The wire gauge of a thermocouple extension cable affects the signal in two ways: resistance and noise pickup. Both matter for temperature accuracy.
Resistance. The thermocouple produces a voltage measured in millivolts. The controller measures that voltage. Any resistance in the circuit drops some of the voltage before it reaches the controller. The controller reads a lower voltage and displays a lower temperature. The error depends on the wire gauge, the wire length, and the input impedance of the controller.
For a typical hot runner installation with a 2-meter extension cable, the resistance of 24 AWG wire is about 0.17 ohms per meter. The total loop resistance is about 0.7 ohms. This is small compared with the thermocouple's own resistance. The error is negligible. But for a 10-meter cable, the resistance is 3.5 ohms. The error becomes measurable. The controller may display a temperature that is 1°C or 2°C low.
Noise pickup. Thinner wires are more susceptible to electromagnetic interference. They act as antennas. They pick up noise from power lines and servo drives. The noise appears as temperature fluctuations. Thicker wires have lower impedance and are less susceptible. But thicker wires are stiffer and harder to route.
The practical choice. For hot runner thermocouples, 24 AWG is the standard. It offers a good balance between flexibility and noise immunity. For long cable runs, use 20 AWG or 18 AWG. For high-interference environments, use shielded twisted pair cable. The shield protects against electric fields. The twist cancels magnetic fields.
The check. Measure the resistance of the extension cable. Disconnect it from the thermocouple and the controller. Measure across the two conductors at one end with the other end shorted. The resistance should be less than 10 ohms for a typical installation. If it is higher, the wire is too thin or too long.
The bottom line. Wire gauge is not the most critical specification, but it is not irrelevant. For short runs, 24 AWG is fine. For long runs or high-interference environments, use thicker wire and shielding. Check the resistance. If it is high, the temperature reading is low.
