Also don't take the original owners word!!!
Even if the wire is not color coded (yellow 12, white 10) it will be printed in the wire if it is that new!
Original owner may think or assume it is 12 but not know for sure. Some contractors will do what they can to save a buck.
That being said I have 3 EMs and a PC all on and old 15 amp circuit with no problem. I know that isn't a great situation but it is in an unfinished part of a 100 year old house.
As stated Do Not just throw a 30 amp breaker at it! I am volunteer fireman and have seen it happen. As someone posted above it is like the ole days of throwing a penny in a fuse.
one thing I would check, if you know how, or pay someoneto see if one of your machines is drawing more current (amps) and it should be.
Transformers deteriorate over time, not just in pinballs.
Please be safe!
Don't hack just to get the games going. Do it the right way.