Every instrumentation site explains that current loops beat voltage signals because current ignores wire resistance. Almost none of them show the actual error a voltage signal produces over real cable.
This guide puts real numbers behind that claim, a voltage divider calculator using real copper wire resistance and receiver input impedance, plus a genuinely balanced look at when a voltage signal is still the right choice.
4-20 mA is used instead of a voltage signal because a current source actively forces the same current through a loop no matter what resistance the cable adds.
A voltage signal, by contrast, loses accuracy to an unintended voltage divider formed between cable resistance and the receiving instrument's input impedance.
That claim gets repeated everywhere, but rarely with the actual numbers behind it, which is the gap this guide closes with a calculator built on real copper wire resistance data.
Why This Is Not Another Generic Comparison List
Two other pieces on this site already cover the qualitative reasons current beats voltage in real depth: noise immunity, live zero fault detection, two wire power, and HART compatibility.
Repeating that list here would add nothing. Instead, this article answers the question those pieces leave open: exactly how much accuracy does a voltage signal actually lose over a real cable run, and under what conditions does that error become negligible instead of serious.
| Signal Type | Effect of Cable Resistance | Typical Best Fit |
|---|---|---|
| 4 to 20 mA current loop | None, by design, as long as loop resistance stays within the transmitter's compliance voltage budget | Long field runs, hazardous areas, noisy plants |
| 0 to 10 V voltage signal | Forms a voltage divider with receiver input impedance, error grows with distance and wire gauge | Short runs inside a single panel or cabinet |
| 0 to 5 V voltage signal | Same divider effect, and a smaller working range makes the same absolute error a larger percentage | Very short, low noise, panel internal wiring |
The Voltage Divider Problem: Actual Numbers
A voltage signal traveling down a cable does not arrive unchanged. The cable's own resistance sits in series with the receiving instrument's input impedance, and together they form an unintended voltage divider.
The measured voltage at the receiver equals the transmitted voltage multiplied by receiver impedance, divided by the sum of receiver impedance and cable resistance. Whenever cable resistance is small compared to receiver impedance, the error stays negligible. Whenever it is not, the error becomes real.
Real copper wire resistance is not trivial over distance. Standard 20 AWG instrumentation cable runs about 34 ohms per kilometer per conductor, meaning a 1 kilometer run, out and back, adds roughly 68 ohms of resistance into that divider.
| Wire Gauge | Resistance per km (single conductor) |
|---|---|
| 18 AWG | About 21.4 ohms |
| 20 AWG | About 34.1 ohms |
| 22 AWG | About 54.1 ohms |
| 24 AWG | About 86.0 ohms |
Why Current Loops Do Not Have This Problem
A 4 to 20 mA transmitter is not a voltage source, it is a current source. It actively adjusts its own output voltage to force exactly the intended current through the loop, regardless of how much resistance the cable and receiver add.
That only works up to a limit, the transmitter's compliance voltage, which sets a maximum total loop resistance the loop supply can push current through.
A separate article on this site works through that maximum loop resistance calculation in detail using the transmitter's compliance voltage rating.
High Impedance DAQ, About 1 Megohm
At 1 kilometer of round trip cable, error stays around 0.0068 percent, effectively negligible.
Typical PLC Analog Input, About 100k Ohms
Same cable run, error rises to about 0.068 percent, still small but no longer trivial.
Older Low Impedance Receiver, About 1k Ohms
Same cable run, error reaches about 6.4 percent, a genuinely serious measurement problem.
Very Low Impedance Circuit, About 250 Ohms
Same cable run, error climbs past 21 percent, unusable for any real measurement.
Voltage Signal Measurement Error Calculator
Enter a cable resistance per kilometer, run length, receiver input impedance, and the transmitter's output voltage to see the actual measurement error the voltage divider effect produces.
Where the Error Comes From
When a Voltage Signal Still Makes Sense
None of this makes voltage signals wrong everywhere. Inside a single control panel, where cable runs are a meter or two and receiver impedance is known and high, the divider error calculated above rounds to zero in practice.
Voltage signals are also simpler and cheaper for that specific case, no current source circuitry needed at the transmitting end, and many low cost sensors and data acquisition boards are built around a voltage input by default.
The decision genuinely comes down to distance and known receiver impedance. Short, panel internal, high impedance receiver, voltage is fine. Long field run, unknown or shared cabling, hazardous area, current loop is the correct choice, not a matter of habit or convention.
Signal Type Selection Do's and Don'ts
✓ Do
- Calculate the actual voltage divider error before assuming a voltage signal is accurate enough over a given run
- Check the receiving equipment's actual input impedance, not an assumed high value
- Use current loops for any run longer than a few meters or with unknown cable resistance
- Confirm loop resistance stays within the transmitter's compliance voltage budget before wiring a current loop
✗ Don't
- Assume a voltage signal is fine simply because the run "looks short," calculate it instead
- Ignore wire gauge, a thinner conductor adds meaningfully more resistance per meter
- Treat a low impedance voltage receiver as interchangeable with a high impedance one
- Forget that 0 to 5 V signals lose a larger percentage of range to the same absolute error than 0 to 10 V signals do
Resources on 4-20 mA Current Loops and Voltage Signals
Why 4-20 mA Is Used Instead of a Voltage Signal, Questions Engineers Ask
External References
- Engineering ToolBox: Copper Wire, Electrical Resistance vs Gauge
- NCD: 4 to 20 mA vs 0 to 10 V, Which Analog Output Should You Choose
What We Learn Today
- A voltage signal loses accuracy through a real, calculable voltage divider formed between cable resistance and receiver input impedance.
- Over 1 kilometer of 20 AWG cable, that error ranges from about 0.004 percent with a high impedance receiver to over 6 percent with a low impedance one.
- A 4 to 20 mA current loop avoids this entirely by forcing constant current, as long as total loop resistance stays under the transmitter's compliance voltage budget.
- Voltage signals remain a reasonable choice for short, panel internal runs with known high receiver impedance, the decision is about distance and impedance, not habit.