Replacing Start Capacitors and Run Capacitors inside a Pump Control Box

There are mainly two types of control boxes.

Example of a typical control box capacitor.
Pictured here is a Barker Microfarad Start Capacitor

(A) Quick disconnect control box. In this type of box, there is only a start capacitor in it. The capacitor will be disconnected through a relay contactor once the motor speeds up. This type of control box is used with some 1HP or smaller pumps where the motor's efficiency is not a big concern.

(B) Dual capacitor control box. There is a start capacitor with higher capacitance and a run capacitor with lower in the control box. In this type of box, the start capacitor is connected to provide high current to quickly start the pump. It will be disconnected once the motor speeds up. The start process usually takes less than a second. The advantages of  a dual capacitor control box are more reliably starting the pump with its high MFD capacitor and running the motor more efficiently with its full-time run capacitor.  Tuhorse Control Box Pro, Franklin Electric's Standard and Deluxe control box are this type of control box.

The connectors of typical control
box capacitor. Pictured here is the bottom
 of a Barker Microfarad Capacitor 

The start capacitors and run capacitor are considered consumable parts in a water pump system. Start capacitor should be replaced every 3 years and every 6 years for the run capacitor. 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 of 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 made with lower capacitance and last longer than start capacitors. We recommend a 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%. A new capacitor usually comes within its rated capacitance range. When the capacitor in the control box is used for years, capacitance will drop below the rated range, near end of life (10% less than rated). 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.

How do you test a pump capacitor? 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 wires terminal 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 terminals (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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