Hi and Welcome to Just Answer,
Ok, let's take a look at this;
This will depend on the water heater, if it is a 20 Amp hookup you will be fine, but if it is a 30 amp hookup you need to use 10 gauge wire.
Here is a commonly used table to go by
Suggested Gauge vs. Breaker Amp
For Copper branch circuits and feeders.
And a Common used Table for Appliances
Common Usage - Breaker / Fuse size:
Stove / Oven
If you need more help Let me know
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