When I bought my house 5 years ago, the home inspection showed the water heater was leaking. I negotiated with the seller to replace it. The WH was a 30 gallon model and I paid to upgrade to a 50 gallon. The seller hired a licensed plumber to do the work. After the closing (dumb me), I noticed that the plumber had reused the existing circuit on the new WH. It was 12-2 on a 20 amp breaker. It was my nickle to upgrade to 10-2, 30 amp.
I guess my point it that there are water heaters that can be used on a 12-2, 20 amp circuit, but you never know what it be changed to in the future. Best to use 10-2, 30 amp.
That makes a lot of sense to me. I guess the best way to do any job is the right way. It's a lot less hassle in the long run.
I know older electric water heaters used to be run on 20 amp 240 volts circuit as long the wattage is under 2500 watts and per NEC code the water heater and electric heaters are condsering a contiounus load which it mean it can stay on more than 3 hours.
I think the Canada codes are simauir so the best bet is go with 6.0mm˛ with 30 amp OCPD and be done with it due most of the newer water heaters are using 4500 somecase 5500 watts.