When to use comma before and?

What are the rules with commas before and?

Two situations:

a list with more than three things (I like chocolate, cheese, and pizza. I enjoy walking, running, eating, and sleeping.)
or
if you're connecting two independent clauses (michael has been running for ten years, and today he won his first race.)

thank you!!

here is a better example:

Michael has been running for ten years, and today he is the fastest man in town.

how about here:

Hurricane Sandy caused many tree limbs to fall on power lines and many people were without electricity for days.

[Hurricane Sandy caused many tree limbs to fall on power lines], and [many people were without electricity for days].