If your transmitter is wired incorrectly, your PLC, display or controller may show the wrong pressure or level, trip at the wrong point, or fail to read the loop at all. We wrote this guide to help you connect a 4-20 mA pressure or level transmitter safely, with enough understanding to protect the instrument, the input card and your commissioning schedule.
Our focus is the practical loop: power supply polarity, 2-wire, 3-wire and 4-wire connections, load resistance, shielding, grounding, testing and calibration. Diversified Technologies recommends checking the transmitter datasheet and the receiving device input type before you energise anything, because small wiring assumptions can create avoidable faults. If you are ordering parts, building a panel, replacing a field device or proving a signal before handover, we want you to know what each terminal does and when to stop and get support.
- A 2-wire 4-20 mA transmitter must be wired as one series current loop, not as a voltage sensor.
- Use 3-wire transmitters when electronics need separate supply and signal reference, and 4-wire units when power and signal are fully separated.
- A 4-20 mA loop works only when supply voltage covers the transmitter, every load, and cable resistance at 20 mA.
What is a 4-20 mA transmitter and why it's used for pressure and level
A 4-20 mA transmitter converts a measured variable, such as pressure or liquid level, into a proportional current signal. In normal scaling, 4 mA represents the lower range value and 20 mA represents the upper range value. It’s used because current loop wiring stays readable over long cable runs when the loop supply has enough voltage headroom.
At Diversified Technologies, we use 4-20 mA devices where a controller, PLC analogue input, indicator or data logger needs a stable industrial signal rather than a raw sensor output. The transmitter does the conditioning at the measuring point, so the receiving instrument doesn’t have to interpret a millivolt bridge signal, a capacitance probe or a resistive element directly. That distinction matters in wet wells, hydraulic packs and tank farms, where field wiring best practices have to account for cable resistance, induced noise and maintenance fault finding.
Why the live zero matters
The live zero is the feature that makes the range especially useful. A true zero-pressure or zero-level reading produces 4 mA, not no current. If the loop reads 0 mA, the fault is usually an open circuit, failed supply, blown fuse or disconnected terminal rather than a genuine process value. According to IEC 60381-1, process-control analogue current signals include the 4 to 20 mA direct-current range, which is why instruments from different manufacturers can usually share the same signal convention.
A pressure transmitter on a hydraulic line sends a current proportional to its calibrated pressure range: the PLC input reads the loop current and scales it into bar, psi or another engineering unit. The signal remains practical over plant cabling when the supply voltage and loop resistance are checked. A level transmitter on a tank or sump sends a current proportional to height, head pressure or interface level: the receiving display converts the current into metres, litres or percentage full. The live-zero signal makes cable breaks easier to distinguish from an empty vessel.
How it compares with other sensor outputs
An RTD changes resistance with temperature, while a transmitter sends an already conditioned process signal. A 0-10 V output can work well, but a current loop is generally more tolerant of voltage drop along longer cable runs. Where a receiving device only accepts voltage, a 4-20 mA converter or signal conditioner can translate the loop into the required input, although that adds another component to document and test.
A loop-powered sensor is the simplest arrangement because the same two conductors carry supply power and measurement current. In practice, three-wire and four-wire devices separate power and signal paths, which can suit higher-load instruments or transmitters with extra electronics. In hazardous areas, the transmitter, barrier and installation method must match the relevant CE, UL, IECEx or ATEX marking, rather than the signal type alone.
Good current loop wiring starts with the right transmitter range, adequate supply voltage, twisted-pair instrumentation cable and sensible shield termination. EMI suppression may also be needed near variable speed drives, contactors or long parallel motor cables. The strongest reason to use 4-20 mA is still practical: it gives pressure and level instruments a standard, diagnosable signal that technicians can measure directly with a meter.
