An electrical subpanel is a secondary circuit breaker panel that feeds off the main panel. It is installed for one of two reasons: either the main circuit breaker panel is already full of breakers and provides no room for installing additional circuits; or the subpanel is installed to put the control point for some circuits in a place close to the area they serve—such as in a woodworking shop or an upstairs apartment. When the goal is simply to add space for more circuits, the subpanel is often placed adjacent to the main panel.
How a Subpanel Works
In modern larger homes, you may sometimes find two main panels, each drawing power from the main utility. These are typically homes with 300-amp or 400-amp service, where there is one 200-amp main panel, plus a second 100-amp or 200-amp panel. In this case, though, neither panel can be described as a subpanel—they both are main panels, and both have their own main shutoff breaker.
A true subpanel is a smaller circuit panel breaker with four to 24 slots that is fed from a 240-volt circuit breaker in the main panel, known as a feeder breaker. In most ways, the subpanel looks just like a smaller version of the main panel, with the same two hot bus bars, but it is equipped with a separate ground bus bar to allow for neutral and ground separation, per National Electrical Code (NEC) regulations. The subpanel may be equipped with a main breaker to allow for power interruption without having to go back to the main panel, but it is not required to have a main shutoff circuit breaker, since the feeder breaker back in the main panel serves this function.
How a Subpanel Is Connected
When an electrician installs a subpanel, the usual process is to first run a feeder cable from the main panel to the subpanel. This is typically a three-wire cable with three insulated conductors plus a bare copper ground wire. The cable must have a wire gauge sufficient to the amperage of the subpanel—a 100-amp subpanel requires #4 copper wires or, more commonly, #2 aluminum wires, for example. (Aluminum is often used for feeder cables because the cost is typically much lower than that of copper wires.)
The electrician connects the two hot wires from the feeder cable to the lugs on the hot bus bars in the subpanel, the neutral wire to the neutral bus bar, and the bare copper grounding wire to the grounding bus bar.
Back at the main panel, the electrician now attaches the hot feeder wires to a new 240-volt circuit breaker, then connects the neutral and ground wires from the feeder cable to the corresponding bus bars in the main panel. Finally, the electrician snaps the feeder breaker into an open double slot in the main service panel.
The subpanel is now ready to accept individual circuit breakers for the added circuits being installed.
There are two capacity issues that must be considered when you install a subpanel. First, the subpanel itself must have an appropriate amperage capacity for the area and circuits it will serve. And second, the main service must be sized so that it is sufficient to supply power for the added demand created by the subpanel.
If you have older 60-amp or 100-amp main service, it is entirely possible that you will need a service upgrade to 150-amps or 200-amps before you are able to adequately power a new subpanel. This is especially true if you are expecting the subpanel to provide the circuits to a major room addition, a comprehensive kitchen remodeling project with several new circuits, or a workshop with heavy power needs. If you already have 200- or 300-amp service, it is generally not a problem to install a 60-amp or 100-amp subpanel, but a service upgrade will likely be necessary if you have just 60-amp or 100-amp service.
The electrician installing your subpanel will carefully calculate loads and recommend the service upgrade if it is necessary. (Be an educated consumer—a service upgrade can cost between $2,000 and $4,000, so get a second opinion if necessary.) If so, the best solution might be to omit the subpanel in favor of installing a new main panel that has plenty of slots for both existing circuits and future expansion. Large main panels may have as many as 40 slots (or up to 80 circuits, when using tandem-style breakers), which may be enough for any conceivable future expansion.