Square D pressure switches are used to control air compressors and water pumps. Water pump pressure switches operate at low pressure, generally in the range of 30 PSI to 50 PSI. Air compressor pressure switches have a much higher operating range and overall pressure, usually between 80 PSI and 120 PSI. Although the two types of switches have different physical characteristics, they are wired in the same fashion.
Correct Pressure Switch
Before any wiring can be performed, locate and identify the type of switch needed and the motor's load requirements. All Square D pressure switches have specifications for the switch located under the cover. Pull the cover off and read the label. The switch label contains information on amperage ratings and pressure ranges. It may also be specific as far as use, for only water systems or air compressors. Read the motor's name plate data tag for its full load amperage capacity. The tag is located on the motor's exterior metal frame or cover. This value must not exceed the rating for the pressure switch. Physically install the new switch into the system's pressure piping. There will be two sets of wires. One pair of wires will come from the power source or main electrical panel. The other set of wires will lead to the pump motor or air compressor motor. In each case, the power source must be shut off.
120 VAC Circuits
Observe the wiring connections of the pressure switch. Note that there are four screws electrically connected to the same number contacts. Contacts for the switch are round copper discs that will make or break the electrical connection. The discs are mechanically connected to a set of pressure springs. Wiring a 120 VAC circuit requires only one set of these contacts and screws. The black, hot wire, will be secured to one set of these contacts. The white, neutral wire, will not be connected to the switch. Strip all the wires, so that one inch of bare copper is revealed from the insulation. Twist the white wires together using wire pliers. Cover the bare wire connection with an insulated wire nut. Secure one black wire, the power feed, to one of the screws. You can use either the right or left set of contacts. The sets of contacts are identified by a small connecting bar attached to the small copper discs. In other words, the two screws to the right are for one set of contacts. The same is for the left set of contacts. Connect the other black wire, feeding the motor, to the other screw for the same set of contacts. Attach the fully bare copper wire, if used, to the green screw located on the metal plate of the pressure switch itself. The fully bare copper wire is used for the earth safety ground of the electrical system.
240 VAC Circuits
The 240 VAC circuit pressure switch is wired in much the same fashion as the 120 VAC circuit. The only exception is that both wires coming from the power panel are hot (contain voltage). Attach one set of wires, the hot wire, from the panel and feed wire to the motor, to one set of the contacts. The other set of wires will be tied to the other set of contacts. Both power feed wires are then controlled simultaneously by the pressure switches contacts.