(The Writing Center) When a coordinating conjunction joins two independent clauses, a comma is used before the coordinating conjunction (unless the two independent clauses are very short).
This is no hard and fast rule; however, in the case of fairly long clauses a comma seems to be used without fail.
(Examples from CoGEL)
- He heard an explosion and he therefore phoned the police.
- Go by air, and save time.
- Join the navy and see the world.
- They are living in England, or they are spending a vacation there.
- She felt ill, and my mother said nothing.
- I spoke to him before the conference was over, but before he started work.
- He tried hard, but he failed.
- John plays the guitar, and his sister plays the piano.