I did a job, and the garage sub Panel was fed with 12-3 wire. This doesn’t seem right to me, however I’m not sure why. I was wondering if someone could help educate me.
If all the circuits were loaded up the current would likely be higher that the “feeder conductors” are rated for. Which would be problematic. I would call it out.
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What size breaker was protecting the feeders at the main?
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It was a 20 amp breaker.
It is being fed from 2 sources, no?
Then the feeders are properly protected. Now the only issue is that someone may mistakenly put in a larger breaker (at the main) if they are tripping the 20 due to the number of circuits and load in the garage. If they switch out the breaker at the main and do not install larger feeders, there could be a big problem!
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What are you seeing Larry?
Where is the 12/3 mentioned in the OP? I see a NM cable (12/2) and a FMC with 3 conductors, and the green being used as a hot leg.
It’s being fed from one source. The conduit coming up from the bottom is the source
I believe there are some restrictions or requirements when using the FMC as the EGC as well, correct Robert? Can you fill us in on those if there are?
What Rob said. ^^^^
Edit: Plus I misread the black hot leg as coming fro the top. 
Yes there are size and length restrictions as to when FMC can be used as the EGC. For 1/2" FMC the maximum permitted length for use as the EGC is 6’. {250.118}
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Thanks Robert. It was definitely longer than 6’.
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I’m assuming this was an attached garage Austin? If it is detached, we have a few more issues going on.
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Um no. This was a detached garage.
Then you need a proper means of disconnect in the garage and also a GES (assuming the main is at or on the house).
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Does that hold true everywhere if one runs a single 20 amp 120 volt circuit to said detached garage?
The disconnect for sure as far as I know. The GES I believe is needed if there are multiple circuits in the garage. Robert may chime in to confirm.
That’s the way it is here. Only if multiple circuits.
However, a disconnecting means, at the detached garage, was not required when I wired the detached garage I built at my last house when I ran a single 20 amp 120 volt circuit out there.
It was simply for one vehicle parking and 20 cords of firewood. 
