House and garage built in 1991. Buried four wire from house panel with breaker to garage. Garage sub panel has six breakers. Ground and white wires from each breaker both go to a single bus bar. Is this ok? Been like that for over two decades.
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Maybe
In the olden days (I'm not quite sure '91 would qualify) you could treat a subpanel in a detached building exactly like a service entrance. You provided a source of ground (UFER, water pipe or ground rod), grounded the neutral and bonded the panel. You didn't even need a ground wire from the service. So if you have a ground source at the garage this would have been OK at one time. Nowadays you need to carry a seperate ground wire to a bonded busbar and provide an isolated bus bar for the neutral. Since you already have a ground wire to the main panel I would bring the garage up to date by seperating the ground and neutral. Be sure to bond the ground to the panel and isolate the neutral. Seperate the ground and white wires to each circuit.
FYI this change in the code came about when multiple wiring connections between buildings became common ie phone, cable TV, alarms etc. It is important for safety that all sources of ground be the same. It probably won't change back no that wireless is becoming the norm.