Was this (1) a direct (bypass) cable from the alt to the batt or (2) OE: alt to/thru bulkhead to the T then to/thru ammeter then to/thru bulkhead to the battery? You have a dead short in the large cable circuit (which you'll need to find/fix first). An alt could dead short internally but that is rare. EDIT Full fielding of the field circuit MAY have dont this but I'm thinking dead short as it pretty much takes a dead short (to ground) to produce that kind of fireworks which comes from ALOT of current flowing.