2-wire vs 3-wire transmitter wiring explained
At the PLC panel, there’s a loop supply, an analogue input card and a glanded transmitter cable. For most pressure and level loops, especially loop-powered PLC inputs, a 2-wire transmitter is usually the simpler choice; use a 3-wire transmitter when the electronics need a separate supply and signal reference.
| Decision factor | 2-wire wiring | 3-wire wiring |
|---|---|---|
| Power and signal share the same path. | The same two conductors carry supply current and the 4-20 mA signal. | Two conductors power the device and a third conductor returns the signal. |
| The PLC input type matters. | It suits passive analogue inputs when the loop supply is wired in series. | It suits active inputs or signal conditioners that expect a shared 0 V reference. |
| Cable choice has to be checked. | A screened twisted pair cable can be enough when voltage drop calculation confirms headroom. | A three-core screened cable is needed because supply negative and signal common must be controlled. |
| Smart functions may need loop resistance. | HART communication normally needs adequate loop resistance, commonly 250 ohms. | HART may still work, but the datasheet decides the correct reference points. |
In practice, voltage headroom often decides the wiring more than terminal count because long cable runs, intrinsic safety barriers and input resistance consume available supply voltage.
Tell us about your loop supply, cable run and PLC input card, and our engineers will confirm the right transmitter and wiring arrangement before you order.
Step-by-step: How to connect a 2-wire 4-20 mA pressure or level transmitter
A 2-wire 4-20 mA transmitter must be wired as one series current loop, not as a voltage sensor. As a baseline, use a correctly rated DC loop supply, a passive PLC analogue input, screened twisted pair cable, and a checked voltage drop calculation.
- Isolate the loop supply and prove it dead before opening the transmitter or marshalling panel. Use approved test equipment, and confirm whether the work area requires ATEX, IECEx, UL, or CE marked devices.
- Confirm the instrument is a 2-wire transmitter. The datasheet should show + and – terminals, with the output listed as 4-20 mA, the direct-current process signal range standardised in IEC 60381-1.
- Select screened twisted pair cable with an insulation and temperature rating suited to the plant area. Terminate the screen at one end only, unless the site earthing design requires a different shield connection.
- Check the available loop voltage before energising. Subtract the PLC input burden, any indicator, any signal isolator, and cable resistance from the supply, because the transmitter still needs its minimum voltage at 20 mA.
- Start with the supply side. Connect +24 V DC to transmitter +, then connect transmitter – to the passive analogue input +, using ferrules where the terminal design allows them.
- Complete the return path from analogue input – to 0 V DC. The same current must pass through every device in the loop, so parallel wiring will give a false reading or a fault.
- Check the PLC card type before adding an external supply. An active analogue input already sources loop power, so connecting a separate 24 V supply can overload the input stage.
- Power the loop and measure current in series. For multimeter mA measurement, break the loop and insert the meter inline, then run the calibration procedure to verify 4 mA at zero and 20 mA at span.
- Record the terminal numbers and conductor colours before closing the enclosure. Brown for loop positive and blue for loop negative is common in panel wiring, but the project drawing should take precedence.
- Only add HART access if the loop design supports it. A HART modem normally needs a suitable load, commonly 250 ohms, and the transmitter datasheet should confirm the communication arrangement.
The step people most often get wrong is treating the transmitter like an RTD or a voltage output device. A 2-wire 4-20 mA transmitter is powered by the same two conductors that carry the signal, so any break in the series path creates an open-loop fault rather than a false low pressure or level reading.
Step-by-step: How to connect a 3-wire or 4-wire transmitter
Most 3-wire and 4-wire transmitters run on 24 V DC, with separate supply and signal paths. The 4-20 mA signal still represents pressure or level; only the wiring changes.
First, isolate power before opening terminals. Check CE, ATEX or IECEx markings where an intrinsically safe barrier is installed. Next, identify terminals against the datasheet. For 3-wire units, connect +V, 0V/common and signal out. Then, wire 4-wire units with supply positive and negative on the power pair. Route mA+ and mA- to the PLC analog input. After that, check the input type before energising. Current source outputs need a sinking input; current sink outputs need a powered input. Finally, test the loop with a multimeter in series on mA, or use a 250 ohm shunt resistor for a 1-5 V input.
In practice, we use single-point earthing to protect analog input scaling.
Power supplies, loop calculations, and maximum cable length
Sometimes the smaller power supply really is the tidy choice. But a 4-20 mA loop only works if the supply voltage can cover the transmitter, every load, and the cable resistance at 20 mA. For a loop-powered sensor, it’s better to check the voltage budget before field wiring than to discover the problem later as a weak signal at the PLC analog input.
