Cable Voltage Drop Calculator
Electricians, panel builders, and instrument techs use this to estimate the voltage lost along a copper cable run before sizing a supply or a loop. Enter the wire gauge, one-way run length, and current, and it returns the round-trip drop so you can check whether the far end still has enough voltage.
Voltage drop grows with cable length, current, and resistance per unit length, which itself depends on the wire gauge. This calculator uses published copper resistance per 1000 ft at 20 C for common instrument gauges and doubles the run to account for both conductors.
The Calculator
Result
Resistance values are for copper at 20 C; hot conductors drop more.
The Formula
The drop is the resistance of both conductors times the current. With resistance quoted per 1000 ft:
The factor of 2 covers the out-and-back path. The copper resistance per 1000 ft at 20 C used here is:
18 AWG: 6.385 20 AWG: 10.15 22 AWG: 16.14
Worked example. A 500 ft run of 16 AWG carrying a 20 mA loop: Vdrop = 2 × (500 / 1000) × 4.016 × 0.02 = 2 × 0.5 × 4.016 × 0.02 = 0.080 V. On a 24 V supply that is about 0.33% and leaves 23.92 V at the load. The same run carrying 1 A drops 2 × 0.5 × 4.016 × 1 = 4.02 V, which matters.
Assumptions and Limits
- Resistances are for solid copper at 20 C. Conductor resistance rises roughly 0.4% per degree C above 20 C, so a warm cable or a hot conduit drops more than shown.
- This is a DC / resistive estimate. It ignores inductive reactance, which can matter on long AC feeders but is negligible for instrument-current loops.
- Stranded wire and different insulation temperature ratings shift the resistance slightly; use the manufacturer's per-foot resistance if you have it.
- Results are engineering estimates. Confirm against the cable datasheet and applicable wiring code before sizing conductors.
FAQ
Do I enter one-way or round-trip length?
Enter the one-way distance to the load. The formula multiplies by 2 to include the return conductor, so you should not pre-double it.
Why does temperature matter?
Copper resistance increases with temperature. The tabulated values are at 20 C; a conductor running at 60 C has roughly 16% more resistance, and therefore 16% more drop, than these figures show.
Is a small drop on a 4-20 mA loop a problem?
Usually not for accuracy, because the loop is a current signal and the receiver reads current, not voltage. The drop matters for the voltage budget: it eats into the headroom the transmitter needs to operate, which the loop voltage headroom calculator handles directly.
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