The heart of a home's electrical system is the main circuit breaker box, also known as the main service panel. This is where the power feed from the utility company first enters the home from the meter and the point at which all the power is distributed to the various branch circuits of the house. All homes have a main service panel.
A subpanel is a smaller service panel that distributes power to a specific area of the home or other building on the property. It is essentially a satellite circuit breaker panel that has its own breakers and is usually installed in an area that is convenient to the area it serves. The subpanel is fed by a double-pole 240-volt breaker at the main service panel, and this single feed circuit is divided into additional branch circuits at the subpanel.
Subpanels are added to a system for three common reasons: space, convenience, or efficiency. Subpanels are usually used to extend the wiring for multiple branch circuits to a specific area of a home or to a building at some distance away from the main panel. A garage, outbuilding, or a room addition might be a place to put a subpanel. The idea is to run a single set of feeder wires from the main panel to a subpanel, where the power will then be divided into multiple branch circuits serving that building or area of the house. The circuits running from the subpanel may power light circuits, outlet circuits, or appliance circuits—just like the main service panel.
The advantage here is that the circuits can be controlled from a more convenient location than returning all the way to the main service panel, which might be at some distance. In a garage with a workshop, for example, power tools might occasionally trip circuit breakers, and resetting them is much easier if you can do it from the garage subpanel rather than returning to the main service panel.
A subpanel also can be a convenient way to add additional circuits when all the breaker slots in the main service panel are full. By running a single 60-amp breaker to a subpanel, for example, you can then divide those 60-amps into several smaller circuits.
Finally, installing a subpanel can save time and construction costs by reducing the number of "home runs" back to the main panel. It costs more in materials and labor to run three or four individual circuits from a remote location back the main panel than it does to run a single high-amperage circuit then divide it into smaller circuits from the subpanel.
How a Subpanel Is Connected
A subpanel requires two hot wires connected to a 240-volt double-pole breaker in the main panel. It also needs a neutral wire and a ground wire. The cable used for this run is known as a "three-wire cable with ground." The two hot wires, called feeder wires, will provide all of the power to the subpanel. This cable run connects to a 240-volt main breaker in the subpanel, which feeds power down through two hot bus bars. Individual circuit breakers will connect to these bus bars to distribute power to the branch circuits running out from the subpanel.