Take a typical service panel using a 24 V DC supply, a 2-wire pressure transmitter that needs 12 V at its terminals, and a PLC input fitted with a 250 Ω shunt resistor so the controller reads 1-5 V. The transmitter acts as a current sink. It regulates loop current, while the supply provides the energy. In this setup, the maximum allowable loop resistance is calculated as (24 V minus 12 V) divided by 0.020 A, giving 600 Ω. Once the 250 Ω input resistor is included, 350 Ω remains for cable, terminals, surge protection, and any signal conditioner.
- The DC supply provides the available voltage for the whole current loop: The supply is 24 V DC in this example. A regulated industrial supply avoids drift caused by sagging voltage.
- The transmitter must retain its minimum operating voltage: The assumed datasheet requirement is 12 V at 20 mA. Below this point, the output may clip before full-scale pressure or level.
- The PLC input converts current to voltage through a load: The 250 Ω resistor produces 1-5 V from 4-20 mA. The load consumes part of the voltage budget.
For cable length, use the loop resistance of the pair, not one conductor. That said, according to IEC 60228 conductor tables, a 0.5 mm² solid copper conductor is commonly rated at about 36 Ω per kilometre at 20°C, so the out-and-back pair is about 72 Ω per kilometre. Dividing the spare 350 Ω by 72 Ω/km gives roughly 4.8 km under clean electrical conditions. Worth noting: that is a calculation limit, not a recommended plant route.
Real installations need margin because terminals corrode, cable warms, and intrinsic safety barriers add resistance. That margin matters. Where CE, UL, IECEx or ATEX requirements apply, the approved barrier or isolator data sheet becomes part of the same loop calculation. Our preference is to leave headroom rather than design to the last ohm, especially where pumps, drives or long trays increase EMI risk. Twisted-pair cable, correct screen termination, and EMI suppression at loads protect the current loop wiring from disturbance.
A 24 V supply is adequate when the transmitter voltage plus all loop loads stays below 24 V at 20 mA. If the calculation fails, we reduce the load, choose a higher supply within ratings, or add a signal conditioner near the measurement point.
Grounding, shielding and noise mitigation best practices
So where should the cable screen be bonded?
Keep the shield continuous, then bond it at one clean instrument-earth point, usually the control-panel end. In a 4-20 mA loop, you want one intentional screen earth, not several competing earth paths. For a loop-powered sensor feeding a PLC analog input, we leave the field end insulated unless the transmitter datasheet states otherwise.
What cable construction best reduces noise?
Use screened twisted pair for current loop wiring, with conductor size and temperature rating matched to the run. Twisting helps reject magnetic pickup from motors, contactors and variable-speed drives. Keep analog wiring away from mains and VFD outputs, because a 250 ohm shunt resistor turns 4-20 mA into 1-5 V and can reveal induced voltage during analog input scaling.
Should transmitter, PLC and supply share common earth?
They need a defined reference, not a chain of protective-earth links. On most jobs, that means keeping protective earth, instrument earth and screen termination separate until the approved bonding point. With current sink vs current source analog inputs, we check the module manual before linking commons, because a wrong bond can create offset, unstable readings or a hidden short.
How does this change in hazardous areas?
Intrinsically safe circuits add another rule set. Where an ATEX or IECEx installation uses an intrinsically safe barrier, wire the barrier earth exactly as its certificate and control drawing require. A CE or UL mark on a transmitter doesn’t override the barrier instructions, so certified drawings remain the wiring authority.
Testing and troubleshooting: common symptoms and fixes
Most 4-20 mA faults can be narrowed down by checking loop power, polarity, load resistance and the measured current at one point in the circuit. A healthy live-zero signal sits near 4 mA at minimum pressure or level and near 20 mA at full-scale input, unless the transmitter has been ranged differently.
