I recently worked on a home with a 100 amp box which the metal bar the breakers stab into is breaking apart. So I carefully changed main feed wire,breakers,elements, thermostats &rewired the old 80gal waterheater to make sure the main wasn't failing. If I got the old 4500watt waterheater operating correctly that meant the 100 amp. service was still holding on and the problem would have to be in the waterheater. I was going to install a 30 gal. 3000watt tank with a lighter 220volt feed wire. I lucked out and the electric pannel was able to handle the 30gal heater and 1500watts less.