[rjniles]

Water heaters are typically 30 amp not 20.

 

[rjniles]

A water heater is a continuous load. Rated current is increased to 125% of normal amps. 18.75 times 1.25. Requires 30 amp circuit.

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[rjniles]

Not all appliances are a continuous load. In residential the WH is about the only one; electric cook top and oven are not.

I would not install a sub unless you are short on breaker spaces. Run 10/2 for the WH, run 10/3 for the cook top, Run 10/3 for the oven. You could use 12/3 for the oven but 10/3 will future proof the install.