Testing starts with the loop supply, not the transmitter body. That said, with the instrument energised and the process safe, measure the DC voltage across the transmitter terminals and compare it with the datasheet minimum. A standard loop supply may be common, but the usable voltage falls as cable resistance, PLC analog input burden, a signal isolator or an intrinsically safe barrier are added in series. On a loop-powered sensor, those extra loads still have to leave enough compliance voltage for the electronics to regulate current.
| Symptom | Likely cause | Practical fix |
|---|---|---|
| PLC reads 0 mA | The loop is open, unpowered, or wired with reversed polarity. | Confirm supply voltage at source, check fuse, then prove continuity from positive supply through transmitter and analog input return. |
| Reading is stuck near 3.6 mA | The transmitter is signalling a fault, often following NAMUR NE43 behaviour. | Check sensor range, process isolation, damaged diaphragm, and any diagnostic message before changing scaling. |
| Reading is always 20 mA or higher | The input may be saturated, or the range may be too low for the process. | Verify applied pressure or head height, then review transmitter span and PLC scaling. |
| Signal moves but is noisy | Electrical interference is getting into the current-loop wiring. | Use screened twisted pair, bond the screen at one end, and separate the cable from drives or contactors. |
| Multimeter shows current only when inserted | The meter has been placed in series correctly, but the normal loop path is open. | Restore the original link, terminal jumper, or analog input connection after testing. |
For a quick current check, break the loop at a convenient terminal and put the meter in series on its mA range. Never place a current meter directly across the supply, because its low internal resistance can short the loop and blow the meter fuse. If the PLC input uses a shunt resistor, confirm the value before interpreting volts as milliamps. As a useful reference, a 250 ohm shunt gives 1 V at 4 mA and 5 V at 20 mA, which is why it’s common on HART-capable loops.
Check analog input scaling after the electrical test, not before it. For a linear loop, calculate percentage as measured current minus 4, divided by 16, then multiplied by 100. If a tank level channel reads correctly on a meter but wrongly in the PLC, the fault is usually in engineering-unit scaling, square-root extraction, or the selected current source vs current sink input mode. Our wiring notes normally label brown or red as positive and blue or black as negative, but the transmitter datasheet takes priority over colour convention. Match ATEX, IECEx, UL or CE markings before energising.
If polarity, supply voltage and terminations all check out and the loop still misreads, our engineers can work through the fault with you before you replace a healthy transmitter.
HART, digital overlays, and special considerations for smart transmitters
On a commissioning bench, a smart pressure transmitter might show 12 mA on the meter while the handheld communicator reports device status. That isn’t a second output; it’s HART data riding on the analogue loop.
According to FieldComm Group, HART superimposes a Bell 202 frequency-shift keying signal, 1,200 and 2,200 Hz, on the 4-20 mA current without changing the measured value. In practice, most communicators need about 250 ohms of loop resistance to read that overlay reliably. Check the voltage drop calculation before adding that resistor, especially where a PLC analogue input, 4-20 mA converter or signal conditioner already consumes loop voltage. With smart transmitters, we keep shielding, twisted pair cable, EMI suppression and intrinsically safe barriers consistent with the device’s ATEX, IECEx, CE or UL approvals. Calibration still verifies current output too.
Calibration: zero/span adjustment and verifying engineering units
Accurate calibration turns a wired transmitter into a trustworthy pressure or level measurement. Before adjusting zero and span, isolate the process, confirm the loop power, and make sure the PLC, indicator or test meter is reading the same current loop wiring you intend to commission.
- Check the transmitter datasheet for the calibrated range, supply voltage, output type and load limit. Remember that a loop-powered sensor acts as a current sink, while some 3-wire devices provide a current source output, so check the terminal function before applying power.
- Connect the calibrated loop to a traceable mA meter, loop calibrator or analogue input with a shunt resistor. A 250 ohm shunt resistor converts 4-20 mA to 1-5 V, which is useful when the receiving input measures voltage.
- Apply the lower range value to the sensor. For a pressure transmitter, vent to atmosphere if the lower range is gauge zero, and for a level transmitter, reproduce the empty condition or its equivalent hydrostatic head.
- Set zero only after the input is stable. At the lower range value, the output should be 4.000 mA, and any PLC scaling change should be handled separately from transmitter calibration.
- Drive the sensor to the upper range value using a deadweight tester, pressure calibrator, hand pump or level simulation rig. The expected output at full scale is 20.000 mA, because the standard current range has a 16 mA live span.
- Adjust span in small increments, then return to zero and check it again. On many analogue transmitters, zero and span affect each other, so one pass usually isn’t enough to prove the instrument is correctly ranged.
