Circuit breakers come in several styles, including the standard single-pole breakers that serve 120-volt household circuits and double-pole breakers that take up two slots in a breaker box and serve 240-volt appliances. A special kind of circuit breaker is the tandem breaker, which is designed to allow two 120-volt circuits to be fit into a single slot in the circuit breaker box. They are normally installed in situations where the breaker box is already full with no available slots for additional circuit breakers.
Identifying a Tandem Breaker
A tandem circuit breaker has a single plastic body, sized the same as a normal 120-volt circuit breaker. But it also has two breaker levers on the face, which operate separate circuits. Internally, the device has separate mechanisms to control and protect separate circuits, and should one of the circuits overload, only one of the handles will trip to the off position.
Tandem circuit breakers are not always allowed in circuit breaker panels. Check first with your local electrical inspector to determine if using tandem breakers is acceptable, and if so, how many are allowed. In most cases, when an existing panel has run out of usable space, a limited number of tandem breakers are allowed. In new construction, however, tandem breakers may not be allowed at all.
Tandem breakers are not the same as another solution that has sometimes been used in situations where there aren't enough slots in the breaker box—the slimline breaker. Slimline breakers are narrow-bodied breakers that can be slotted side-by-side in a single slot in the breaker box. The effect is the same—two circuits in a single breaker slot—but tandem breakers have a single body with two internal breaker mechanisms, while slimline breakers are simply extra-narrow breakers that can be fit side-by-side in one slot.
Limitations for Tandem Breakers
Since a tandem circuit breaker uses only one space in the breaker box, it is connected to one of the hot bus bars and therefore uses only one phase of power. This limits the ability to use a single 3-wire cable to feed both circuits, as is sometimes done for kitchen countertop circuits and in other applications. In such configurations, where two hot wires feeding separate circuits share a common neutral wire, the hot wires must be connected to different hot bus bars. This is impossible to do with tandem breakers.This means that tandem breakers are suitable only where both circuits are fed by individual cables, not a single cable with a shared neutral.
Another limitation of the tandem circuit breaker is that it cannot take as much heat as a standard single circuit breaker. With two circuits encompassed in the same frame, each wire can heat the same circuit breaker while being under load.
No AFCI or GFCI Option
While tandem breakers are sometimes found in older breaker panels where there isn't room for all the necessary standard circuit breakers, they are rarely, if ever, used in new construction, due to new requirements requiring that most residential circuits have AFCI (arc-fault circuit interrupter) protection. This protection is usually offered by special AFCI circuit breakers, which are not currently offered in tandem styles. Nor are tandem breakers currently available in GFCI models, so in situations where a circuit requires either AFCI and/or GFCI protection, the only option is a standard circuit breaker. Tandem circuit breakers are not an option in these circumstances.
In addition, when an electrician makes repairs or upgrades to an existing electrical system, he is required by law to install whatever AFCI and GFCI protection is required by current code. This means that he may be required to remove tandem breakers and install the code-required AFCI circuit breakers. If your panel is already full, this can lead to some fairly major changes in order to fit all the circuits, possibly even requiring the installation of a subpanel or all-new main service panel.