As a safety precaution, always discharge any capacitor before handling, because capacitors can hold a charge for a prolonged period of time. Discharge a capacitor by holding onto the insulated handle of a screwdriver and touching the metal blade between the two posts of the capacitor

There are two basic types of capacitors used in electric motors, start capacitors and run capacitors. Start capacitors can usually be distinguished by their black plastic case, and run capacitors are contained in a silver metal canister, or in the case of WEG capacitors, a gray plastic case. Start capacitors are designed to be in the circuit for only a few seconds as the motor starts and comes up to speed, while run capacitors are designed to stay in the circuit continuously while the motor is running.

As materials and insulation have improved through the years, new materials have allowed manufacturers to make the insulation thinner, and thereby reduce the physical size of the case on capacitors. If the new capacitor is physically smaller, you can assume that your capacitor is older, and as long as the MFD value and voltage are the same, the capacitor will work fine. If the capacitor is loose in the case due to size reduction, foam rubber or any non-conductive material can be added to keep the capacitor from vibrating or moving inside the capacitor cover.

Checking the capacitance of start capacitors or run capacitors requires a meter designed to read capacitance, and most meters designed for electronics do not have high enough ranges to read the values in start or run capacitors. Standard digital meters , if equipped with a capacitance scale, will usually not read above 1 MFD, far below the value of motor start and run capacitors.

All capacitors have two values of importance: Capacitance and Voltage.

Capacitance is rated in microfarads, abbreviated MFD, or uf ,or a symbol that looks like a backwards small case y with an F after it. The size of the capacitor is chosen, in an electric motor circuit, to offset the inductance of the winding, a characteristic determined by the size of the wire and the number of turns in the winding.  Changing the value of the capacitor, going either larger or smaller in MFD value, will negatively affect the starting power of the motor because the capacitance will no longer offset the inductance correctly, so always replace the capacitor with one of the same value and voltage.

In start capacitors, capacitance will be listed either as a single value, or a range of values. For example a capacitor may be marked  450MFD  or 400-480 MFD. If marked with one value, the capacitor will normally state a plus or minus percentage. If plus or minus 10 %, the 450 MFD capacitor, if measured, will have a value somewhere between 405MFD and 495MFD. The capacitor with a range listing of 400 to 480 MFD will simply fall somewhere within the listed range. Different manufactures have slightly different range designations, and no problems will be encountered if you have one with a slightly different range or an overlapping range.

Run capacitors will show only one value with a plus or minus percentage stated, and a 10 MFD should be replaced with a 10 MFD. Note however that run capacitors can be manufactured in either round or oval cases. They are interchangeable, but mounting hardware or size limitations may make one shape preferable over the other.

The voltage rating on the capacitor is the maximum allowable voltage the capacitor is designed to handle. Older capacitors sometimes use 110 volt rating or 220 volt rating while many newer capacitors will show 125 volt, 165 volt,  250 volt , or 330 volt ratings. 110 , 115, 120, or 125 volt rated start capacitors are interchangeable, as are 220, 230, 240,or 250 volt capacitors. Run capacitors are rated 370 volt, or 440 volt, or in  WEG 400 volt. In 120 volt circuits, 370 Volt rated run capacitors are the norm as voltage spikes seen by the capacitor can exceed the 120 Volt input Voltage.

Our parts numbering system uses the smallest number of the capacitor’s range followed by the voltage designation. So our part # C400E125 would be a 400 to 480 MFD capacitor rated for 125 Volts. All start capacitors in our inventory are round.  In run capacitors a C10E370 is a 10 MFD oval capacitor rated for 370 Volts AC. If the run capacitor part number ends in R, it shows that the case is round.

Always check the physical size of the capacitor to be replaced and compare it with the size specifications in our listings. Note that the WEG capacitors are for the most part smaller than the standard capacitors and can be used to replace capacitors that are smaller than normal as long as the capacitance and voltage are the same.

Due to the possibility of damage to the capacitor installed in motors with other problems, we cannot accept returned capacitors for any reason.

All capacitors are listed in our online store, available by clicking the link on this page, please note that the WEG capacitors are listed separately in the store.

We hope you have found this information helpful in the selection of your capacitors. If you have any questions, please feel free to email us at sales@stoutselectric.com or call us at 970-243-3944 and we will be happy to answer them to the best of our ability. If you are emailing, please include  Capacitor Question in your subject line so we will see it more easily and will be able to promptly reply.

Capacitor Operation, Selection, and Information