What size breaker do I need for a 6000 watt water heater?

If you have a heater that uses a total of 6000 watts in a 240 volt connection, you'll need a minimum of 25 Amps. However, because the system will use all 25 Amps, it's safer to get a circuit breaker of 30 Amps. If you need to plug anything else into the same circuit, a circuit of 35 Amps will cover multiple devices as well as the water heater.