You absolutely cannot do that. Any person working in your panel would have
no way to know that the 12 awg had been reduced somewhere downstream and in
doing a service upgrade for example would install the wire to a 20 amp
breaker
" wrote in message
oups.com...
Greetings,

I would like to confirm that, according to the NEC, there is no reason
why you cannot use 12/3 connected to 15 and 20 amp breakers (one on
pole A the other on pole B) and then (say 30 feet away) branch into two
circuits (a 15 amp circuit on 14/2 and a 20 amp circuit on 12/2).

Thank you for your time and energy,
William