- Verify intermediate points at 25%, 50% and 75% of range. For a 0-10 bar transmitter, those checkpoints should read 8 mA, 12 mA and 16 mA before any analog input scaling converts the signal into bar.
- Check the receiving device in engineering units, not just milliamps. A display scaled 0-6 m for tank level must show 3 m at 12 mA, while a PLC channel scaled 0-100 bar must show 50 bar at 12 mA.
- Inspect the loop under normal cable routing before signing off. Once the cabinet door is closed and drives are running, shield termination, twisted pair cable, CE, UL, IECEx or ATEX boundary conditions, and EMI suppression can all affect the reading.
- Record the final mA values, applied inputs and displayed engineering units in the commissioning sheet. The important point is that the calibration record should match the transmitter range, not an undocumented workaround in the controller.
Series connection with a display, PLC, or controller
In a 2-wire loop-powered 4-20 mA setup, every device that measures loop current sits in the same series path, not across the loop in parallel. A typical chain is +24 VDC → transmitter + → transmitter − → display mA+ → display mA− → PLC AI+ → PLC AI−/COM → 0 VDC, as long as both receivers are rated for series insertion.
The key detail is the power source. Some powered displays supply the loop; passive loop-powered indicators add burden resistance. So check whether the PLC input is active or passive before wiring, to avoid supply conflicts. Datasheets usually show receiver types such as PLCs, controllers, panel meters, ADCs, signal conditioners and data loggers. The important catch is that extra receivers raise voltage drop, so our voltage drop calculation covers burden, cable length and transmitter minimum. If it fails, use a higher supply within datasheet limits or a signal isolator/splitter.
Connecting to a PLC analog input module
A 4-20 mA pressure or level transmitter can look easy to land on a PLC, and sometimes it is that simple. The catch is the input type. In practice, connecting a 4-20 mA transmitter to a PLC analog input module means matching the transmitter wiring to the module’s active or passive current-input terminals, then configuring that channel for 4-20 mA current.
An active PLC input supplies the loop power. A passive PLC input only measures loop current, so the loop needs a separate DC supply. Manufacturer labels matter here: Siemens SIMATIC S7-300 SM 331 documentation uses MANA for the analogue reference, while Allen-Bradley Compact I/O manuals use channel terminals such as IN0+ and IN0-. Other modules may show AI+, I+, COM or return, so check the module drawing before wiring.
The PLC input is active: the module feeds the loop and measures the returned current. PLC loop + goes to transmitter +, transmitter − to PLC mA input or return, depending on the terminal labels. The PLC input is passive: the module measures current only and needs an external supply. Supply + goes to transmitter +, transmitter − goes to PLC AI+, and PLC COM goes to supply −.
Take a worked setup: a loop-powered sensor is being added to a tank level system, and the PLC card is a passive current-input module. The technician isolates the panel, confirms the instrument range and supply voltage, and lands the positive supply wire on transmitter +. The transmitter − conductor then goes to AI+, and COM returns to the power-supply negative. Brown and blue are used for DC positive and negative in IEC-style field wiring, but existing plant standards should win over colour memory.
Just as often, commissioning trips up in software rather than in the copper. During commissioning, set the PLC channel to current input and scale it as 4 mA at the live zero and 20 mA at full-scale pressure or level. Don’t leave the card set for 0-10 V, thermocouple or RTD input, because those modes read different electrical behaviour and can make a loop look faulty. If the input is voltage-only, a resistor, 4-20 mA converter or signal conditioner may be needed.
Before you apply power, verify polarity with a meter, check that every current-measuring device is in series, and confirm the PLC common reference matches that module. Shield termination should follow the site earthing plan, usually at one end unless the manufacturer specifies otherwise, and twisted-pair cable helps with EMI suppression. If the PLC shows zero, over-range or a frozen value, fault finding starts with loop voltage, open terminals, blown input fuses and the channel’s count before anyone changes the transmitter.
Current-to-pressure or level formula
The 4-20 mA scaling formula: what calculation converts current to a measured value?
Start with the lower range value, then add the share of span represented by the current above 4 mA. Building on this, use: value = LRV + ((mA − 4) ÷ 16) × (URV − LRV). In this scale, 4 mA represents 0% of calibrated span and 20 mA represents 100%.
What do LRV and URV mean in practical scaling?
