Ok, cool. So if the romex is rated at 15A then I’m going to assume that’s the rated safety limit before things start to get sketchy.

This panel is going to provide 1200w at (I’m assuming) 120v which is 10A.

At that point I plug in something that pulls 1500w (120v for 12.5A total) like a hairdryer.

You’re saying the load on the circuit breaker will be 1500w-1200w for a total of 300w (2.5A), correct? The load on the romex in the walls after the breaker will still be 1500w (12.5A), correct?

What happens if I then plug in a space heater on that same circuit which consumes 1500w (12.5A). The breaker load should then be 15A (the net 2.5A + 12.5A) as it’s rated for and shouldn’t trip, but the load on the romex after the breaker would be 25A (12.5A + 12.5A), 167% its rated capacity.

Am I misunderstanding something? I’m honestly not trying to argue. I’m trying to understand.