100A main breaker trips when using 60A heater
The fact that the 100-Amp main is tripping, supplying a load of about 45-Amps through a 60 &amp; 50-Amp breakers doesn't really say anything other than the 100-Amp breaker has failed and conceivably needs to be change.  However, given that you have a 45-Amp load on a heater, which we consider to be a continuous load, you are very smart to upgrade to 200-Amp capacity.  This higher capacity needs to go all the way to the weather head.