Two dedicated 20 AMP Circuits...

[milwaukeesteve]

Re: Two dedicated 20 AMP Circuits...

Back to Charlie's response. (Don't know where Allen was on that one.)
The fact that 2 different sized OCPD feed these two circuits in a MWBC is not that uncommon. As long as the neutral is sized to the largest return current. (I wouldn't suggest larger than 15 and 20 Amps, but that is my suggestion).
Even if you were max'd out on both circuits (12A and 16A) your return current is only 4A.

 

[charlie b]

Re: Two dedicated 20 AMP Circuits...

You don't max out at 12 and 16, but rather at 15 and 20. The loads need not be continuous.

But you are right about sizing the neutral for the larger of the two. If the loads on the 15 amp breaker were all off, and the loads on the 20 amp breaker were at max, then the neutral would carry 20 amps. So that same 20 amp load could easily damage a neutral that had been sized for 15 amps, even if it were non-continuous (i.e., 2 hours would be enough to lead to damaged insulation).

 

[milwaukeesteve]

Re: Two dedicated 20 AMP Circuits...

Again, that is why the neutral would have to be sized to carry the largest unbalanced return current. In this example, that neutral would have to be a 12, to cover the potential 20 A unbalanced return current.