Lots of factors in that...which is inline first, which if either is lower rated amperage ( if you have 20 amp fuse fed from a 15 amp breaker...breaker will trip...I know, but I've seen weird shit people have done). Like I said, best way to get an idea of what you have to work with is to test what light/outlet goes to what breaker....might get lucky like me and find a 20 amp circuit feeding a single outlet that's behind where the couch is, was nothing to pull the fuse and tap into that circuit adding a outlet.