1. Examine the original capacitor and match its ratings in Voltage and capacitance, uF or microfarads.
Look for and record all of the markings on the capacitor.
Typically you'll see a rating in microfarads like the 25uF in our capacitor. and you'll also see a voltage range rating such as the 370VAC (maximum voltage).
2. Take the capacitor or the whole electric motor to a motor rebuild-shop or to your local electrical supplier.
If the capacitor markings are legible, they'll be able to match it.
If the capacitor markings are not legible the electrical supplier or motor rebuilder will recommend a start, run, or combination capacitor based on the motor data tag information.
3. Check the motor type & motor part number and data tag specifications and use a motor part substitution or OEM part catalog.
For readers whose capacitor has no markings whatsoever, your electrical supplier will want to know the technical details about the capacitor's intended use. Those can be found on the motor data tags for the motor served by the cap.
Capacitor Voltage Rating Selection Limits
The voltage rating of a capacitor indicates the highest nominal voltage at which it is designed to operate.
Run capacitors must not be subject to voltages exceeding 10 percent of the nominal rating, and start capacitors must not be subjected to voltages exceeding 30 percent of the nominal rating.
Capacitor Frequency (Hz) Rating Limits
Most capacitors will be rated and marked as 50/60 Hz meaning that the capacitor can run at either Hz or frequency. But do not use a cap marked only 50Hz on a 60Hz circuit and vice versa.
4. Choose a capacitor based on Electric Motor Type, Motor Horsepower or Motor Kw or Kilowatt rating.
If you cannot find any data giving the motor's brand, model, specifications, and you have no markings nor even the actual old start/run capacitor from an electric motor, you might get into the right ballpark by choosing a capacitor based on the motor's horsepower and voltage and application.
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