Keta,

In a perfect world, you might be correct. In the real world, a 15A breaker (especially one that's more than a few years old) is probably going to trip at 80-85% of its rated load. And it only goes downhill from there as the breakers age further.

I'm in the same boat as MeanGene....an older house with older wiring/breakers. I'm just going to have a couple of new 20A circuits pulled with new (and shielded...cuz I'm anal that way) 10/3nstranded cable. It's more expensive to do things that way, but I prefer to over-engineer than try to scrimp and save a few bucks. I can reliably trip my current 15A amp circuit by playing a Bach organ fugue at high volume. I would imagine that very compressed rock music would be even more brutal on the circuit. The old BX in my plaster walls belongs in a museum...when we remodel, I'll probably bite the bullet and have it all replaced along with all new panels.

Cheers,