Signal Loop Wiring Calculator

Loop resistance · voltage budget · max cable length

Loop

Supply Voltage (V)

Min Voltage at Device (V)

Loop Current (mA)

Wire

Gauge

Resistance (Ω/1000ft, ★ editable)

Cable Length (ft, one way)

Resistance is per 1000 ft, one way — the tool doubles it for the round trip. Resistance rises with temperature; increase the value for hot environments.

Series Devices (burden)

Add each device in the loop with its burden resistance (Ω). Sense resistors, PLC AI cards, indicators and recorders all add up in series.

Result

Enter your supply, wiring and series devices to get the total loop resistance, total voltage drop, a Pass/Fail voltage-budget check with the exact margin, or switch to reverse mode to find the maximum cable length the loop can support

The voltage budget is checked at the worst case — maximum loop current, which is 20 mA for a 4-20mA loop — because that is when the voltage drop across the wiring and burden is greatest. Total resistance is the round-trip wire resistance plus every series burden; the transmitter itself also needs a minimum operating voltage, so include that as the minimum-voltage figure or as a burden as appropriate. Built-in gauge resistances are typical copper values at about 20 °C, and real resistance rises with temperature, so raise the value for hot runs. HART communication needs at least 230–250 Ω of loop resistance to work, which is a separate consideration from this voltage check. Always confirm against the device datasheets and your cable specification.

Signal Loop Wiring Calculator: Loop Resistance, Voltage Budget and Maximum Cable Length

A 4-20mA loop looks simple, two wires and a current, but it fails silently if the total resistance around that loop gets too high, since the supply voltage just can’t push enough voltage through at the worst-case 20mA to satisfy every device in the circuit. This loop resistance calculator sums up the round-trip wire resistance, from a built in gauge table you can override, plus every series device burden, transmitter, PLC input card, indicator, recorder, then runs the actual voltage budget: available supply minus loop-current voltage drop must stay above what the receiving device actually needs, shown as a clear Pass or Fail with the exact margin in volts. Multiple device mode adds each device in series with it’s own burden. As a genuine max cable length calculator, the reverse mode answers the practical field question directly, given everything else, how far can I actually run this cable before voltage drops too low, by solving for the maximum supportable length.

How to Use

This tool has two tabs: Voltage Budget and Max Cable Length.

Setting up your loop (shared across both modes)

Enter your Supply Voltage (V), the loop power supply, commonly 24V DC.
Enter the Min Voltage at Device (V), the minimum operating voltage your receiving device, transmitter or otherwise, actually needs, taken straight from it’s datasheet.
Enter your Loop Current (mA), the default is 20mA since that’s the worst-case current for a 4-20mA loop and voltage drop is always greatest at maximum current.

Setting up your wiring

Select your wire Gauge from the dropdown, this auto-fills a typical resistance value in Ω per 1000ft.
Check the Resistance (Ω/1000ft) field and edit it directly if you have a more specific figure for your actual cable, or if you’re accounting for a hot environment, since resistance rises with temperature.
Enter your Cable Length (ft, one way), the tool automatically doubles this for the round trip since current has to flow out and back.

Using Voltage Budget mode

Under Series Devices, tap Add Device for each device sitting in the loop, sense resistors, PLC analog-input cards, indicators, recorders, entering it’s burden resistance in ohms. All of these add up in series.
Tap Check Loop. The result shows total loop resistance, wire resistance and burden resistance broken out separately, total voltage drop at your worst-case current, and a clear Pass/Fail against your minimum device voltage, with the exact margin in volts either way.

Using Max Cable Length mode

Switch to the Max Cable Length tab. Enter the same supply, minimum voltage, current, wire gauge and series devices as before, but leave cable length itself as the unknown.
Tap Check Loop. Instead of a Pass/Fail, the tool solves backward and tells you exactly how far, in feet, you can run this specific cable before the voltage budget fails.

