I had an electrician install a 50-amp sub-panel in my basement a few years ago. He used an existing 3-wire (black, red, white, no ground) 6 gauge cable. It is fed from a 50 amp breaker in the main panel in the attached garage. The ground bus on the sub-panel is connected to the main copper water line by #4 green wire.
Is this proper? I have read on other posts that a sub-panel should be connected to the main panel with 4 wires.