I have a 20 amp breaker in my 1890’s home with 12 gauge wire attached to it. Further down the line (Where I can not see) there is some old 1960’s 14 gauge Romex wire attached to it within the walls of the house. This old 14 gauge wire is attached to a plug I want to tap into with 14 gauge wire to add a ceiling fan in my bedroom. Is this ok?
Since your 20A circuit has 14ga wire attached to it, it's already illegal -- adding more 14ga wire to it won't change that situation as far as the code is concerned. But adding load to it may well start a fire inside your walls and burn your house down, since a 20A breaker or fuse can't properly protect 14ga wire. You're lucky in that it hasn't overloaded so far, but your new ceiling fan may well be the straw that breaks the camel's back.
To correct this problem, and make both your existing installation and your proposed extension legal and safe, you should change the breaker on that circuit to 15A.
Some allow for prior use but it is a touchy subject. If you have a history, fairly long and not between various users, under 20 amp breaker with 14 gauge wire, there is not a major problem if you do not change your current use??? Best to change all of the wiring in the circuit or change the breaker to 15 amp. I am trying the “all the wiring” but it is a chore fishing the wiring, not so bad, I suppose, but some of the metal boxes don’t like the tolerance of the twelve wiring, so use a plastic box when you get to ground arcing