What is a circuit?
Inside the panel, connections are made to the incoming wires. These connections are then used to supply power to selected portions of the home. There are three different combinations: 1) one hot, one neutral, and ground: 110V circuit. 2) two hots, no neutral, and ground: 220V circuit. 3) two hots, neutral, and ground: 220V circuit + neutral, and/or two 110V circuits with a common neutral.
(1) is used for most circuits supplying receptacles and lighting within your house. (3) is usually used for supplying power to major appliances such as stoves, and dryers – they often have need for both 220V and 110V, or for bringing several circuits from the panel box to a distribution point. (2) is usually for special 220V motor circuits, electric heaters, or air conditioners.
(1) is usually wired with three conductor wire: black for hot, white for neutral, and bare for grounding.
(2) and (3) have one hot wire coloured red, the other black, a bare wire for grounding, and in (3) white wire for neutral.
You will sometimes see (2) wired with just a black, white and ground wire. Since the white is “hot” in this case, both the NEC and CEC requires that the white wire be “permanently marked” at the ends to indicate that it is a live wire. Usually done with paint, nail polish or sometimes electrical tape.
Each circuit is attached to the main wires coming into the panel through a circuit breaker or fuse. There are, in a few locales, circuits that look like (1), (2) or (3) except that they have two bare ground wires. Some places require this for hot tubs and the like (one ground is “frame ground”, the other attaches to the motor). This may or may not be an alternative to GFCI protection.
Learn more about What is Grounding vs Grounded vs Neutral? here>>