I had a 60 amp breaker that was connected to an oven in the kitchen. Since this is no longer needed, I'm trying to re-purpose this #6 wire to connect to a new Electric water heater I'm putting in the basement. Putting any new wires will require me to break up the basement ceilings which I'd avoid if possible. So, I decided to put up a sub panel in the basement (about 15 feet from the main panel) and connect this 60 amp 6 gauge wire to the subpanel and add a 30 amp breaker connected to 10/3 wire to the new heat pump/water heater (35 ft away in the basement). But the original #6 wire only had a 2 hot and a neutral coming into the sub panel, no separate ground wire included. This house was built in the 80's and probably these wiring were done around the same time.
So, my question is this... how do I take care of the ground in the subpanel. The new electric water heater only takes 2 hot and a ground connection (no neutral needed). But, I'm only getting the neutral(and 2 hots) from the main panel and I don't really have anything to ground it to in the basement where this sub panel will be located. Is it mandatory to ground the subpanel separately? What's my best option here?