I am getting ready to install an upstairs washer/dryer combo and have run 10/3 with ground wire for dryer. I have an older home and my breaker box is an older Square D with a bunch of "piggyback" breakers. (no room left) so I want to install a sub box. However I noticed that the box is rated at 125 amp and it has 60 amp main breaker. Is there a reason someone would have installed a smaller main breaker and would I be able to replace it with a 100 amp main breaker?