When a submersible pump stops, the usual assumption is that the pump has died and the well needs pulling. Often it has not. The part that failed is sitting on the wall of the well house, at eye level, and costs a fraction of a pump.
That part is the control box, and understanding what it does is the difference between a cheap afternoon and an expensive one.
What Does a Well Pump Control Box Do?
A well pump control box holds the starting components for a three wire submersible motor: the start capacitor, the relay, and on larger motors an overload. The motor will not start without them. They live above ground rather than down the well precisely so they can be replaced without pulling the pump.
Key Takeaways
- Only three wire motors use a control box. Two wire motors do not.
- The box must match the motor horsepower, voltage and phase exactly.
- A humming motor that will not start usually means a failed start capacitor.
- The box is above ground, so check it before paying to pull the pump.
- Wiring and testing involve line voltage and belong to an electrician.
Two Wire or Three Wire? Check First
This single distinction causes more wasted orders than anything else on the topic.
| Two wire motor | Three wire motor | |
|---|---|---|
| Control box | None required | Required, and matched |
| Starting components | Sealed inside the motor | In the box, above ground |
| Replacing a failed capacitor | Pull the pump | Open the box |
| Wires down the well | Two plus ground | Three plus ground |
| Typical use | Smaller residential | Most sizes, larger duty |
Read the wire count off the motor nameplate rather than counting the cable at the wellhead, and confirm horsepower and voltage at the same time. A box rated for the wrong horsepower will start the motor and then damage it, which is the expensive version of this mistake.
What Fails Inside the Box
The start capacitor
This is the most common failure by a wide margin. A capacitor that has bulged at the top, leaked, or reads well under its printed rating no longer supplies the starting torque. The symptom is a motor that hums and trips rather than one that is silent. The underlying role of a motor start capacitor explains why: it creates the phase shift the motor needs to begin turning at all.
The relay
The relay switches the start winding out once the motor is running. Pitted or welded contacts leave the start winding energized, which overheats the motor, or prevent it engaging at all.
Heat damage
Scorched terminals and melted insulation mean something has been running hot, often a loose connection. Replacing the burnt part without finding why it got hot usually buys a few weeks.
All three are surface-level parts. The pump itself may be perfectly serviceable, which is why the box is worth opening before anyone books a rig. If the box checks out clean, the diagnostic order continues in our guide to telling whether a well pump is bad.
Franklin Control Boxes and Motor Protection
Franklin Electric builds control boxes to match its submersible motors across the residential and light commercial range, using quick disconnect designs on the smaller horsepowers and capacitor start capacitor run construction as sizes increase. The box carries its own model number, and matching that number to the motor is what keeps the pairing correct.
Motor protection is a separate device rather than part of the box. Franklin’s Pumptec monitors load and voltage and shuts the pump down on dry running, low yield or supply problems, which are the conditions that destroy motors quickest. On a well with a falling water table it pays for itself the first time it acts.
Boxes, capacitors and protection devices are all part of the wider Franklin Electric range, and the parts side is covered in our guide to sourcing Franklin parts.
Wiring: What to Know Before You Open It
The wiring diagram for any Franklin box is printed inside its cover and repeated in the manual. Follow that diagram rather than a color convention remembered from another pump, because terminal markings and lead arrangements vary between models and manufacturers.
Everything inside the box is at line voltage, and a start capacitor can hold a charge after the power is off. This is qualified electrician work. What an owner can safely do is read the nameplate, photograph the label, and check whether the capacitor looks bulged before calling anyone.
Once a pump is running again, the thing most likely to shorten its life is not the box but the pressure tank, as covered in our notes on keeping a well pump serviceable.
Frequently Asked Questions
What does a well pump control box do?
A well pump control box holds the starting components for a three wire submersible motor, which are the start capacitor, the relay and on larger sizes the overload. The motor cannot start without them. They sit above ground in the box rather than down the well so they can be replaced without pulling the pump, which is the whole point of the design.
How do you tell if a well pump control box is bad?
Tell if a well pump control box is bad by opening it and looking before testing anything. A bulged, leaking or burnt start capacitor, pitted relay contacts or a scorched terminal all point at the box rather than the pump. If the motor hums but will not start, the box is the first suspect, because the capacitor supplies the starting torque.
How do you test a well pump control box?
Test a well pump control box by checking line voltage at the input terminals, then measuring the start capacitor with a meter set to capacitance and comparing it against the rating printed on its case. A capacitor reading well below its rating has failed. Testing involves live line voltage, so it is work for a qualified electrician rather than a homeowner.
Do all submersible pumps need a control box?
No, not all submersible pumps need a control box. Two wire motors have their starting components sealed inside the motor and run without one. Three wire motors need a matching box, and the two are not interchangeable. Check the motor nameplate before ordering, because the wire count decides whether you need a box at all.
Control Boxes and Submersible Parts
Ken’s Distributing supplies Franklin Electric submersible motors, control components and the repair parts that go with them. Send the motor nameplate details, horsepower, voltage and wire count, and we will confirm the matching box before you order.
Control boxes are one piece of a much wider catalog, mapped out in our guide to the Franklin Electric pump range.