Sub Panel - Main Disconnect

Sub panel in attached garage has a 100A main disconnect. It is fed from a 100A breaker on the main panel.
I know it is not required to have a main disconnect on the sub, but what will it do having a 100A on each end?

You have redundant overcurrent protection and two disconnects. Nothing wrong with either.

Thanks Chuck

Did they wire the entire box like a main? I’ve see a few were the sub had a redundant main and the neutral and ground bar were connected. What’s your take on that Chuck? I called those out as incorrect for a distortbution box but only because there suppose to be separate not a clear answer as to why. Would like to know.

The rest of the box was wired as a Sub, just the main disconnect was there.

Quite often it’s cheaper to buy the panel at a big box store with the main and a bunch of branch circuit breakers. Nothing wrong with it, the main/panel can even be of a larger ampacity than the size of the OCPD ahead of the feeder.

The grounded/neutral should be isolated from the panel and equipment grounding except at the service equipment. Bonding on the load side can put current on the EGCs so it’s important to keep them separated on distibution panels and other load side equipment. I didn’t mention it because it wasn’t the question that the OP was asking, but you are correct that it is important.

A graphic from Mike Holt to expand on what Chuck stated and depict the definition of objectionable current:

Chuck and Larry, Thanks. Based on what I know about electricity what you shared was what I assumed, but nice to have it confirmed. Good information.

One caveat, the code previously avowed 3 wire feeders to detached structures with certain conditions . Those panels were wired like a service panel.