Tandem breakers, often called split, twin or double breakers, provide two separate circuits in the space of a regular sized breaker opening. Every circuit breaker panel (a.k.a. your home's breaker box) has a limited number of circuit breaker slots available. The problem is that when the openings are all used up and you still need to add another circuit, what do you do? You could upgrade the electrical panel or add a sub-panel, and this makes sense if you need to add a lot of circuits.
If you need to add just one or two new circuits, sometimes it's possible to use tandem breakers, but your panel must be designed for them.
What Is a Tandem Breaker?
A tandem breaker is the same size as any other standard, or single-pole, breaker but it has a key difference. This breaker sports two smaller breakers built into one regular-sized breaker body. Each has its own toggle switch, and the breaker snaps into the panel just like a regular breaker. A tandem breaker supplies two separate 120-volt circuits. This is not the same as a double-pole breaker, which serves a single 240-volt circuit that is used for high-voltage appliances like dryers and ranges.
Where Tandem Breakers are Allowed
Tandem breakers are safe and legal to use only when the panel is designed for them and only in the slots that accept tandem breakers. Look at the manufacturer's labeling on the inside face of the panel door.
If the panel accepts tandem breakers, the label should indicate how many you can use and in which slots you can use them. Some panels allow tandems on just two or four slots; others may allow them on all of the standard slots. But no matter how many tandems are allowed, you still must stay under the safe load limit on the panel.
In other words, just because there is space for more breakers doesn't mean you can't overload the panel by installing them. Checking the safe capacity of your panel is a job for an electrician.
Problems with Tandem Breakers
Being a smaller version of a typical circuit breaker has its drawbacks. At half the size, these smaller breakers seem to heat up quicker and easier when under load. I have found tandem breakers to trip more easily than a normal-sized circuit breaker. The other downfall is that the pecking order of the phases is disturbed by adding tandem circuit breakers. You see, every other standard circuit breaker is on a different phase of the power. That's why the breakers are arranged in two equal rows and why the slot numbers alternate from side to side (evens down one side, odds down the other). This helps create a balanced load on the panel. Tandem breakers have two connections on only one side of the panel, and if you have too many tandems on one side the load can be out of balance.
A More Complete Solution
For simply adding a new circuit in your home with its own neutral, a tandem circuit breaker can do the trick. Now for the news you do not want to hear. I would not recommend adding the tandem circuit breaker.
If the circuit breaker panel is full, the better option is to add a sub-panel or upgrade your electrical panel. This may seem extreme, but I've seen where the use of tandem breakers can lead. I've actually seen a circuit breaker panel half full of tandem circuit breakers. This is not a good sign. Not only are you likely overloading the circuit breaker panel, but it's likely the percentage of fill of electrical wires in your panel is way overboard. Upgrading to a larger panel or adding a sub-panel is a safer solution that gives you more flexibility for future expansion of your electrical system.