LRV is the lower range value, or the measurement shown at 4 mA. URV is the upper range value, or the measurement shown at 20 mA. In IEC 60381-1 practice for analogue process signals, current-loop instruments use a defined current range; transmitter datasheets then map that range to the calibrated engineering span.
- 4 mA: The analogue input reads the configured lower range value.
- 12 mA: The signal is halfway through the 16 mA live span, so the value is 50% of range.
- 20 mA: The analogue input reads the configured upper range value.
How do we calculate the expected current from a known value?
Use the reverse formula: mA = 4 + 16 × ((value − LRV) ÷ (URV − LRV)). This is useful when checking analog input scaling, proving a loop-powered sensor with a signal generator, or confirming whether a PLC channel behaves as a current sink or current source.
Does the same formula work for every 4-20 mA measurement?
For any linear variable, the same calculation applies to pressure, level, differential-pressure (DP), temperature, or position. Some cases need separate logic: square-root flow extraction, custom tank strapping tables, and non-linear level conversions shouldn’t be treated as a straight line. If a shunt resistor, signal isolator, or intrinsically safe barrier is in the loop, scale the current value after confirming the analogue input is receiving the correct signal.
Testing a 4-20 mA signal
You can test a 4-20 mA signal by measuring loop current in series, or by using a process meter or loop calibrator to read, source or simulate current. For a live pressure or level transmitter, compare the measured milliamps with the expected engineering value from the tank level or pressure reference.
Safe testing starts with the right instrument. Use a digital multimeter with a fused mA input, a loop calibrator, a process meter, a signal generator or simulator, and test leads rated for the site voltage and environment. We prefer clips that can’t slip from terminals during fault finding, especially inside crowded PLC panels.
There are three common test methods, and each one checks a different part of the loop.
- Series current measurement: The live transmitter output matches the actual pressure or level. Commissioning checks after wiring.
- Calibrator reading current: The loop is powered and stable while the transmitter remains connected. Calibration procedure and drift checks.
- Current simulation: The PLC, indicator or controller scaling is correct without using the transmitter. Input card testing at 4 mA, 12 mA and 20 mA.
To measure a live loop, break the circuit at a convenient terminal and place the meter in series so the loop current flows through the mA input. Never place the meter across the loop supply while it’s in current mode. That mistake can short the loop and blow the meter fuse.
According to Fluke’s 789 ProcessMeter specifications, the milliamp input is protected by a 440 mA, 1000 V fast-acting fuse, and the instrument can measure current ranges used in process loops. Different meters have different protection, so we check the exact maximum current and fuse type printed in the manual before live-loop testing.
A 0 to 10 bar pressure transmitter should produce about 4 mA at zero bar, 12 mA at 5 bar and 20 mA at 10 bar.
If you’re checking the controller rather than the transmitter, disconnect the field signal at the analogue input and inject 4, 12 and 20 mA from a signal generator. With the transmitter out of the loop, the PLC display should show low scale, midpoint and full scale. If it doesn’t, the fault is in scaling, input configuration, wiring polarity or the receiving resistor.
Cable condition matters during testing because a correct transmitter can still look wrong at the panel. Twisted pair cable helps reject induced noise, the shield should be bonded according to the site earthing design, and EMI suppression may be needed near variable-speed drives. The practical check is the voltage drop calculation: it confirms that the transmitter still has enough compliance voltage at 20 mA.
A final check is terminal function, not wire colour alone. The key distinction: red often carries positive DC and black or blue returns negative, but panel standards vary. Treat an RTD input, voltage input and current input as different circuits.
Use a multimeter without damaging the loop
With the loop live, place the multimeter at a test point. To measure current safely, insert it in series with the 4-20 mA loop on the fused mA/current input, never across the transmitter or power supply like a voltage meter. Measuring current in parallel creates a low-resistance short path, risking the fuse, input or transmitter output.
If you can, de-energise first: move the red lead to the mA jack, select DC mA, open the loop at a test point, connect the leads in series, energise, read current, then restore the wiring. According to Fluke’s 87V Industrial Multimeter manual, its mA/µA input uses a 440 mA, 1000 V fast fuse and carries CAT III 1000 V/CAT IV 600 V ratings. Use loop test terminals or clamp-style mA meters. Afterwards, put the red lead back in the voltage jack.
