Here's why:

* Calculating Amperage: To determine the appropriate circuit breaker size, you need to calculate the amperage draw of the water heater. You can do this using the following formula:

Amps = Watts / Volts

In this case:

* Amps = 1500 watts / 120 volts

* Amps = 12.5 amps

* Circuit Breaker Sizing: Electrical codes generally require a circuit breaker to be sized at 125% of the continuous load. A continuous load is anything that runs for 3 hours or more.

* 12.5 amps x 1.25 = 15.625 amps

* Standard Circuit Breaker Sizes: Since you can't get a breaker for 15.625 amps, you round up to the next standard size, which is a 20-amp breaker.

Important Considerations:

* Local Electrical Codes: Always consult your local electrical codes to ensure you are meeting all safety requirements.

* Dedicated Circuit: It's essential to dedicate a separate circuit for your water heater. Don't connect it to the same circuit as other appliances.

* Wire Gauge: The wire size used for the circuit should match the circuit breaker rating. A 20-amp circuit requires 12 AWG wire.

Remember, electrical work can be dangerous. If you are not comfortable working with electrical systems, consult a qualified electrician.