What color is the negative wire of a thermocouple?

• A. Blue
• B. White
• C. Red
• D. Green

Answer: (C)

Understanding polarity in a thermocouple is essential because the tiny voltage it produces is referenced to the two conductors. The color coding on the wires tells you which conductor is the negative leg. In many standard instrumentation conventions used here, the red conductor is designated as the negative lead. That means connect the red wire to the instrument’s negative input, and use the other wire as the positive leg. If the leads are swapped, the reading can be inverted or otherwise misinterpreted by the instrument. Keep in mind that some standards and thermocouple types use different color schemes, so always check the specific type and equipment documentation. But for this question, red is the negative wire.