Motor Capacitor Size Calculator: Run and Start Capacitor µF for Single-Phase Motors
Estimate run and start capacitor µF and voltage rating for a single-phase motor from HP or kW, voltage and 50/60 Hz, or test a capacitor by its current.
How it works
Start capacitor ≈ FLA × 10⁶ ÷ (2π f V) µF (= 2,653 × A ÷ V at 60 Hz, 3,183 × A ÷ V at 50 Hz) · run capacitor ≈ 30–50 µF per kW at 230 V, 50 Hz, scaled by (230 ÷ V) × (50 ÷ f) · test: C = I × 10⁶ ÷ (2π f V) · energy: C = 2E ÷ V²
A single-phase motor needs help to start turning, and many use a capacitor to shift the current in the auxiliary (start) winding so that the two windings make a rotating field. A start capacitor is large (often 100–800 µF), works for a second or two and is then switched out. A run capacitor is small (usually 5–80 µF), stays connected and improves running torque and efficiency. Capacitor-start motors have only the first, PSC motors only the second, and capacitor-start capacitor-run motors have both. The best value is always the one printed on the motor or the old capacitor. When that is missing, this calculator gives starting estimates. The start capacitor is sized so that it passes roughly the motor’s full-load current at line voltage, C = I ÷ (2πfV). Full-load current comes from your nameplate or from NEC Table 430.248 (its 115 V column for 110–120 V and its 230 V column for 220–240 V supplies), and the result lands inside the standard catalog ranges used for motors of that size. The run capacitor uses the common rule of 30–50 µF per kW at 230 V and 50 Hz, scaled for your voltage and frequency, so treat it as a range to try. The test mode works the other way: measure the current through a running capacitor and the voltage across it, and C = I ÷ (2πfV) tells you its real capacitance (the HVAC "2,653 × amps ÷ volts" rule at 60 Hz), which you can compare with the label. The energy mode solves E = ½CV², as some online tools do. Unlike the power-factor correction calculator, which sizes kVAR banks for a whole installation, this one is for the capacitor inside one single-phase motor.
Worked example
A 1 hp capacitor-start, capacitor-run motor on 230 V, 60 Hz draws 8.0 A at full load (NEC Table 430.248). Start capacitor ≈ 8.0 × 1,000,000 ÷ (2π × 60 × 230) = 92.3 µF, inside the standard 88–106 µF range, rated 250 VAC or higher. Run capacitor ≈ 0.746 kW × 30–50 µF/kW × (230 ÷ 230) × (50 ÷ 60) = 18.6–31.1 µF, so start with about 24.9 µF, nearest standard size 25 µF at 370 VAC.
Sources: Elliott Electric: Full-load current for single-phase AC motors (NEC Table 430.248) · InspectAPedia: Motor start and run capacitor selection (30–50 µF/kW run rule, tolerance and voltage ratings) · Omni Calculator: Motor capacitor size (E = ½CV², 9 J at 110 V example) · Wikipedia: Electrical reactance (capacitive reactance)
Frequently asked questions
What size capacitor do I need for a 1 hp motor?
On 230 V a 1 hp capacitor-start motor typically uses a start capacitor around 88–106 µF and, if it has one, a run capacitor of about 20–30 µF. On 115 V the start capacitor is about four times larger in µF (roughly 340–408 µF), because the same reactive power needs C ∝ 1/V². The nameplate value always wins.
How do I calculate capacitor µF from amps and volts?
C (µF) = amps × 1,000,000 ÷ (2π × frequency × volts). At 60 Hz this simplifies to 2,653 × amps ÷ volts; at 50 Hz, 3,183 × amps ÷ volts. Example: 4.7 A through a capacitor with 360 V across it at 60 Hz is 2,653 × 4.7 ÷ 360 = 34.6 µF.
Can I use a higher µF capacitor on my motor?
For a start capacitor, up to about 20% higher is usually acceptable. For a run capacitor stay within the rated value and its ±5–6% tolerance: too many µF overheats the start winding and too few reduces torque. A higher voltage rating is always fine; a lower one is not.
What voltage rating should the capacitor have?
Run capacitors see more than the line voltage, so 230–240 V motors use 370 or 440 VAC run capacitors and 115–120 V motors use 250 or 370 VAC. Start capacitors are rated closer to the line voltage: 110–125 VAC for 115 V motors and 220–250 VAC or 330 VAC for 230 V motors.
What is the difference between a start and a run capacitor?
A start capacitor is a large electrolytic (usually a black plastic case) that is in the circuit only for the first second or two. A run capacitor is a smaller film capacitor (usually metal-cased) that stays connected the whole time the motor runs. They are not interchangeable.
Related calculators
Capacitor values here are estimates. Always follow the motor nameplate or the manufacturer’s data, and have a qualified electrician work on motors and capacitors, which can hold a dangerous charge.