It’s okay to break one wire

A common question received on the Hotline is whether it is acceptable to break only one wire with a pressure switch, float switch or probe system? As a water system’s primary operating control, the control switch senses when to turn off the water supply, either by pressure or level. Improperly wired switches may lead to control box or submersible motor failure.

When connecting the power supply to a Franklin submersible motor and control box, all local, state and national codes must be followed. The National Electrical Code (NEC) states in section 430-84: “The controller shall not be required to open all conductors to the motor.” However, the preferred method of supplying power to a Franklin submersible 2-wire or 3-wire motor is to break both incoming power lines before the control box on a 3-wire with your control device.

Typically, using a control device which only breaks one line does not cause a problem. However, if the wire which is interrupted develops a ground fault or insulation breakdown between the control device and the motor, the uninterrupted wire will supply low voltage to the motor and/or control box thru the ground fault (i.e., a 230 volt supply would have 115 volts to ground). This low voltage can cause premature failure of the motor and control box components. A submersible motor and control box will try to run as long as the low voltage is available. However, it will typically draw high amps and eventually fail. Breaking both lines eliminates this potential problem by removing all power from the control box and motor. This can be done with most standard control switches or floats.

If you are using a float switch or control switch that only has one set of contacts, then you can use a 2-pole magnetic contactor or relay. When a magnetic contactor or relay is used, the coil voltage of the contactor is controlled by the original switch, float or probe. The power supply to the motor or control box is supplied through the contacts of the relay or contactor.

Another problem when only one line is broken is that a voltage potential may exist at the control box or motor, even though the control device is open. This could cause a work hazard during repair.

Only breaking one line to control a submersible motor is done frequently, typically meets National Electrical Code requirements and may meet local codes. However, it is not the preferred method for the reasons mentioned. You should interrupt both in-coming power legs whenever possible.