The wiring (at least in the US) to the 120V outlets is just one half of the split-phase 240V. If you want to send 240V down a particular wire, you can do that, by changing the breaker, but then you lose the neutral. You also make the wires dangerous to people who don't realize that the white wire is now energized at 120V over ground. (Though it's best to test to be sure anyway, as polarity gets reversed by accident, etc.) Live wires should be black or red.