How did I fail to see this thread was on the "Canadian"forum? Simple- I was grabbing a thread from the 'recent post' sidebar.
Now, pool, I'd like a shot at changing your opinion of plumbers!
In your house, it's common for the plumbing to be laid out like a tree ... a big line at the start, getting smaller as you near the final appliance. This is so the first pipe can supply the shower, the toilet, and the kitchen sink all at once. By the time you get to the final run to any of those things, the line is much smaller- as it needs only to supply that one.
I submit that's exactly what you're doing with your wire sizing. What can go wrong?
Well, there's a big difference with electrical work, in that we have the possibility of dead faults, and circuit breakers.
Break a pipe,and only a certain amount of water will flow. There's no sort of break that will draw more water than the pipe can pass. Not so with electricity.
The wires have to be able to handle hundreds - even thousands- of amps until the breaker trips. That dead short has the same maximum fault current, pretty much regardless of the wire size. Your only hope of saving a smaller wire is to have a smaller breaker on it.
The same matter comes up when someone suggests that we use a smaller wire for the switch leg. After all, it's only a 60 watt bulb - why not use bell wire?
That's why "down South" we don't do as you propose. One might be able to do so,if there is a fuse in the heater (tap rules).
It's probably why Mike reminds you that you still need a full-size ground wire. It's all about the fault current.