You could power limit the 3090s to fit a standard 120V*20A = 2400W outlet if you really want to. The default power limit is 350W each so you'll only lose a little perf.
Also most rooms have multiple circuits. Just connect half the GPUs to each outlet. I already do this with my desktop PC because it has 2 PSUs.
Also most homes in the US have 30A*240V = 7200W dryer/stove outlets in the kitchen, laundry room, garage, etc. That's where you want to keep loud computers.
In the bay area every home I've lived in had 20A circuits for everything.
The NEC 80% rule is only relevant in data centers where someone enforces it. The time-current curve for any 20A breaker shows that it will take infinite time to trip when you draw 20A. This is correct behavior because 12awg wire can handle 20A indefinitely
Some outlets have the 20A sideways part, but from a safety/code perspective that's irrelevant.
Consider what happens when you plug two 10A devices into a power strip. In the US there are no true 15A rated outlets. All outlets must handle 20A because the electrical code allows any outlet to be on a 20A breaker.
Also most rooms have multiple circuits. Just connect half the GPUs to each outlet. I already do this with my desktop PC because it has 2 PSUs.
Also most homes in the US have 30A*240V = 7200W dryer/stove outlets in the kitchen, laundry room, garage, etc. That's where you want to keep loud computers.