Signal generator for connection and testing
A current-loop signal generator lets you source or simulate known current values, so you can test the receiver wiring, PLC input, display scaling, and controller response without relying on the transmitter. That makes it useful during connection checks because it separates a transmitter fault from a cable, signal isolator, intrinsically safe barrier, or analogue input scaling fault.
First, set the calibrator to source mode when you want the instrument to become the current source for the input under test. In plain terms, it pushes 4, 12, or 20 mA into a passive receiver, such as a PLC analogue input with its own terminals disconnected from the transmitter. Next, select simulate mode when the existing loop supply must remain in circuit. The calibrator then behaves like a two-wire pressure or level transmitter, varying the current drawn from the loop rather than supplying power itself.
Then, check what’s powering the loop before you connect the leads. If a 24 VDC supply, PLC input, or loop-powered indicator is already energising the circuit, use simulate mode or isolate that supply, because opposing current sources can damage inputs or distort readings. After that, inject 4 mA and confirm the receiver shows zero, the lower range value, or LRV, or the empty tank point. Use a multimeter mA measurement in series only after opening the loop deliberately, because meter burden can break compliance.
After that, inject 12 mA and confirm the display reads 50% of span. For a pressure transmitter ranged 0 to 10 bar, that should be 5 bar after the PLC scaling calculation has converted current into engineering units. After that, inject 20 mA and confirm full scale, URV, or full tank indication at the receiver terminals. If the PLC reads correctly at 4 mA but not at 20 mA, check the voltage drop calculation across cable resistance, barriers, isolators, and input impedance.
After that, check HART loops with the digital overlay in mind. A HART modem commonly needs 250 Ω loop resistance, so don’t remove the resistor or signal isolator needed for communication while proving the analogue value. Finally, compare your setup with the calibrator manual. According to the Fluke 707 Loop Calibrator Users Manual, the instrument provides mA source and simulate functions over 0 to 24 mA, and its 24 V loop supply sets the compliance limit when it powers the loop.
The step most often missed is mode selection. That aside, mark the test point on your wiring diagram, record the three injected values, and reconnect the twisted pair cable as found before returning the transmitter to service.
Maximum 4-20 mA cable run
At 20 mA, the loop can run only as far as the voltage budget, cable resistance, electrical noise, and installation rules will let it. The exact distance isn’t universal. Transmitter load capacity, wire gauge, intrinsically safe barriers, and receiver resistance set the limit.
As the run gets longer, cable resistance creates voltage drop at 20 mA. For example, the Belden 8761 datasheet lists 22 AWG twisted shielded instrument cable at 16.5 ohms per 1,000 ft conductor resistance, so we calculate the out-and-back length and include it in the loop budget. The Rosemount 3051 datasheet defines allowable load as 43.5 times supply voltage minus 10.5, about 587 ohms at 24 V. Separation from power or VFD wiring improves noise performance. For extreme runs, use a signal isolator, repeater, or remote I/O.
Cable, shielding, and grounding practice
4-20 mA transmitter wiring can sometimes survive with ordinary unscreened cores over a short, quiet panel run, and that’s the honest argument for keeping cable simple. For field wiring, though, use twisted-pair instrument cable with a shield grounded at one end, unless your plant standard, NEC or CEC installation rules, or the transmitter manual says otherwise.
Twisting the loop conductors makes external magnetic interference induce similar noise in both wires, so the receiver rejects more of it. That matters on a loop-powered sensor feeding a PLC analog input because small noise errors can appear as unstable pressure, level, or analog input scaling. In electrically noisy areas, specify screened instrument cable, especially on long runs near motors, VFDs, solenoids, contactors, or high-voltage trays.
Take a maintenance engineer wiring a tank level transmitter to a remote controller. Before pulling cable, they check the hazardous-area drawing, device manual, terminal schedule, and site earthing standard. It’s also worth checking whether the loop passes through an intrinsically safe barrier or a signal isolator, because the barrier documentation may control screen bonding and segregation. The same check should confirm whether the transmitter is a current source or current sink, and whether a shunt resistor is fitted at the analog input.
