Compare an illustrative run/start capacitor estimate or calculate ideal power-factor-correction capacitance per capacitor for a balanced delta or wye bank.

Illustrative single-phase conversion estimate only. The original 70 µF/kW rule assumes 230 V, 50 Hz; it is not a verified sizing rule for your motor.

Legacy illustrative default: 70. Use a manufacturer-supported factor for the specific motor and supply.

3 Phase Motor Capacitor Formula

The following equation gives capacitance per capacitor in a wye bank from total three-phase reactive power and line-to-line voltage. For delta, divide this capacitance by 3.

C = (Q × 10⁹) / (2 × pi × f × V²)
  • Where C is the capacitance of each of three equal wye-connected capacitors (µF)
  • Q is total three-phase reactive power to compensate (kVAR)
  • f is the frequency (Hz), typically 60 Hz
  • V is line-to-line RMS voltage (V)

Choose a calculation. For correction, supply total reactive power or real input power and existing/target power factors, then voltage, frequency, and bank connection. For power-factor inputs, Q = P × [tan(acos(PF1)) − tan(acos(PF2))].

What is a 3 Phase Motor Capacitor?

Definition:

A power-factor-correction capacitor bank supplies reactive power to reduce upstream reactive current. It does not inherently improve motor efficiency or balance an unbalanced load. Single-phase motor conversion is a different application: the retained 70 µF/kW run estimate and 2.5× start estimate are unverified rules of thumb, not universal sizing recommendations.

How to Calculate 3 Phase Motor Capacitor Parameters?

Example Problem:

The following example outlines the steps and information needed to calculate the missing capacitor parameter.

Choose correction from reactive power, with 50 kVAR total, 400 V line-to-line, 60 Hz, and a wye bank.

Next, calculate the squared value of the line voltage: 400² equals 160,000.

Finally, determine the capacitor value using the formula above:

C = (Q * 10^9) / (2*pi*f*V^2)

C = (50 * 10^9) / (2*pi*60*160000)

C ≈ 828.932 µF per wye capacitor; a delta bank would require approximately 276.311 µF per capacitor.

FAQ

What parameters can be calculated using this calculator?

The calculator estimates run/start capacitance, or calculates correction capacitance from reactive power or a desired power-factor improvement. It does not reverse-solve voltage or reactive power from a capacitor value.

Why is the frequency set to 60 Hz?

Most three-phase systems in many regions operate at 60 Hz; however, if your system uses a different frequency, you may need to adjust the calculation accordingly.

How do I ensure an accurate calculation?

Enter the known values shown for the selected calculation and choose the actual bank connection. Check manufacturer requirements, capacitor duty, voltage rating, harmonics, switching and motor self-excitation before selecting equipment.