A thermal sensing system for providing thermal protection to an electronic circuit breaker that does not require any additional components. The layout of the conductive traces on the printed circuit board are dimensioned and configured so that the ratio of primary current to primary conductor thermal mass heating is correlated with the ratio of secondary current to secondary thermal mass heating. A voltage at a semiconductor junction in the rectifier behind the secondary transformer is measured, and a temperature or heat is calculated based on the voltage. Because this temperature or heat calculation is correlated with the temperature or heat of the primary conductor, a trip routine can be added based on the measured voltage for thermal protection of the primary conductors. A thermal history of the ‘primary thermal mass’ is also provided for robust thermal protection of the circuit breaker load and primary conductors.