Thermocouple Wiring Diagram: Types, Color Codes, and Connections
A thermocouple is two dissimilar metal wires joined at one end. The junction generates a small voltage proportional to the temperature difference between that hot junction and a reference point -- the Seebeck effect. The millivolt output is tiny (roughly 41 µV/°C for a Type K), which means correct wiring matters more than it does for most sensors. Wrong polarity, wrong extension wire, or a bad cold-junction compensation (CJC) setup can introduce errors of tens of degrees without triggering any obvious fault.
This guide covers the major thermocouple types, ANSI and IEC color codes, how to connect a thermocouple to a controller or transmitter, and what cold-junction compensation actually does.
The Seebeck Effect and the Cold Junction
When two different conductors are joined and their junctions are at different temperatures, a voltage appears across the open ends. This is the Seebeck effect. In practice:
- Hot junction (measuring junction): the tip inserted into the process -- a furnace, engine exhaust, solder pot, etc.
- Cold junction (reference junction): wherever the thermocouple wires terminate, usually at the instrument terminals.
The instrument measures the voltage difference and computes temperature -- but only relative to the cold junction temperature. If the cold junction is at 25 °C and the hot junction is at 300 °C, the output corresponds to a 275 °C difference. The controller adds the cold-junction temperature (measured by an on-board thermistor or RTD) to get the true 300 °C reading. Omit or miscalculate CJC and every reading is offset.
Thermocouple Types
Type K (Chromel/Alumel)
The most widely used. Range: -200 °C to +1260 °C. Sensitivity: ~41 µV/°C. Good for general industrial use, HVAC, and food processing. Susceptible to oxidation above 1000 °C in some atmospheres.
Type J (Iron/Constantan)
Range: -40 °C to +750 °C. Sensitivity: ~52 µV/°C. Common in older US industrial equipment. Iron leg rusts above 500 °C in humid conditions. Not suited for new designs in most cases.
Type T (Copper/Constantan)
Range: -200 °C to +370 °C. Sensitivity: ~43 µV/°C. Excellent for cryogenics and food industry. Copper leg conducts heat away from the junction, which can cause errors in low-thermal-mass applications.
Type E (Chromel/Constantan)
Range: -200 °C to +900 °C. Sensitivity: ~68 µV/°C -- the highest of the common base-metal types. Good for differential temperature measurements.
Types R, S, B (Platinum)
High-temperature industrial and laboratory use. Ranges up to 1700 °C (Type B). Sensitivity is much lower (~10 µV/°C), requiring low-noise wiring and instrumentation.
Color Codes
Color codes differ between the ANSI (American) and IEC (European) standards -- and between types within each standard. The table below covers the types most commonly encountered.
ANSI/ASTM Color Code (US)
| Type | Positive Wire | Negative Wire | Connector Body |
|---|---|---|---|
| K | Yellow | Red | Yellow |
| J | White | Red | Black |
| T | Blue | Red | Blue |
| E | Purple | Red | Purple |
Note: In ANSI convention the negative wire is always red, regardless of type. This trips up engineers familiar with IEC wiring.
IEC 60584-3 Color Code (Europe/International)
| Type | Positive Wire | Negative Wire | Overall Jacket |
|---|---|---|---|
| K | Green | White | Green |
| J | Black | White | Black |
| T | Brown | White | Brown |
| E | Purple | White | Violet |
IEC uses white for the negative conductor across all types.
Label or photograph your wiring before you start if you are working in a facility that mixes US and European equipment -- misidentifying polarity causes a reading error rather than an obvious fault.
Extension Wire vs. Thermocouple Wire
A thermocouple circuit must use matched metal conductors from the hot junction all the way to the instrument terminals. Running ordinary copper wire from a junction box to the controller introduces a new, uncompensated junction -- effectively a second thermocouple in series -- and the reading will be wrong.
Two options:
- Thermocouple wire: The same alloy as the sensor element. Used when you want to maintain full temperature accuracy through the extension run. More expensive.
- Extension-grade wire (compensating cable): A less expensive alloy with a similar Seebeck coefficient over a limited temperature range (typically 0--200 °C). Acceptable for most industrial runs inside conduit where the wire temperature stays within that range. Marked with an "X" suffix in the ANSI system (KX, JX, TX, etc.).
