Coming as someone who did the same themselves, basically all mains wiring is good up to 600v in the US, and all main and sub panels have breakers precisely because you can overload them just by using a decent portion of your circuits to their fullest.

Putting in new circuits or plugs isn’t exactly uncommon or particularly difficult. The biggest thing to watch out for being the extra 20% safety margin the NEC requires on top of a circuits rated capacity that if I remember correctly puts you a gauge up from what the circuit itself requires, but if the state certified inspector signed off on it then it’s almost certainly good to go.