Thirty-amp fuses can handle both 120 and 240 volts, which are the common voltages in the United States. Fuses or circuit breakers prevent too much current going through a circuit and reduce the risk of fire.
Amps, or amperage, indicate the amount of current in a wire. Most appliances and lighting in a home require a 15-amp circuit, but air conditioners and electric dryers require a 30-amp circuit. If more electronics are connected to a circuit than it can handle, the wires may overheat and start a fire. A fuse breaks the circuit when the current exceeds the amperage, preventing the wires from overheating.