Yes, the wire is quite hefty. At least 10 ga.
Is it possible there are some insulators that should be on the ammeter terminals that are missing? The gauges have been out of the truck for a long time, and could have been misplaced. Or, are there insulators between the gauge cluster and the dash I've left off? something simple like that (he hopes in vain)?
Also, is it normal for this to happen just by leaving the ignition switch in the "on" position? This doesn't seem likely, but I have to ask.