rotating field of its own, only a pulsating one. To get starting torque you need a second winding carrying current out of phase with the first, and a capacitor in series with that winding is the cheapest way to create the shift. In a permanent split capacitor motor the capacitor stays in circuit while the motor runs, which is why it’s called a run capacitor and why it has to live in a continuous-duty, oil-filled or metallised-film can. A capacitor-start capacitor-run motor adds a much larger electrolytic start capacitor in parallel, switched out by a centrifugal switch or relay.
The sizing rule on this page ties capacitance to power: C = P × η / (2π × f × V²). It gives microfarads that supply roughly the reactive current the auxiliary winding needs. It’s a starting point, not a substitute for the nameplate. Always replace a failed capacitor with the printed value. A run capacitor 10% under makes the motor run hot; one far over overheats the auxiliary winding.
Start cap (CSCR) ≈ 3.5 × run capacitance (3.0–5.0 ×) • Vcap ≥ 1.4 × Vsupply
NEC & Standard References
IEC 60252-1 is the standard for AC motor capacitors, giving tolerances (usually ±5% or ±6%), duty and safety construction. UL 810 covers capacitors in North America. NEMA MG-1 Part 1 covers single-phase motor ratings, and NEC 110.3(B) requires listed equipment to be installed and maintained per its marking, which includes the nameplate capacitor value. Any capacitor should be treated as live until you have discharged it. The voltage rating on the can is the working voltage, and the winding voltage with the capacitor in series can run well above the supply.1. 2π × 60 × 230² = 19.94 × 106.
2. C = 746 × 0.70 / 19.94 × 106 = 26.2 µF. Nearest standard value: 25 µF (−4.5%).
3. Minimum working voltage = 1.4 × 230 = 322 V, so choose 370 V AC.
4. Reactive power with the 25 µF can = 2π × 60 × 25 × 10−6 × 230² = 0.50 kVAR.
If this were a capacitor-start motor, expect a start capacitor of about 75–125 µF (nominal 88 µF) in addition.
- Discharge before you touch. A capacitor holds charge after power is removed. Short the terminals through a resistor, never a screwdriver.
- Don’t swap voltage ratings downward. A 250 V can on a 230 V motor can fail from dielectric breakdown, often with a bulging or venting case.
- Match the microfarads, within tolerance. Keep within 5–10% of the original value, and never use a start capacitor as a run capacitor.
- Heat kills them. A capacitor mounted on a hot compressor shell ages quickly; replace in pairs after repeated failures.