Help - I want to be sure to get this correct. I am replacing my 100 Amp sub-panel in our In-Law quarters. The sub-panel is being feed by a 4 wire system and common and ground in Sub-panel box are not connected. Does the 3rd wire for 3 wire stove, hot water heater, etc go to the ground lug or the common lug?

If I remember correctly I think the 3rd wire was hooked to ground lug but I want to be 100% sure. When feeding off main panel I know this doesn't matter but when feeding 220 (3 wire system) from sub-panel I know this need to be correct.

I guess at some point it would be good to change everything over to a 4 wire system but for now I need to get electric turned back on for the night.

Thanks!

First off, you need to get straight that there is a 120V circuit, a 240V, AND a 120/240V circuit.

120V = Hot, neutral, ground
240V = 2 hots, ground
120/240V = 2 hots, neutral, ground

A sub-panel is 120/240V.
A newer range is 120/240V
A newer dryer is 120/240V
A water heater is 240V
A larger motor or welder is 240V

Older dryers and ranges were typically a "3-wire" 120/240V circuit, where the neutral also served as the ground. These circuits cannot be fed from a sub-panel. They must originate from a main panel.