No, they each need their own breaker of the right amp. Neither of them would cause the breaker to trip if there was a problem.
30 amp breaker
I think you need to do some checking, A GAS water heater does not normally need electrical service that large- an ELECTRIC water heater will. So- just what do you have- gas or electric?
i have 30 amp on mine works just fine
10/3 wire with 30 amp double pole breaker from panel box to water heater.
Assuming you are referring to a 240 volt residential hot water heater you will need to install 10/2 NM cable (Romex) with ground from the service panel to the water heater wired to a 30 amp circuit breaker in the service panel. If you cannot see the water heater from the service panel you will need to install a service disconnect at the water heater.
try higher amp breaker
20 amp is perfect, 15 is fine
Look on the heater and see what amps it is pulling. That will determine the wire size and breaker size. It must be on a dedicated circuit. 15 amps = AWG # 14 wire with 15 amp breaker 20 amps = AWG # 12 wire with 20 amp breaker 30 amps = AWG # 10 wire with 30 amp breaker 40 amps = AWG # 8 wire with 40 amp breaker
If the water tank is a standard house tank you will need a two conductor #12 cable protected by a two pole 20 amp breaker.