How to build a pump control panel: size the contactor, set the overload relay, wire the float switch, and protect submersible and sewage pumps.
A pump control panel sits between the supply and the motor. Its job is easy to state and easy to get wrong: switch the pump safely, protect the motor from overload and dry-run, and let a float switch or pressure switch decide when to run. Whether you are building a panel for a submersible borehole pump, a sewage lift station, or a booster set, the same core components appear every time: a contactor, an overload relay (or a motor protection circuit breaker), and a level-control device. This guide walks through how to size and select each one, with a bill of materials you can hand straight to a panel shop.
The contactor carries the motor's full-load current every time the pump starts. Size it from the motor nameplate, not from the pipe size or the pump curve. A 3-phase 4 kW motor at 400 V draws roughly 8.5 A full load; a 7.5 kW unit draws about 15 A. Pick a contactor rated for at least the motor full-load current at the duty cycle, and allow for inrush — motors pull 5–7 times their running current for a fraction of a second at start. For pumps that cycle frequently (level-controlled sewage or sump pumps), choose a contactor with an AC-3 utilization category and a comfortable margin, because each start is a wear event on the contacts. Single-phase pumps use a correspondingly rated single- or two-pole contactor. Match the coil voltage to the control circuit, typically 230 V AC, or 24 V DC where intrinsically safe control is required.
Two common ways protect the motor from overheating: a thermal overload relay paired with the contactor, or a motor protection circuit breaker that combines switching and tripping in one device. A thermal overload relay (OLR) is adjustable and tracks the motor temperature; set it to the motor full-load current. A motor protection circuit breaker (MPCB — ranges such as GV, 3UV, 3VE, M611 and 3RV) gives adjustable magnetic and thermal trips in one compact unit and can also act as a manual disconnect. For pump duty the MPCB is often the cleaner choice in a small panel because it saves space and removes a wiring joint. Whichever you use, the trip must sit above inrush but below the motor's safe temperature rise — that is why a correctly set overload is the single most important safety device in the panel.
The float switch decides when the pump runs. A cable float switch is a mechanical level sensor: it tilts at a set angle and closes or opens a contact that energises the contactor coil. Wire it so the pump starts at high level and stops at low level (or the reverse for filling duties). For dirty or viscous liquids — sewage, slurry — a magnetic or ball float switch in a stilling tube avoids fouling. In a duplex (two-pump) pit, two floats give lead/lag alternation so both pumps share the wear. Use a liquid-level controller where you need adjustable hysteresis or where the float must drive a PLC rather than switch the coil directly.
| Component | Example | How to size |
|---|---|---|
| Main isolator | Rotary disconnect switch | ≥ motor current |
| Contactor | AC contactor, AC-3 | ≥ motor full-load current |
| Overload | Thermal OLR or MPCB | = motor full-load current |
| Float switch | Cable / magnetic float | by liquid type & pit depth |
| Control transformer | 230 V / 24 V | by number of coils |
| Enclosure | IP65 distribution box | by component count |
Only for very small single-phase pumps whose float is rated for the motor current. For anything above roughly 1–2 kW, the float should switch the contactor coil, not the motor directly — the contactor handles the inrush and provides proper overload protection.
AC-3 for standard direct-on-line squirrel-cage motors. Pumps are classic AC-3 duty. If the pump is started with a soft starter or VFD, the contactor sees reduced current and an AC-1 rating may apply on the output side.
Add a separate level or pressure sensor that drops the contactor when the source runs low, in addition to the float. A dry-run is the fastest way to cook a submersible motor, so a dedicated low-level cutoff is worth the extra float.
Borehole and most submersible pumps above about 1.5 kW are three-phase for efficiency and starting torque. Single-phase is fine for small domestic booster and garden pumps. Match the contactor and overload to whichever the motor is.
If the panel is outdoors or fed from a long rural line, a Type 2 surge protective device on the supply side protects the contactor coil and controls from lightning-induced spikes. It is cheap insurance for remote pump houses.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.