DC Voltage Drop Calculator

Calculate DC voltage drop over a cable run for remote radios, cameras and injectors, and check the equipment still receives a usable voltage.

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Result

Voltage drop
2.51 V
Voltage at equipment
45.49 V
Drop percentage
5.22%
Loop resistance
1.672 Ω
80 m of conductor
Power lost in cable
3.76 W
5.2% of delivered power
Maximum length at this gauge
95.7 m
to stay above 42 V

Gauge comparison at 1.5 A over 40 m

GaugeDropAt equipmentMax length
10 AWG0.39 V47.61 V611.6 m
12 AWG0.62 V47.38 V384.9 m
14 AWG0.99 V47.01 V241.9 m
16 AWG1.58 V46.42 V152.3 m
18 AWG2.51 V45.49 V95.7 m
20 AWG3.98 V44.02 V60.2 m
22 AWG6.33 V41.67 V37.9 m
24 AWG10.07 V37.93 V23.8 m
Within tolerance
45.49 V arrives, above the 42 V minimum. Note that copper resistance rises roughly 0.4% per °C - a cable in direct sun or a hot riser will drop more than this calculation suggests.
Why 48 V
Higher voltage is the cheapest fix. At constant power, doubling the voltage halves the current and quarters the loss - which is exactly why remote radios are fed 48 V rather than 12 V.

About DC Voltage Drop Calculator

DC voltage drop is Ohm's law over the round trip of the cable. It is why a radio on a mast browns out at dusk when the heater kicks in, and why 12 V equipment at the end of a long run is a recurring field problem.

Temperature and the numbers that shift

Copper resistance rises about 0.393% per degree Celsius above 20 C. A cable in a sun-exposed conduit at 60 C carries roughly 16% more resistance than the datasheet figure, which is exactly when a marginal design fails — on the hottest afternoon, at peak load. Size for the worst-case ambient you expect, not for a bench measurement, and remember that inrush at power-on can double the steady-state current for a few hundred milliseconds.

The arithmetic

Resistance = ρ × (2 × length) ÷ area, where ρ for copper is 0.0172 Ω·mm²/m. Both conductors carry the current, hence the doubling. Drop = current × resistance. Keep it under 5% of supply voltage for comfort, and never exceed the equipment minimum under peak load - inrush at power-on is often double the steady-state figure.

Common use cases

  • Sizing the DC feed for a tower-mounted radio or PTZ camera.
  • Deciding between a local injector and a longer DC run.
  • Diagnosing equipment that resets under load.

Edge cases and gotchas

  • Aluminium conductors have roughly 60% higher resistance than copper for the same area.
  • Inrush current at power-on can far exceed steady-state draw - size for the peak.

Frequently asked questions

Does the calculation change for AC circuits?
Yes. AC adds reactance and, on single-phase, uses a different multiplier than the three-phase case. This tool covers DC feeds, which is what remote radios, cameras and injectors use.
How much drop is acceptable?
3% is comfortable, 5% is a common design limit, above 10% and you are wasting power as heat and inviting intermittent faults.