Calculate the right capacitor size for power factor correction, motor start/run, DC filtering, and audio crossover applications.
Choosing the right capacitor size is critical for performance, efficiency, and safety. An undersized power factor correction capacitor wastes energy; an undersized motor start capacitor causes the motor to stall. An oversized DC filter capacitor is wasteful; too small allows excessive ripple.
Each application has a different formula. Power factor correction requires Qc = P × (tan θ₁ − tan θ₂), motor start capacitors are oversized for torque, DC filters use C = I/(f × V_ripple), and audio crossovers use C = 1/(2πfZ). Voltage ratings must include safety margins, and the capacitor type (electrolytic, film, oil-filled) must match the application.
This calculator handles all five common applications with application-specific formulas, recommends standard capacitor sizes, and includes voltage rating and technology recommendations. It saves time by combining application knowledge with capacitor physics in one tool. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case.
Capacitor sizing involves application-specific formulas, trigonometric functions, unit conversions, and knowledge of standard component values. Getting it wrong can cause poor performance, equipment damage, or safety hazards.
This calculator combines five different sizing methods in one tool with standard value recommendations, voltage rating guidelines, and capacitor type suggestions. It eliminates the need for separate calculations and lookup tables for different applications.
PF correction: Qc = P(tan θ₁ − tan θ₂), C = Qc/(2πfV²). DC filter: C = I/(f×V_ripple). Audio: C = 1/(2πfZ). Motor start: 2× PF correction value. Voltage rating ≥ 1.25 × operating voltage.
Result: 68.6 µF capacitor, recommend 100 µF standard
θ₁ = arccos(0.7) = 45.6°, θ₂ = arccos(0.95) = 18.2°. Qc = 10000 × (tan 45.6° − tan 18.2°) = 10000 × (1.020 − 0.329) = 6914 VAR. C = 6914/(2π × 60 × 480²) = 79.5 µF. Next standard: 100 µF at ≥600V.
Inductive loads (motors, transformers, fluorescent lighting) draw reactive power that increases the apparent power and current. Utilities charge for apparent power (kVA) or penalize poor power factor, making correction economically important.
The capacitor size for PF correction is: Qc = P × (tan θ₁ − tan θ₂), where θ₁ and θ₂ are the angles corresponding to the current and target power factors. The capacitance is then C = Qc/(2πfV²). Target PF of 0.95-0.98 is typical; overcorrecting to unity or leading PF should be avoided.
Single-phase induction motors require a capacitor to create a rotating magnetic field. The start capacitor provides high torque during startup (typically 50-100 µF per HP at 120V) and is disconnected by a centrifugal switch. The run capacitor remains in circuit for efficiency and PF improvement (typically 15-50 µF per HP).
Capacitor failure is one of the most common causes of single-phase motor problems. Symptoms of a failed start cap: motor hums but won't start. Failed run cap: motor runs hot and draws excess current.
In power supplies, the filter capacitor smooths the pulsating DC from the rectifier. The ripple voltage is approximately V_ripple = I_load/(f × C), where f is the ripple frequency (2× line frequency for full-bridge). Larger capacitance means less ripple but higher inrush current and slower startup.
Power factor correction adds capacitance to cancel the inductive reactive power drawn by motors, transformers, and fluorescent lights. This reduces apparent power (kVA), lowering electricity costs and reducing losses in wiring and transformers.
Start capacitors (electrolytic, high capacitance) are used briefly during startup for high torque. Run capacitors (film/oil, lower capacitance) operate continuously for efficiency and power factor. Using a start cap as a run cap will cause overheating and failure.
Use at least 1.25× the operating voltage as a minimum. For motor applications, voltage spikes during switching can reach 2-3× rated voltage, so higher ratings are safer. AC-rated capacitors are required for motor and PF correction.
For PF correction, overcorrection makes the power factor leading (capacitive), which utilities also penalize. For motors, too much capacitance causes excessive current and overheating. For filters, only cost and size increase.
Yes — parallel capacitors add capacitance directly. This is common in PF correction banks where capacitors are switched in steps to match the varying load.
PF correction: self-healing metallized polypropylene. Motor run: oil-filled or metallized film. Motor start: AC-rated electrolytic. DC filter: aluminum electrolytic (low ESR). Audio: polypropylene film.