The wire running to the main breaker in my panel has the following written on the side of the wire. I am trying to figure out what gauge it is. The wire says, "GENERAL CABLE 3 TYPE THW 600V MTW GUARDIANT 090 3 MACHINE TOOL WIRE OIL RESISTANT 60 C"

Thanks ahead of time.

Somewhere on the wire should state something like "#14 AWG".

Wonder if it's a number 3, Do you have about a 125 Amp service?
The number and size of strands will help determine.

I know it is much larger than #14. It is feeding a 100a main breaker & it has 7 strands.

My aerial line comes from my meter (meter on pole) and ends just before it goes into the conduit on the side of my house. This is where 6 runs of this wire start by being clamped to the end of the aerial line. THe 6 runs go through the conduit and come into the house, where 3 of each feed my two separate 100amp load center panels. I wanted to upgrade one of the panels to 150 amp but I did not know if this wire would handel it. You might wonder why I wante to upgrade when I already have 200 amp service with the two 100 amp panels. Well, the reason is, I would like to install a tankless water heater that requires three, 50 amp breakers. I had considered installing one of the 50amp breakers in one panel and the other two in the other, however I figured if I could upgrade on panel to 150 amp, it could house the two 50 amp breakers and I would be better off.

1/0 Copper used for 150 Amps, You have a #3 now.