Does anyone know when 4 wire conductors were required for the feed between the service panel and the sub panel. I inspected a home today that was built in 1974. It has one service panel with a 225 AMP breaker and 3 sub panels fed from the main with a 100 AMP, a 125 AMP and a 150 AMP breaker. All three panels are fed with only 3 conductors so the neutrals and grounds are connected to the same buss in the panels.
I know this is not permitted now but was it then?
Appreciate any help.