A control system will normally use AC and DC power at different voltage levels. Control cabinets are often supplied with single phase AC at 220/440/550V, or two phase AC at 220/440Vac, or three phase AC at 330/550V. This power must be dropped down to a lower voltage level for the controls and DC power supplies. 110Vac is common in North America, and 220Vac is common in Europe and the Commonwealth countries. It is also common for a controls cabinet to supply a higher voltage to other equipment, such as motors.

An example of a wiring diagram for a motor controller is shown in See A Motor Controller Schematic (note: the symbols are discussed in detail later). Dashed lines indicate a single purchased component. This system uses 3 phase AC power (L1, L2 and L3) connected to the terminals. The three phases are then connected to a power interrupter. Next, all three phases are supplied to a motor starter that contains three contacts, M, and three thermal overload relays (breakers). The contacts, M, will be controlled by the coil, M. The output of the motor starter goes to a three phase AC motor. Power is supplied by connecting a step down transformer to the control electronics by connecting to phases L2 and L3. The lower voltage is then used to supply power to the left and right rails of the ladder below. The neutral rail is also grounded. The logic consists of two push buttons. The start push button is normally open, so that if something fails the motor cannot be started. The stop push button is normally closed, so that if a wire or connection fails the system halts safely. The system controls the motor starter coil M, and uses a spare contact on the starter, M, to seal in the motor stater.

A Motor Controller Schematic

The diagram also shows numbering for the wires in the device. This is essential for industrial control systems that may contain hundreds or thousands of wires. These numbering schemes are often particular to each facility, but there are tools to help make wire labels that will appear in the final controls cabinet.