Pot Limit - Official Ruling Question

My home games are PL and we play hold'em and Omaha. The following ruling for this situation has plagued me for some time.

What's the official ruling on calculating the POT size when theres a main pot and a side pot as result of a player being 'all in'?

Let's say player 1 is all in and players 2 & 3 continue betting with a side pot. If the main pot = $20 and side pot = $5, which pot amount does players 2 & 3 use as their POT limit?

a. the side pot of $5 because that's the active pot.

b. the total amount of both pots [$25] because both players 2 & 3 have interest in both pots and therefore can use both pots in their calculation.

I say 'a' but an argument can be made for 'b' due to the interest in both pots. I have not played in a PL game at a casino nor online to experience this situation.

Thanks for the help.