Most of the time, you should not use a comma before because when it connects two clauses in a sentence. Because is a subordinating conjunction, which means that it connects a subordinate clause to an independent clause; good style dictates that there should be no comma between these two clauses. An exception can and should be made when the lack of a comma would cause ambiguity.
Because has a straightforward job to do in the English language. It is one of several words and phrases used to introduce a “clause of purpose.” A clause beginning with because answers the question “Why?” and that clause is automatically subordinate to an independent clause. There should generally be no comma between the two.
When to make an exception for clarity
If a sentence will be ambiguous without a comma before because, it is better to insert one and avoid misleading your reader. The most problematic sentences containing because are often those that begin with a negative statement. Take this example:
This sentence might imply that Alex won the race, but that ability was not the most important factor contributing to his victory. Perhaps Alex has an extraordinary ability to dig deep and persevere over long distances.
Or perhaps Alex is a rascally scoundrel and won for completely different reasons.
Either way, without a comma before because, it can be implied that there was no relationship between Alex’s victory and his level of ability. However, if your intention is to convey that Alex lost and that his level of ability was the reason, insert a comma to make the causation clear.
With the comma in place, it becomes clear for the first time that Alex was in fact not the winner of the race, and that his ability is probably not at a sophisticated level. This sentence might lead to another sentence supporting the idea that Alex’s running ability is quite low.
See what a difference a comma can make?