grammar - Comma before or after "unless"

Had a discussion with a colleague on the following sentence today:

By default situation A will happen, unless you choose to activate option B.

I have set the comma as indicated above, but my colleague is convince the comma should be after the "unless".

As a non-native speaker I don't feel I have the authority to overrule my colleague but my gut says the comma should be as indicated above. Can someone help me? Thanks.

Answer

The normal usage dictates no comma at all:

By default, situation A will happen unless you choose to activate option B

(the comma after "default" is there because "by default" is an adverbial phrase, and these require a comma if they start a sentence)

Anyway, "unless" here is a dependant word, and as such does not use a comma.

However, in case of a long sentence, you may use a comma to break the sentence into more manageable chunks for easier comprehension:

By default, situation A [, including things D, E and F, as well as potential danger G,] will happen, unless you choose to activate option B [, which will disable things D and F and protect you from G].