Motor Start Capacitor Calculator
Enter the run capacitor (CSCR) or motor power (CSIR) to find the start capacitor in µF and its voltage rating.
Step-by-step
- Enter valid values to begin.
Cstart = 3.5 × Crun (typical band 3.0–5.0×). CSIR motors have no run capacitor, so a reference run value C = Pη / (2π f V²) is used first. Start capacitors are intermittent-duty parts (roughly 3 s, about 20 starts/hour). The motor manufacturer’s replacement part number always overrides this rule of thumb.
Motor Start Capacitor: Big Microfarads for a Short Job
Core Engineering Principles
A start capacitor exists to do one thing: put the auxiliary winding almost ninety degrees out of phase with the main winding for the two or three seconds the rotor needs to get moving. It has to be large to do that, because starting current is high and the capacitor must supply a lot of reactive volt-amperes. As a working rule we take the start capacitor at about 3.5 times the run capacitor, with 3.0 to 5.0 times as the sensible band. The low end suits easy loads such as fans, and the high end suits hard starters like compressors and conveyors against pressure.
The part is built for intermittent duty, not continuous duty. Its electrolytic construction heats up quickly with current flowing, which is why the centrifugal switch or the potential relay has to drop it out at about 75 to 80% of speed. Leave it in, and it cooks in under a minute, often with a small bang. On a capacitor-start induction-run motor there’s no run capacitor at all, so the 3.5× rule has nothing to hang on. In that case we base it on the motor’s power, using a reference run value from P × η / (2πfV²), and then apply the same ratio.
Cref = P × η / (2π f V²) (CSIR, no run cap)
Vstart ≥ 1.5 × Vsupply • IC = 2π f C V
NEC & Standard References
NEMA MG-1 Part 1 defines capacitor-start and capacitor-start capacitor-run motors and how each should perform at locked rotor. IEC 60252-1 sets the construction, endurance and safety requirements for motor capacitors, including the duty classification of the start capacitor. UL 810 is the North American capacitor standard. NEC 430.32 and 430.52 protect the motor and its branch circuit regardless of what the capacitor does. Confirm the adopted edition, and let the manufacturer’s own replacement value override this rule of thumb.1. Nominal start value = 3.5 × 25 = 87.5 µF.
2. Range = 3.0 × 25 to 5.0 × 25 = 75 to 125 µF.
3. The nearest standard can is 88 µF.
4. Voltage rating = 1.5 × 230 = 345 V, so choose a 370 V part.
5. Peak starting current through the capacitor = 376.99 × 88 × 10−6 × 230 = 7.63 A while it is in circuit.
- Never swap start and run capacitors. A run can in the start position blows up from continuous duty current, and a start can left in run service burns out in minutes.
- Check the switch. A welded centrifugal switch or a stuck potential relay keeps the start capacitor in circuit and is the number one cause of swollen cans.
- Limit the starts. Typical start capacitors tolerate about 20 starts per hour, three seconds each. Short-cycling a compressor exceeds that.
- Use the bleeder resistor. A 15 kΩ resistor across the can lets it discharge, and it protects the switch contacts from welding.
- Don’t go smaller to save money. An undersized can gives weak starting torque and a motor that hums without turning, which burns the winding in seconds.