John TX,
If you can install a 125 amp breaker in the main service panel, and it will fit correctly, then you can do it. That's what Steve [Sparky] was referring to about the breaker.
Normally, the largest frame size that can be installed into a panel of 200 amps and less - used in Residential - would be 100 amp.
The next frame size is 225 amp - which would be for breakers rated from 125 amp to 225 amp. Make completely sure the breaker will fit the panel before buying it!
If you are able to install the 125 amp breaker, that's fine - just make sure to use wire that is no less than 125 amp.
If you cannot use the 125 amp breaker, just use a 100 amp breaker. Once again, use wire that is no less than 100 amp for the 100 amp breaker [this is the wire feeding the new panel - AKA Subfeed].
It's highly unlikely that you will ever load that subpanel to 100 amps @ 240 Volts [or 100 amps per Line], unless you are running a lot of real heavy loads from it. This would be equal to maybe 2 heat pumps, plus a few 5 horse motors, and the welder - all on the same panel and running at maximum draw at the same time. Then you would run the risk of tripping the 100 amp subfeed breaker.
The better way would be running the subfeed from the main service - that will keep the surges and voltage drop from effecting other stuff too much.
The Welder Woes thread kind of covers this more, plus the possible Harmonic Distortion that could be reflected back into the power system.
I, and most likely others, would like to have you post a follow-up message that explains the way you are going to hookup the new subpanel, such as ampacities of all the conductors, conduit - or cable, plus most importantly the way to ground everything.
Feel free to ask anyone here if you are unsure of ANYTHING that concerns the new panel's installation.
Good luck!
Scott SET
