Replacing Start Capacitors and Run Capacitors inside a Pump Control Box

There are usually 2 capacitors inside a pump control box, a start capacitor and a run capacitor. A start capacitor is considered a consumable part in a water pump system, and should be replaced every 3 years. Why? Similar to a phone's battery, after charging so many times or being in use for years, capacitors reach their end of life. In situations with heat (control box in hot summer sun) or frequent on-off cycling (pressure tank set up), you may need to replace the capacitor even sooner. Run capacitors are similar but are made with lower capacitance and last much longer than start capacitors. We recommend a run capacitor change when 10% degradation to capacitance is observed with a multi-meter. We'll focus on start capacitors, since replacement is more of a concern for them.

Degraded Start Capacitor Performance and Risks over Time and Use

A start capacitor’s capacitance is usually stated in a range, for example, 105-126UF (MFD), or 105UF+20%. When the capacitor in the control box is used for years, the capacitance decreases below 10% of the range, near the end of life. That is the time you should change the capacitor.

Another reason to replace the start capacitor every 3 years is to prevent capacitor internal leaking that may cause an explosion. What? Explosion? Not to worry, most capacitors are engineered to allow a soft explosion, this kind of explosion does not harm you, but can cause problems for the pump.

When capacitance is below its rating, the capacitor will supply less current to the motor during each start. This could cause serious problems to the motor due to a longer start process. Longer starts may result in motor burnout or overheating, and damage to the bearings.

Capacitance can be tested with a multi-meter that has the feature built-in. Most $150 or higher price multi-meter includes the feature to test capacitance.

Capacitor Replacement

Changing the capacitor is easy and can be done by the user. You MUST disconnect the power source before replacing the capacitor!! This includes shutting off the 2 breakers (for 2 hot lines) in the power panel and any power switches for your pump system. Most control boxes are made for easy capacitor replacement. Simply unplug the wire terminals connecting to the old capacitor and re-plug them back to the new one and you are ready to go. Start Capacitors have two leads, but each lead provides two pins. Be careful to re-connect the wires in the same manner. Pliers should be used to crimp the wire terminal(connector) before plugging to a new capacitor to ensure tight fitting and good connectivity.

Among the capacitor manufacturers in the USA, Barker Microfarad Inc (BMI) is one of the top companies for the quality of their products.

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