Single pole switch VS double pole switch
The single-pole switch is the general-purpose workhorse of switches. It is used to control a light, receptacle, or other device from a single location. A characteristic feature of a single-pole toggle switch is that it has on and off markings on the toggle. This is something you will not find on three-way or four-way switches. However, some styles of single-pole switches (notably, rocker style switches) do not have on/off markings.
A single-pole switch has two brass-colored screw terminals that are connected to the hot, or power-source, wires. These wires are usually black. One brass terminal is designated for the incoming hot wire from the power source, and the other is for the outgoing hot wire to the fixture. Most single-pole switches also include a ground terminal for connecting the circuit's ground wire.
As a general rule, neutral (usually white) wires are not connected to switches. If two neutrals are present in the box, these wires typically are joined so that they continue through the box without touching the switch. Or, you may see a single neutral wire passing through the box. Sometimes, however, you may see a white wire attached to the switch, and this is when it is functioning as a hot wire. In this case, the white wire should have a wrap of black tape on it near the switch terminal to indicate that the wire is operating as a hot wire and not a neutral wire.
The double-pole switch is commonly used in industrial applications but can be found in some home wiring systems. Like the single-pole switch, a double-pole switch has on/off markings and controls a device or equipment from one location. The main difference is that a double-pole switch has four hot brass terminals, instead of two, plus a ground terminal. This allows the user to connect it to two pairs of hot wires from a 240-volt circuit.
Double-pole switches are commonly rated for 30 amps, compared to 15 or 20 amps with standard switches. This allows the switches to control power feeding higher-demand appliances, motors, and machinery.