[sourdough]

Originally Posted by JRTJH

Unless you're planning to rip out the 12 gauge ROMEX that the factory installed in the ceiling as part of the "2nd A/C option", then it's academic whether you put a 15 amp or a 20 amp circuit breaker in that position. The wiring is sized properly for 20 amps and the air conditioner likely will require 20 amps. Trying to "balance the load" by installing a 15 amp breaker on a 20 amp circuit is very likely going to lead to frustration and possible equipment failure (your frustration from dealing with tripped breakers and having to reset them and equipment failure from repeatedly overheating the breaker as well as the interrupted startups on the air conditioner compressor). Remember, there is a 90 second wait time from startup interruption before you can apply power to the compressor again without possible damage. Counting that down every time the 15 amp breaker trips on a 20 amp load will eventually get very, VERY old.....

Why not just install the air conditioner on the 20 amp circuit that was designated for it and deal with what to do with upgrading the other 15 amp circuit separately.

In nearly every application, a 5 amp difference in "total circuit breaker ratings" between the two input sources (line 1 and line 2) that 5 amp difference is minor and of no consequence.

Maybe explaining what your "other plans might be" would shed light on why your 5 amp concern is important ???? From what you've shared so far, it seems to be a "non starter for concern".....

^^^This is the question to me. Without explanation of the the goal this conversation is sort of pointless to me.