How to Wire a 4-20mA Pressure Transmitter: A Step-by-Step Guide for Industrial Engineers
Learn 4-20mA pressure transmitter wiring step by step, including power supply, PLC connections, current-loop testing, troubleshooting and best practices.
How to Wire a 4-20mA Pressure Transmitter
4-20mA pressure transmitter wiring is a fundamental part of industrial instrumentation and process automation. Pressure transmitters are widely used to measure pressure in hydraulic, pneumatic, water treatment, manufacturing, HVAC and process control systems.
A correctly wired transmitter converts pressure into a reliable 4-20mA electrical signal that can be monitored by a PLC, DCS, SCADA system, digital indicator or controller.
While the basic principle is simple, incorrect wiring can cause inaccurate readings, unstable signals, loss of communication or equipment damage. This guide explains the wiring process and key considerations for industrial engineers and technicians.
What Is a 4-20mA Pressure Transmitter?
A 4-20mA pressure transmitter measures pressure and converts it into a standard current signal.
Typically:
-
4mA represents the lower measurement range.
-
20mA represents the upper measurement range.
- Values between 4mA and 20mA represent proportional pressure.
- A two-wire transmitter commonly uses the same two wires for power and signal.
- The output can be connected to a PLC, DCS, indicator or other compatible analogue input.
For example, a transmitter with a 0-10 bar range will typically produce:
| Pressure |
Output |
| 0 bar |
4mA |
| 2.5 bar |
8mA |
| 5 bar |
12mA |
| 7.5 bar |
16mA |
| 10 bar |
20mA |
The exact output behaviour should always be confirmed using the manufacturer's technical documentation.
What Is a 4-20mA Pressure Transmitter?
A pressure transmitter is an industrial measuring device that detects pressure and converts it into an electrical signal.
The 4-20mA output standard is particularly popular because it provides reliable signal transmission over industrial cable runs and works well with automation equipment.
Typical applications include:
- Hydraulic pressure monitoring
- Pneumatic systems
- Water and wastewater plants
- Pump monitoring
- Compressor systems
- Process automation
- Manufacturing machinery
- HVAC systems
- Industrial tanks and pipelines
The transmitter may measure gauge pressure, absolute pressure or differential pressure depending on its design.
What You Need for Wiring
Before starting installation, make sure you have:
- 4-20mA pressure transmitter
- Suitable DC power supply
- PLC, DCS or analogue indicator
- Suitable instrumentation cable
- Multimeter or loop calibrator
- Terminal blocks
- Appropriate cable glands
- Screwdriver and installation tools
Before wiring, verify the transmitter's:
- Supply voltage
- Output type
- Terminal configuration
- Pressure range
- Process connection
- Electrical connection
- Maximum loop resistance
- Environmental rating
Step-by-Step 4-20mA Pressure Transmitter Wiring
Step 1: Switch Off the Power
Turn off and isolate the relevant power supply before making electrical connections.
Follow your site's electrical isolation and lockout/tagout procedures where applicable.
Step 2: Identify the Transmitter Terminals
Check the markings on the transmitter.
Common terminal labels include:
+-V+V-I+I-PWR+PWR-
However, manufacturers do not use identical terminal arrangements. Never assume that wire colours or terminal positions are universal.
Step 3: Verify the Power Supply
Many industrial pressure transmitters operate from a DC supply, commonly 24VDC, but the correct voltage depends on the transmitter.
A typical two-wire current loop can be represented as:
+24VDC
|
|
Pressure Transmitter
|
|
PLC Analogue Input
|
|
0VDC
The actual wiring must match the transmitter and PLC manufacturer's specifications.
Step 4: Connect the Positive Supply
For a conventional two-wire transmitter, connect the positive side of the DC supply to the transmitter's positive loop terminal.
The terminal may be marked +, V+, P+ or another designation.
Step 5: Connect the Transmitter to the Analogue Input
Connect the transmitter's other loop terminal to the appropriate analogue input on the PLC or control system.
A simplified configuration is:
+24VDC
|
↓
TX +
Pressure Transmitter
TX -
|
↓
PLC AI+
|
↓
PLC AI-
|
↓
0VDC
The receiving input must be electrically compatible with the transmitter.
Step 6: Check Whether the PLC Input Is Active or Passive
This is one of the most important steps in 4-20mA pressure transmitter wiring.
A passive analogue input does not normally provide loop power, so an external power supply may be required.
An active analogue input may provide power to the current loop.
Connecting an external supply incorrectly to an already powered input can cause electrical problems or equipment damage.
Always check the PLC analogue input documentation.
Step 7: Check Polarity
Before switching on the system, carefully verify polarity.
Check:
Positive → Positive
Negative → Negative
Do not rely solely on cable colour. Follow the manufacturer's terminal diagram.
Step 8: Power Up and Test the Loop
After completing the wiring:
- Check all terminals.
- Verify polarity.
- Check for shorts.
- Confirm supply voltage.
- Energise the circuit.
- Measure the current loop.
- Check the PLC analogue input.
- Compare the displayed pressure with the actual pressure.
A multimeter or dedicated loop calibrator can be useful during commissioning.
Understanding the 4-20mA Signal
The current output normally corresponds linearly to the pressure range.
For a 0-10 bar transmitter:
- 0 bar = 4mA
- 2.5 bar = 8mA
- 5 bar = 12mA
- 7.5 bar = 16mA
- 10 bar = 20mA
The relationship can be expressed as:
Current = 4mA + [(Measured Pressure ÷ Pressure Span) × 16mA]
This allows the PLC to convert the analogue signal into an engineering value.
Common Wiring Problems and Troubleshooting
No Current Signal
Possible causes include:
- No power supply
- Open circuit
- Incorrect polarity
- Incorrect terminal connection
- Faulty transmitter
- Incorrect PLC configuration
Start by checking the power supply and loop continuity.
Signal Stuck at Approximately 4mA
If the signal remains around 4mA, verify that the transmitter is actually exposed to the expected pressure.
Check:
- Process isolation valves
- Pressure connection
- Blocked impulse lines
- Sensor configuration
- Pressure range
Unstable 4-20mA Signal
An unstable signal may result from:
- Electrical interference
- Loose terminals
- Poor cable installation
- Grounding problems
- Pressure pulsation
- Mechanical vibration
- Damaged cable
Use suitable instrumentation cable and keep analogue signal wiring away from high-power cables where practical.
FAQs
What does 4mA represent?
In a standard 4-20mA pressure transmitter, 4mA generally represents the lower range value.
What does 20mA represent?
20mA generally represents the upper range value.
Can I use 24VDC?
Many industrial transmitters are designed for 24VDC systems, but the manufacturer's specified supply range must be confirmed before installation.
Why is 4mA used instead of 0mA?
The 4mA "live zero" provides a useful diagnostic reference. A 0mA reading can indicate conditions such as an open loop or loss of power, depending on the system.
Can a pressure transmitter connect directly to a PLC?
Yes, provided the transmitter and PLC analogue input are electrically compatible and the loop is powered correctly.
Conclusion
Correct 4-20mA pressure transmitter wiring is essential for dependable industrial pressure measurement. The installation process involves selecting the correct power supply, identifying the transmitter terminals, understanding the PLC input configuration, checking polarity and verifying the current loop.
A well-designed 4-20mA system provides a robust interface between the pressure measurement point and the industrial control system. However, successful commissioning requires more than simply connecting two wires. The transmitter range, loop resistance, analogue input, cable installation and PLC scaling should all be verified.
