@bigtwin Thanks for posting. That is exactly the simple (and correct as far as it goes) understanding that causes people to buy too small a cable. If you go by your amps power rating, lets say 200 watts, then you need less than 2 amps. An ampacity table will tell you that 26 awg is all you need. Can you imagine a 26 gauge power cord? A small to medium amp would run on it--and sound like crap.
In reality, your load is dynamic, not static. You need to be able to change current values very quickly. It isn't the steady state current that sizes your cable. That is the steady state calculation you posted.
But if people keep using power cables sized the way you calculated, there will be used amps for sale for the rest of us to buy.
Jerry