A temperature limiting device for a thermal system includes a modular unit that is configured to connect to a two-wire heater of the thermal system. More particularly, the modular unit includes a heater interface configured to connect to a two-wire heater of the thermal system, a power interface configured to connect to a power source to receive power; and a controller including a sensor circuit. The sensor circuit is configured to measure an electrical characteristic of the two-wire heater, which includes voltage, current, or a combination thereof. The controller is configured to calculate a temperature of the thermal system based on the measured electrical characteristic and determine whether the temperature is greater than a temperature setpoint.