The general rule of thumb is as follows: If a coordinating conjunction (for, and, nor, but, or, yet) joins two independent cluases, a comma is added before the conjunction in order to separate the independent clauses:
Independent clause: The stock market collapsed.
Indpendent clause: Jenna lost all of her money.
EX: The stock market collapsed, and Jenna lost all her money.
Originally Posted by Webster.commnet