Is the garage attached or detached?
No point in using a 125a breaker for a 100a sub
You need to check your main breaker panel label to see what the max breaker size is.
If it is a detached garage, you will find the most common recommendation is to use 2-2-2-4 AL Mobile Home Feeder (MHF) in trenched conduit and a (max) 90 amp breaker at the main panel. If its inside the house you could use 2-2-2-4 SER cable. It is a very common cable size, so its available from multiple sources for a reasonable price. The only downside is that it can only handle 90 amps.
You can use your 100 amp panel to get more breaker spaces, but the 90 amp breaker at the main will keep you code compliant. MHF is available dual rated, so it can be buried outside and run in conduit (required) inside the house. SER cable cannot be buried or run in a wet area.
Bruce
It all depends what your going to have connected to that subpanel. Maybe that extra 25 amps would make a difference. If you would go with 125 amps, and being a separate building, You have to have a main in the garage. You would have to probably take a main lug panel, which most are rated for 125 amps, and back feed it with a 125 amp breaker with a hold down kit installed. I have never seen a 125 amp main breaker panel, that doesn't mean they don't exist. Just make sure you size your underground feeders for 125 amps
That's the hard way and wastes a couple of branch circuit spaces.
Use a 200a rated panel with a 200a main cb. As long as the rating of the panel equals or exceeds the rating of the feeder ocp, the main cb in the panel acts as a disconnect & provides short circuit protection only. The upstream feeder ocp provides short circuit and o/l protection for the feeder.
If you are a one man shop/garage, 100 amp feed is a lot of current!
I see your point, but that would not be legal in my area. Has to be 125 amp breakers at each end.
I see your point, but that would not be legal in my area. Has to be 125 amp breakers at each end.