How does a tandem breaker work?
Regarding this, is it OK to use tandem breakers?
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. In other words, just because there is space for more breakers doesn't mean the panel can safely handle the extra load of new breakers.
Subsequently, question is, can you use a tandem breaker for 240v? As others have mentioned, it's not possible to get 240 volts from a single pole in a 120/240V split phase system. Each tandem breaker provides 2 120 V circuits, this is true. However, if you measure between the terminals on a single tandem breaker, you'll get 0 volts.
Considering this, how many tandem breakers are allowed in a panel?
The model or part number of the electric panelboard usually will indicate whether or not the electric panelboard is designed to accept tandem breakers and how many can be used. Here are a few examples: G3040BL1200 = 30 spaces, 40 total circuits allowed. Up to 10 tandem circuit breakers can be used.
How do you wire a tandem breaker?
Locate the wiring you wish to connect the tandem circuit breaker to. There should be two "hot" wires, usually insulated in black, and two grounding wires, usually insulated in green. Test them all with a multimeter set to the "Volts AC" position to make sure there is no electricity running through them.