The first thing to check is the voltage on the heater. I have seen this too many times to ignore the obvious. You may have wired 120vac heaters to 220vac. I am not to worried that you wired the heaters in series becuase that would d-rate the output not make them hotter. Your load should not exceed 12~13amps. So get an amp probe and check you load. If it is in excess of 13amps open the breaker and check the voltage of the baseboard. Residential baseboard is typically around 250watts a linear foot.