A head mounted temperature transmitter has only a handful of terminals, yet wrong wiring causes many loop faults at commissioning. This guide covers every connection, from the sensor leads to the DCS card, with real terminal tables and a calculator.
Temperature transmitter wiring joins the sensor on one side and a 4 to 20 mA loop on the other. We cover RTD and thermocouple terminals, loop voltage, the HART resistor, shield earthing and common faults.
What Is Temperature Transmitter Wiring?
Temperature transmitter wiring is the set of connections that joins an RTD or thermocouple to the transmitter input terminals and joins the transmitter output to a 24 V DC, 4 to 20 mA loop. In the usual 2 wire loop powered design the same two wires carry both the supply and the signal, so the transmitter, the power supply and the DCS input are simply connected in series.
- Output: 2 wire loop, transmitter + to supply +, transmitter minus to the analog input, then back to supply minus
- RTD input: 2, 3 or 4 wires to the sensor terminals, with 3 wire the common industrial choice
- Thermocouple input: two terminals, polarity matters, use matching extension or compensating cable
- Loop budget: maximum load = (supply minus transmitter minimum voltage) ÷ 0.022 A
- HART needs at least 250 Ω in the loop, and the cable shield is earthed at one end only
A head mount temperature transmitter sits inside the sensor head, so the sensor leads are short and the long cable carries a robust current signal. If the basic idea is new to you, start with what is a temperature transmitter.
How Do You Wire a 2 Wire Temperature Transmitter?
In 2 wire temperature transmitter wiring the transmitter acts as a current regulator, drawing 4 mA at the lower range value and 20 mA at the upper range value from the supply. The difference between this and a separately powered device is explained in 2 wire and 4 wire transmitters.
Many DCS and PLC cards provide loop power on their own terminals, so you only land the two field wires on the card. In that case check the card manual for which terminal is the powered input, as covered in our note on field instrument wiring to the system cabinet.
The 4 mA live zero lets the transmitter power its own electronics from the loop. That is why a 2 wire device can never output 0 mA during normal operation, and a reading of 0 mA always points to an open circuit or a dead supply.
Temperature Transmitter Wiring Terminal Table
Terminal numbers for temperature transmitter wiring differ between brands, so always follow the label printed on the transmitter. As a real example, the Pepperl+Fuchs HUT head transmitter data sheet uses the terminal layout below.
| Connection | HUT terminals | Notes |
|---|---|---|
| Loop supply and output | 1 and 2 | 8 to 35 V DC, 4 to 20 mA |
| RTD 2 wire | 3 and 6 | Lead resistance adds to reading |
| RTD 3 wire | 3, 5 and 6 | Lead resistance compensated |
| RTD 4 wire | 3, 4, 5 and 6 | Best accuracy |
| Thermocouple | 4 and 6 | Polarity sensitive |
| Resistance input (Ω) | Same as RTD | 2, 3 or 4 wire |
Pepperl+Fuchs also lists a maximum of 11 Ω per line for RTD inputs on this model. The Endress+Hauser TMT182 allows up to 20 Ω per cable in 3 and 4 wire systems, so the limits differ by model.
Take a photo of the transmitter label and the terminal block before you start any temperature transmitter wiring. When the head is closed later, the photo saves a trip back to the field during loop checks.
RTD 2, 3 and 4 Wire Connections
DwyerOmega explains that with a 2 wire RTD the lead resistance is included in the measurement, so the error grows with cable length. A 3 wire RTD cancels the lead resistance by assuming the leads are equal, and a 4 wire RTD uses separate pairs so the leads have almost no effect.
2 wire leads: 2 × 5 Ω = 10 Ω
Error ≈ 10 ÷ 0.385 = 26 °C high
3 wire: error cancels if leads match
4 wire: lead error practically zero
A Pt100 changes about 0.385 Ω for every degree, so even a few ohms of cable shows up as several degrees. Our guide on 2 wire, 3 wire and 4 wire RTD compares the three methods in more detail.
Lead colours vary by maker and standard, so always confirm with a meter rather than trusting colour alone. For more on testing, see how to test an RTD Pt100.
Thermocouple Polarity and Extension Cable
A thermocouple produces only millivolts, and swapping the two leads makes the transmitter read in the wrong direction. Type K under IEC 60584 uses green for positive and white for negative, while the ANSI code uses yellow for positive and red for negative.
| Type | IEC positive | IEC negative | ANSI positive | ANSI negative |
|---|---|---|---|---|
| K | Green | White | Yellow | Red |
| J | Black | White | White | Red |
| T | Brown | White | Blue | Red |
Note that red is negative in the ANSI code, which surprises many electricians used to red for positive. Our article on thermocouple polarity identification shows how to confirm it with a magnet and a meter.
In thermocouple temperature transmitter wiring between the sensor and a remote transmitter, use matching thermocouple extension wire and not copper. Copper creates a new junction at the terminals and spoils the cold junction compensation done inside the transmitter.
Loop Resistance Budget and Minimum Voltage
A 2 wire transmitter needs a minimum voltage across its terminals to work. Endress+Hauser gives the TMT182 a supply range of 11.5 to 35 V and a maximum load of (supply minus 11.5 V) ÷ 0.022 A.
R max = (24 minus 11.5) ÷ 0.022
R max = 568.2 Ω
R loop = 250 + 2 × 0.5 km × 12.1 Ω/km
R loop = 262.1 Ω
V at transmitter = 24 minus 0.022 × 262.1 = 18.23 V
In this example a 250 Ω input resistor and 500 m of 1.5 mm² copper pair use only 262.1 Ω of the 568.2 Ω allowed. Add the resistance of any isolator or barrier from its own data sheet, and see our intrinsic safety entity calculation for hazardous area loops.
