In article , cayoung61-
says...
Well, supposing I'm running a space heater (15 amps) on the black wire and
the bare wire. And running a hair dryer (14 amps) on the white wire and the
bare wire. That's 29amps in a bare piece of 12 ga.

How would you wire it differently?

First, I'd replace the dangerous and illegal [for this use] 12-2 cable
with 12-3, with the red and black wires connected to a two-pole 240V
breaker. Then I'd wire the space heater across the black and white, and
the hair dryer across the red and white.

That's 15A in the black wire, 14A in the red wire, and (15A - 14A) = 1A
in the white wire.

If you don't understand how that works, and why it's safe, and why the
setup you described is not ... then you oughta stop offering electrical
advice.