A 24 V loop with a 250 Ω receiver
At 20 mA the 270 Ω of receiver and cable drop 5.4 V, so the loop needs 10.5 + 5.4 = 15.9 V. The 24 V supply leaves 8.1 V of headroom, enough for up to 675 Ω in total.
Voltage headroom 8.1 V Open in the calculator
4-20 mA loop voltage calculator
Check that a loop-powered 4-20 mA transmitter keeps enough voltage at full current once the receiver, cable and barriers take their share.
Voltage headroom
8.1V
Check the values to see a result.
- Supply needed at the design current
- 15.9 V
- Largest loop resistance this supply drives
- 675 Ω
- Loop resistance entered
- 270 Ω
- Pass: The supply drives the loopat 20 mA8.1 V spare
Keep some headroom for supply tolerance and ageing.
How it’s calculated
- Vloop = I × Rtotal=0.02 A × 270 Ω=5.4 V
- Vrequired = Vtransmitter + Vloop + Vother=10.5 + 5.4=15.9 V
- Headroom = Vsupply − Vrequired=24 − 15.9=8.1 V
Vrequired = Vtransmitter + I × Rtotal + Votherheadroom = Vsupply − Vrequired
At 20 mA the loop needs 15.9 V, so a 24 V supply leaves 8.1 V spare.
Tip: Check at the highest current the loop will carry: 20 mA, or 21 mA and above if the transmitter signals faults high.
How to check 4-20 mA loop voltage
A loop-powered 4-20 mA transmitter takes its power from the loop, so the supply has to cover the transmitter’s own minimum voltage plus the voltage dropped across everything else in the loop at full current. A 24 V supply driving a transmitter that needs 10.5 V, through a 250 Ω receiver and 20 Ω of cable, leaves 8.1 V spare at 20 mA.
Voltage the loop needs
Vrequired = Vtransmitter + I × Rloop + Vfixed
Vtransmitter is the minimum terminal voltage from the transmitter’s datasheet. Rloop is every resistance in series: the receiver’s input, the cable out and back, barriers and indicators. Vfixed covers parts that drop a fixed voltage, such as diodes and some isolators.
Headroom
Headroom = Vsupply − Vrequired
Positive headroom means the transmitter still has enough voltage at the design current. Negative means it will stall below full scale and the reading will clip.
Largest loop resistance
Rmax = (Vsupply − Vtransmitter − Vfixed) ÷ I
The same budget turned round: the most resistance this supply can drive. Transmitter datasheets often draw it as a load line.
Check at the highest current the loop will carry, not 12 mA. Many transmitters signal a fault by driving the loop to 21 mA or more, following NAMUR NE 43, and the loop has to carry that too, or the fault signal is lost.
4-20 mA loop examples
The same loop on 12 V
A 12 V supply is 3.9 V short: the loop still needs 15.9 V. The transmitter would stop regulating well before 20 mA. Use a 24 V supply or a receiver with a lower input resistance.
Voltage shortfall 3.9 V Open in the calculator
A barrier and an indicator at the fault current
With a barrier and a loop indicator adding 280 Ω, and the loop checked at 22 mA for the fault signal, 560 Ω drops 12.32 V. With a 12 V transmitter minimum the loop needs 24.32 V, a shortfall of 0.32 V.
Voltage shortfall 0.32 V Open in the calculator
Largest loop resistance at 20 mA
Total series resistance each supply can drive at 20 mA, for three transmitter minimum voltages, with no fixed drops.
| Supply (V) | 8 V transmitter (Ω) | 10.5 V transmitter (Ω) | 12 V transmitter (Ω) |
|---|---|---|---|
| 12 | 200 | 75 | 0 |
| 15 | 350 | 225 | 150 |
| 18 | 500 | 375 | 300 |
| 20 | 600 | 475 | 400 |
| 24 | 800 | 675 | 600 |
| 28 | 1,000 | 875 | 800 |
| 30 | 1,100 | 975 | 900 |
Questions about 4-20 mA loop voltage
Why do receivers use 250 Ω?
Because 4-20 mA through 250 Ω is 1-5 V, a range a voltage input can read. It also takes 5 V of the loop’s budget at 20 mA, so on low supply voltages a smaller resistor, and a matching input range, leaves more for the transmitter.
What minimum voltage does a 4-20 mA transmitter need?
It varies by model, often between about 8 V and 12 V, and more for some with HART or displays. Use the figure from the transmitter’s own datasheet, at its worst case over temperature.
Does cable length matter?
Only through its resistance. A long run of thin cable adds tens of ohms out and back; the conductor resistance calculator gives it from the length and size.
What happens if the headroom is negative?
The transmitter cannot push the full current, so the signal stops rising at some point below 20 mA. Readings at the top of the range, and any high fault signal, are lost.
Limits of this result
- Use the transmitter’s own minimum voltage at its worst-case temperature, and check the loop at its fault current too.
- Include every barrier, isolator, indicator, surge protector and input resistor in the loop.
- Intrinsic safety, earthing and EMC need their own design; this checks the voltage budget only.
Log the signal continuously
A ZIO reads 4-20 mA and 0-10 V signals, so its input resistance is one of the drops in this budget.
