This paper presents an ultra-high-frequency (UHF) passive sense tag for electrical grid and substation thermal monitoring, with emphasis on the tag system optimization and the design of a low power embedded temperature sensor. The designed tag achieves a sensitivity of -12.3 dBm under active temperature monitoring operation, which is the state of the art among existing UHF passive temperature sense tag products. The sensing inaccuracy of the tag is ±2.5 °C (3σ) from -25 to 120 °C after a low-cost wireless single-point trim. An antimetal ceramic-packaged tag was tested by attaching to a ring main unit in the substation and complete tag system demonstrated robust wireless operation with a sensing distance of 3.5 m. The combination of batteryless and wireless operation, high sensitivity, wide sensing range, and small incident-power-dependent error (±0.2 °C) makes this tag suitable for the target applications.