Negative terminal welding after big 3

Hi guys, New to the forum so I apologise if this is in the wrong section. I have just installed the big 3 wiring upgrades and have a problem. At first when everything was wired up, I connected positive battery terminal and then negative, and the negative made a pop sound and the cable was "welded" to the terminal. The cable became so hot that it melted probably 50% of the cable and has now been solely replaced by the 0 gauge wire. However, the problem still exists every time I try to troubleshoot and reconnect the negative; strong sparks which destroy some of the negative battery post. I am hoping you guys might have a possible solution as to what is going on! Thanks.

Having the negative terminal connected and touching each lead that was connected to the pos terminal individually to the post, makes the alternator charging cable spark big time and take chunks out of the pos terminal. Does this mean my alt is fried?

Try disconnecting the battery and then remove the 12 volt feed from the back of your alternator. Now reconnect the battery and see if the "big spark" goes away.My ex once told me I have a perfect face for radio.

I am betting on you used a larger ring terminal on the alternator wire and it is touching the case of the alternator. Performing the above suggestion will help you to see if this could be the case.