If there's even an inch of #14, old or new, anywhere in the circuit,
then that circuit needs to be protected with a 15A breaker or fuse,
and there is essentially zero advantage to splicing in any #12, unless
it's all you have and the stores are closed. Possible exception is if
it's a very very long run, like to an outbuilding, in which case you
maybe need to use #12 or heavier with 15A protection.
If it's #12 that's visible leaving the panel, then there's a danger
that someone will assume the whole circuit is #12 and thus they might
put in a 20A fuse. So if your new wire is going to go into the panel,
then there's a very good reason to make it the same was what's already
in the rest of the circuit.
(Besides, some will tell you that old wire is HEAVIER than new stuff
of the same nominal gauge. No matter. Old #12 or new #12 should ok
with 20A, unless it's damaged, in which case it shouldn't be used at
You got to tell us what the heck is going on here.
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