Temperature Transmitter Wiring Loop Voltage Calculator
Enter the supply voltage, the transmitter minimum voltage, the input resistor, the one way cable length, the cable resistance per km and the worst case current. The calculator checks the temperature transmitter wiring loop budget with the same numbers as the example above.
Use 22 mA or the highest alarm current of your transmitter as the worst case, since the voltage drop is largest there. For a deeper check, our HART loop voltage budget calculator adds barriers and other loop devices.
HART Resistor and Shield Earthing
Endress+Hauser states that HART communication needs a minimum load of 250 Ω in the signal circuit. Most analog input cards already include a 250 Ω resistor, but a bench test with only a supply and meter needs one added, as explained in HART loop resistor in HART communication.
For temperature transmitter wiring, use twisted pair cable with an overall shield, and earth the shield at one end only, normally at the control room or marshalling cabinet. Earthing both ends lets current flow between two earth points and creates a ground loop that adds noise to the signal.
- Twisted pair with overall shield for every loop
- Shield earthed at the control room end only
- Shield cut back and taped at the field end
- Separate instrument cables from power cables
- Ferrules and terminal numbers matched to the loop drawing
- Shield earthed at both ends
- Shield drain left touching the transmitter housing
- Copper wire used to extend a thermocouple
- 2 wire RTD on long cable runs
- Unlabelled cores and spare pairs left floating
Most smart transmitters have a galvanic isolation between input and output. The Endress+Hauser TMT182 is rated at 2 kV AC, which helps when a grounded thermocouple meets an earthed loop.
Troubleshooting Temperature Transmitter Wiring
When temperature transmitter wiring fails at commissioning, measure the current first and read the value against the table below. The alarm currents listed are those Endress+Hauser gives for the TMT182, and other models follow similar NAMUR style values.
| Symptom | Likely cause | Fix |
|---|---|---|
| 0 mA | Open loop, reversed polarity or no supply | Check supply and polarity at the terminals |
| 3.6 mA or 21.0 mA | Sensor break or short circuit | Check sensor leads and terminals |
| Reading falls as process heats | Thermocouple leads reversed | Swap the TC leads at the transmitter |
| Reading high by a fixed amount | 2 wire RTD lead resistance | Use 3 wire or enter lead compensation |
| Reading drops at high current | Loop voltage too low | Check the loop budget and supply |
| Noisy or jumping value | Shield earthed at two ends, cable near power | Earth the shield at one end, reroute cable |
The sensor break current is set by the burnout function in a temperature transmitter, and the range is set during temperature transmitter configuration and ranging.
- Loop polarity verified at both ends
- RTD lead pair identified with a meter
- Sensor type and wire count set in the transmitter
- Loop voltage at the transmitter above the minimum at 22 mA
- Shield earthed at one end only
- Loop test done at 4, 12 and 20 mA
During the loop check, simulate the input with a calibrator at three points and read the value at the DCS. Our instrument loop checking procedure gives the full sequence.
For field cable colours and core identification, see cable colour codes for field instruments. The overall loop concept is explained in 4 to 20 mA current loop explained.
Temperature Transmitter Wiring FAQ
For temperature transmitter wiring, connect the supply positive to the transmitter positive terminal, then take the transmitter negative to the analog input and back to the supply negative. The sensor connects separately to the input terminals marked for RTD or thermocouple.
Many DCS and PLC cards supply loop power themselves, so only two field wires land on the card. Check the card manual for the powered input terminal.
Yes, the loop terminals of most 2 wire transmitters are marked positive and negative. Many models include polarity protection, so a reversed connection simply gives no current instead of damage.
Thermocouple inputs are also polarity sensitive because the sensor produces a small voltage. Reversed thermocouple leads make the reading move the wrong way as the process heats up.
Find the two leads of the same colour with a meter, since they read almost zero ohms between them. Land that pair on the two terminals marked for the common side of the element.
Connect the single odd colour lead to the remaining RTD terminal. Then set the sensor type and the 3 wire option in the transmitter configuration.
It depends on the model, so read the data sheet for the exact value before you finish the temperature transmitter wiring. Endress+Hauser lists 11.5 volts for the TMT182, while the Pepperl+Fuchs HUT works from 8 volts.
The voltage must stay above this minimum even at the highest loop current. Check the budget at 22 mA with all resistors and cable included.
Earth the shield at one end only, normally at the control room or marshalling cabinet earth bar. At the field end, cut the shield back and insulate it so it cannot touch the housing.
Earthing both ends lets current flow along the shield between two earth points. That current couples noise into the signal and causes unstable readings.
HART signals ride on the 4 to 20 mA current as a small frequency shift voltage across a resistance. Without enough resistance in the loop, the communicator cannot see the signal clearly.
Endress+Hauser asks for at least 250 ohms in the signal circuit for this reason. Most analog input cards already include a 250 ohm resistor inside.
Related Articles
- Head Mount Temperature Transmitter
- Difference Between 2 Wire, 3 Wire and 4 Wire RTD
- Thermocouple Polarity Identification
- HART Loop Resistor in HART Communication
- 4 to 20 mA Current Loop
External References
- Endress+Hauser: iTEMP HART TMT182 Technical Information (PDF)
- DwyerOmega: RTD 2, 3 and 4 Wire Connections
- Wikipedia: Current loop
What We Learn Today
- Temperature transmitter wiring uses a 2 wire loop where the transmitter, the 24 V DC supply and the analog input are connected in series.
- A 3 wire RTD cancels matched lead resistance, while a 2 wire RTD on long cable adds about one degree for every 0.385 ohm.
- The loop budget must leave the transmitter above its minimum voltage at 22 mA, with the HART resistor and cable resistance included.