The detail that trips people up most often is shield termination
First, before pulling cable, verify the plant standard, hazardous-area drawing, NEC or CEC requirements where they apply, and the manufacturer’s shield-termination instructions. That’s the safest way to avoid unsupported universal grounding rules. Next, run the instrument cable separately from power wiring and keep it out of the same tray as motor feeds where the installation allows. If you can’t avoid a crossing, cross the power cable at right angles rather than running alongside it.
Then, terminate the shield at the control system earth or another designated single point, unless the approved documentation tells you to bond it differently. Hazardous-area installations using ATEX, IECEx, CE, or UL certified equipment may require specific bonding at barriers, gland plates, or earth bars. Finally, fit suitable strain relief and cable glands, then seal against water ingress before corrosion reaches terminals. Outdoor pressure and level transmitters fail more often from damp junction boxes, loose glands, and green copper oxide than from the mA principle itself.
For the tank example, the engineer lands the positive and negative loop cores on the transmitter terminals, follows the site’s colour schedule rather than guessing from core colours, and leaves the shield insulated at the field end. Back at the PLC, they confirm continuity, then use a multimeter mA measurement in series only when the loop can be opened safely. If the reading is noisy, they review cable route, shield continuity, isolator/barrier wiring, and analog input scaling before changing the transmitter in error.
When to call Diversified Technologies: ordering parts, commissioning, and support
So when should you involve us rather than keep fault finding on site?
Involve us if the loop current is still reading 0 mA, 3.6 mA or over 21 mA after you’ve checked polarity, supply voltage and terminations. A healthy 4-20 mA loop should be measured in series with a multimeter on the mA range, not across the transmitter terminals. That distinction prevents blown fuses and damaged inputs.
What parts should be specified before ordering?
Before choosing a transmitter, match the range, process connection, wetted material, output, supply voltage and approval. For hazardous areas, confirm whether ATEX or IECEx approval is required. Cable matters too, so specify twisted pair cable with the correct temperature rating, screen termination and conductor size for the voltage drop calculation.
A HART modem is useful when the transmitter supports digital configuration over the analogue loop: it allows range, damping and diagnostics checks without removing the device from service. A 4-20 mA converter is useful when an older controller expects voltage, resistance or another input format: it prevents unnecessary PLC replacement when only the signal interface is wrong.
What commissioning support is worth arranging?
For commissioning, it’s worth covering loop verification, PLC analogue input scaling, zero and span checks and clear calibration records. Colour conventions should be recorded on drawings, because brown for positive and blue for negative is common but not universal. If HART is present, we’ll also check that the analogue value and digital process variable agree.
Speak with a Diversified Technologies engineer on 585-461-2110 for help with transmitter selection, loop calculations, cable specification and commissioning support.
Frequently Asked Questions
A single 4-20 mA transmitter should not be connected directly in parallel to two analogue inputs; use a loop splitter, signal isolator, or approved retransmission output. Parallel inputs can steal current from each other and create unstable or inaccurate readings. A splitter also helps when one system needs isolation, different scaling, or a separate maintenance signal.
Transmitters installed in hazardous areas must be wired through protection methods specified by the area classification, equipment certification, and the project’s approved control drawing documents. Intrinsic safety barriers, galvanic isolators, glands, segregation, and earthing all affect how the loop is connected. The barrier or isolator also adds voltage burden, so it must be included in the loop power calculation.
A 4-20 mA transmitter can feed a voltage-only input only through a suitable precision resistor or signal conditioner that converts current into the required voltage range. The resistor value, tolerance, power rating, and input impedance all affect accuracy. A signal conditioner is often the cleaner option when isolation, offset scaling, or a non-standard voltage range is required.
The transmitter range should cover the real operating limits while keeping normal operation well within span, because wiring cannot compensate for a badly selected measurement range. For pressure, include vacuum, surge, and overpressure conditions. For level, consider density, tank geometry, mounting height, wet or dry legs, and whether the PLC expects direct level, volume, or percentage.
The fault-current direction should match the plant’s control philosophy, so alarms and interlocks treat transmitter failure differently from genuine low or high process conditions safely. Many smart transmitters and input cards allow low-fail or high-fail behaviour to be configured. Document the chosen direction in the PLC scaling and alarm design so maintenance staff can interpret abnormal currents correctly.