Using ANSI/ISA standards, the red lead from a thermocouple or extension wire is always the _____ lead.

In ANSI/ISA standards, the red lead from a thermocouple or extension wire is designated as the negative lead. This convention stems from the fact that thermocouples generate a voltage based on temperature differences between two junctions—one hot and one cold. The color coding helps standardize connections in instrumentation to avoid confusion and ensure that wiring is consistent across different installations.

In thermocouple configurations, the reference junction (where the temperature is known or held constant) is related to the voltage produced at the hot junction. By convention, the red wire represents the reference junction, which is tied to the negative terminal. This association is critical in maintaining accurate readings and preventing errors in temperature measurement due to incorrect wiring.

Understanding the lead designations is essential for technicians working with temperature sensors and control systems. Correctly identifying the negative lead ensures that the thermocouple is wired properly, allowing for accurate temperature readings and effective response in control systems.