OK, I have to fess up to being wrong (sort of
). I was having a hard time wrapping my head around the power issue.
It IS the power across the fuse that causes it to blow, but I was assuming that the full voltage of the circuit (6V or 12V) factored into that equation. But the power across a fuse is developed by the voltage drop ACROSS THE FUSE which is a fraction of the circuit voltage. The power across a fuse can be expressed as P = I squared x R where R is the fuse resistance and I is the current running thru the fuse. Resistance is fixed, and so the power (and amps) to blow the fuse is also fixed for a particular fuse.
So the bottom line is don't use a 20A rated circuit breaker in a 6V circuit that had a 30A circuit breaker in it, as it would be under-protected and might cause a fire.