Exporting your result

Use Print / PDF for a clean printable copy or Copy to paste the figures into your loop drawing or calibration file.

Key Features

Built-in wire-gauge resistance table, auto-filling a typical value, always fully overridable for your actual cable or a hot-environment temperature correction
Multi-device series burden mode, adding sense resistors, PLC input cards, indicators, and recorders all together in one running total
Voltage-budget Pass/Fail check at the worst-case 20mA current, with the exact margin shown in volts
Reverse max-cable-length mode, solving directly for how far you can run the cable before the loop fails
Fully custom wire resistance, entered in Ω per 1000ft, editable for any real cable and temperature condition
Built in reminder that HART communication needs a minimum of roughly 230-250Ω loop resistance to work, a separate consideration from the voltage check alone
Export as PDF or copy the result.

Formula / Logic Used

Total Loop Resistance

$R_{total} = R_{wire} + \sum R_{device}$

Where $R_{wire}$ is the round-trip resistance, twice the one-way cable length times the per-1000ft resistance figure.

Voltage Drop and Budget Check

$V_{drop} = I_{loop} \times R_{total}, \qquad V_{available} = V_{supply} – V_{drop}$

The loop passes if $V_{available} \geq V_{min}$ , with the margin shown as $V_{available} – V_{min}$ .

Reverse Mode: Maximum Cable Length

 $R_{allowed} = \frac{V_{supply} - V_{min}}{I_{loop}} - \sum R_{device}, \qquad Length_{max} = \frac{R_{allowed}}{2 \times R_{per\ 1000ft}} \times 1000$

The factor of 2 accounts for the round trip, current flows out to the device and back through the second wire, so only half of the allowed wire resistance is actually available per one-way foot of cable.

Who Should Use This Tool

Instrumentation and electrical engineers checking whether a planned 4-20mA loop will actually work over it’s real cable run before installation. Also useful for diploma and B.Tech Instrumentation and Electrical Engineering students learning loop resistance, voltage budget and series burden concepts for their process control coursework.

Frequently Asked Questions (FAQs)

1. Why is 20mA used as the worst case for a 4-20mA loop voltage check?

Voltage drop across wiring and burden resistance scales directly with current, so it’s largest right at the top of the range, 20mA, and this is exactly when the loop is most likely to fail the voltage budget. This tool always checks the worst case current by default, since a loop that passes at 20mA will always pass at every lower current too.

2. How do I calculate the maximum cable length for a 4-20mA loop?

Subtract the minimum device voltage from the supply voltage, divide by the loop current to get the allowed total resistance, subtract any series device burden, then divide the remaining resistance by twice the wire’s per-foot resistance, since current has to travel out and back. This tool’s reverse mode does this calculation instantly.

3. Why does the wire resistance table need to be edited for hot environments?

Copper resistance rises measurably with temperature, so a cable run through a hot equipment room or outdoor conduit in summer will have noticeably higher actual resistance then the standard 20°C reference value in a typical gauge table. This tool keeps the resistance field always editable specifically so you can apply this correction yourself.

4. What counts as a “device burden” in a 4-20mA loop?

Any component that drops voltage as current passes through it, sense resistors, a PLC or DCS analog input card’s internal resistor, indicators, and recorders all add their own burden resistance in series around the loop. This tool lets you add each one individually so the total burden is accurate for your specific configuration.

5. Does passing the voltage budget check guarantee HART communication will also work?

No, these are two separate checks. The voltage budget confirms the loop can supply enough DC voltage to every device at worst-case current, while HART digital communication additionally needs a minimum loop resistance, typically around 230-250Ω, to properly modulate it’s signal on top of the DC current. A loop can pass the voltage check and still fall short of this separate HART resistance requirement.

Related Tools

Sensor Range/Accuracy Converter (4-20mA) – convert the signal carried by this same loop into engineering units
Control Valve Cv Sizing Calculator – size the valve at the far end of a control loop
PID Controller Tuning Estimator – tune the controller using the measurement this loop carries