For maker and embedded applications -- connecting a thermocouple to an Arduino-based controller using an MAX31855 or MAX6675 breakout board -- the thermocouple usually terminates directly at the PCB, which handles CJC internally. No extension run is needed.
Wiring a Thermocouple to a Temperature Controller
Dedicated Thermocouple Input Controller (e.g., Autonics, Omega, Watlow)
These instruments have clearly marked thermocouple input terminals, usually a screw-type terminal block.
- Set the controller to the correct thermocouple type in the menu (K, J, T, etc.).
- Connect the positive wire (ANSI: type-specific color; IEC: green/black/brown/purple) to the positive (+) input terminal.
- Connect the negative wire (ANSI: red; IEC: white) to the negative (--) input terminal.
- The terminals are usually labeled TC+, TC--, or IN+, IN--.
- Do not connect the shield (if present) at both ends -- ground the shield at one end only to avoid ground loops.
4-20 mA Transmitter
A thermocouple transmitter (DIN rail or head-mount) converts the thermocouple millivolt signal to a 4--20 mA loop for PLC or SCADA input.
Wiring has two parts:
Thermocouple side (input):
- Connect TC+ and TC-- as above.
Transmitter output (loop):
- The loop supply voltage (typically 24 V DC) connects to the transmitter's supply terminals.
- The 4--20 mA signal wire runs in series with the loop to the PLC analog input.
- 4 mA = minimum temperature (e.g., 0 °C), 20 mA = maximum temperature (e.g., 500 °C).
Arduino with MAX31855 or MAX6675 (Maker/Embedded)
The MAX31855 reads Type K thermocouples and handles cold-junction compensation on-chip. It communicates via SPI.
Connections:
- T+ (TC+): Positive thermocouple wire (yellow in ANSI K)
- T- (TC-): Negative thermocouple wire (red in ANSI K)
- VCC: 3.3 V (5 V tolerant on most breakouts)
- GND: Ground
- SCK, CS, SO: To Arduino SPI pins
Keep the thermocouple leads short between the board terminals and the junction. Do not route them parallel to power wires -- pickup on the microvolt-level signal causes noisy readings.
You can prototype the signal conditioning side in CircuitDiagramMaker -- draw the MAX31855 breakout, the SPI connection to an Arduino, and annotate the wire types before you start soldering.
Common Wiring Mistakes
Reversed polarity: Connecting positive to the negative terminal gives a reading that decreases as temperature rises -- or goes negative. The magnitude of the error equals the actual measurement. Reversed polarity is the single most common thermocouple wiring error.
Copper extension wire: Using ordinary wire for a long run from the thermocouple to the controller introduces uncompensated junctions. The error grows with the temperature at those junctions.
Ground loops: Connecting the shield at both ends creates a loop that picks up noise and offsets the reading. Ground one end only, typically at the controller/instrument.
Incorrect type selection: Setting a controller to Type K when a Type J sensor is installed. At 300 °C, the calibration error is roughly 30 °C.
Create Your Own Thermocouple Wiring Diagram
CircuitDiagramMaker is well suited for documenting instrumentation wiring:
- Draw transmitter terminal blocks with labeled TC+/TC-- inputs
- Show 4-20 mA loop wiring from transmitter to PLC
- Annotate wire types (thermocouple wire vs. extension grade) and color codes
- Draw MAX31855 SPI connections for embedded designs
- Export to PDF for control panel documentation
Create your own thermocouple wiring diagram -- free
Key Takeaways
- All thermocouples rely on the Seebeck effect: a small voltage proportional to the temperature difference between the hot junction and the cold junction.
- Cold-junction compensation is essential -- instruments measure it internally with a thermistor or RTD and add it to the reading.
- In the ANSI color system, the negative wire is always red regardless of thermocouple type; in IEC it is always white.
- Extension and compensating wire must use the correct alloy to match the sensor -- ordinary copper wire introduces uncorrected measurement error.
- For embedded use, an MAX31855 or MAX6675 breakout handles CJC on-chip and speaks SPI directly to a microcontroller.
- Ground shields at one end only to prevent ground-loop noise from corrupting the microvolt-level signal.
- Reversed polarity causes the reading to decrease with rising temperature -- always verify polarity before commissioning.