At 24 V DC the same power means more current, and the same voltage drop is a bigger share: 1 V is 0.4 % of 230 V but about 4.2 % of 24 V. Voltage drop is the first thing to check when sensors, remote I/O or RS-485 devices sit far from the panel.
Formula
ΔV = 2 × I × ρ × L ÷ A I: current (A), L: one-way length (m), A: cross-section (mm²) ρ: copper at 20 °C 1/58 ≈ 0.01724 Ω·mm²/m
The 2 counts both conductors, out (+) and back (0 V). Enter the one-way length (supply to device) and multiply by 2. One kilometre of 1 mm² copper is about 17.24 Ω at 20 °C.
Example: 2 A over 50 m one way
| Size | Loop resistance | Drop | Percent | Voltage at device |
|---|---|---|---|---|
| 0.75 mm² (≈ 18 AWG) | 2.30 Ω | 4.60 V | 19.2 % | 19.40 V |
| 1.5 mm² (≈ 16 AWG) | 1.15 Ω | 2.30 V | 9.6 % | 21.70 V |
| 2.5 mm² (≈ 14 AWG) | 0.69 Ω | 1.38 V | 5.7 % | 22.62 V |
| 4 mm² (≈ 12 AWG) | 0.43 Ω | 0.86 V | 3.6 % | 23.14 V |
How much drop is acceptable is set by the device. Take the lower limit from its supply specification (e.g. «24 V DC, allowed range … V») and check the voltage at the device stays comfortably above it. Some supplies let you raise the output slightly, but the drop changes with current, so a thicker wire or a closer supply is the real fix.
What gets overlooked
- Inrush: relays, solenoid valves and motors draw much more current when switching on. If they share the wire, the voltage dips just then and devices restart. Calculate with the largest current, not the average.
- Shared common (0 V): when several devices share one 0 V wire, their currents add up in it, increasing the drop and shifting the reference of the other devices.
- Temperature: copper resistance rises about 0.393 % per °C, so 50 °C above 20 °C means about 20 % more. Use a higher temperature for wiring in hot ducts or panels.
- Terminals and connectors: contact resistance is not in this calculation. One loose terminal can drop more than a long wire.
- Power over thin signal wires: sending power through thin conductors in a communication cable (e.g. 24 AWG ≈ 0.2 mm²) drops a lot even over short runs.
What it looks like on a communication bus
A device that restarts for lack of voltage does not reply while it boots, so it shows up as «timeouts that come in bursts». Log the times of the timeouts and compare them with when other loads on the same supply (valves, motors, heaters) switch on to tell a power problem from a communication one.
4–20 mA is checked differently
A 4–20 mA signal is a current, so wire resistance does not change the reading. A 2-wire transmitter, however, needs «supply − voltage lost in the loop resistance» to stay above its minimum operating voltage, so calculate the loop load rather than a percentage drop.
In PortDock
- «Voltage Drop & Wire Size Calculator» web tool: DC, single- and three-phase, mm² or AWG, copper or aluminium, temperature; gives drop and percent, plus minimum size and maximum length for an allowed drop.
- «4–20 mA Loop Load Calculator» web tool: voltage left at the transmitter and maximum cable length from loop supply, transmitter minimum, 250 Ω and cable resistance.
- Modbus Master's «Log to file» keeps the bytes with timestamps, and timeouts appear as event lines in the traffic log — line them up with when other loads switch on.
Published 2026-10-23