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Wire Harness Voltage Drop and Power Loss Calculator by AWG

Long wire runs on vehicles, shelters and aircraft quietly consume supply voltage and turn it into heat. This calculator gives the voltage drop, percentage loss and I squared R heating for a copper run by gauge.

Your numbers

Wire gauge (copper)

Solid copper DC resistance at 20 C. Stranded wire is slightly higher; use the wire spec value for final design.

Load current

One-way run length

Length from source to load; the return path is added automatically.

Supply voltage

For example 28 V DC aircraft or vehicle bus.

Conductor temperature

Copper resistance rises about 0.39 percent per degree C above 20 C.

Allowed voltage drop

% of supply

Your results

Voltage drop

1.5963

Volts lost in the wire.

Voltage drop percent

5.7%

Round trip resistance

0.3193

Ohms for supply and return conductors at the stated temperature.

Voltage at the load

26.4038

Power lost as heat in the wire

7.9813

Watts.

Maximum one-way length within allowed drop

13.1558

Feet.

DC resistance estimate for copper. It does not cover ampacity, bundle derating, altitude, insulation temperature rating or connector and contact resistance. Use the applicable wire specification and your harness design rules for current ratings.

Save results:

Voltage drop vs one-way length

Sweeps One-way run length from 1 to 100, holding your other inputs fixed.

Harness and BOM data in one private search

Optional: discuss making wire lists, BOM and ECO history searchable inside your network.

Optional. The tool and every export work without this. No spam.

How to use it

Select the wire gauge, enter load current and one-way length, then supply voltage and conductor temperature. The tool doubles the length to include the return conductor, applies a temperature correction to copper and returns the drop in volts and percent, the load voltage, the wire heating loss and the longest run that stays within your allowed drop. Use the chart to compare how drop grows with length.

Enter one-way length; do not double it yourself.
Use hot conductor temperature for worst case.
Set the allowed drop from your interface requirement.

The formulas

Resistance equals the wire resistance per 1000 ft times round trip length divided by 1000, corrected for temperature with R(T) = R20 x (1 + 0.00393 x (T - 20)) for copper. Voltage drop is I x R and wire loss is I squared x R. The AWG resistances used are standard values for bare copper at 20 C. Real wire to a military wire specification may be tin or silver plated and stranded, so check the specification resistance.

Worked example

A 5 A load on 25 ft of AWG 18 at 28 V has round trip resistance 6.385 x 50 / 1000, or 0.319 ohm. Drop is 1.6 V, about 5.7 percent of supply, and the wire dissipates 8 W. If the budget is 3 percent, the longest run is about 13 ft, or you can move to AWG 14 which cuts the drop to roughly 0.63 V. Voltage drop is separate from the current carrying limit, so always check ampacity too.

Common mistakes

Using only the one-way length is the classic error and halves the answer. Others ignore connector and splice resistance, which can add as much drop as several feet of wire in a harness with many mating pairs. For AC or high frequency, inductance and skin effect change the picture. For safety, size wire for both ampacity and voltage drop and take the worse case.

Frequently Asked Questions

Why is the length doubled?

Current flows out on the supply conductor and returns on the ground or return conductor, so the voltage drop includes both. If a chassis return is used, replace the return length with the chassis path resistance.

Does temperature matter?

Yes. Copper resistance increases about 0.39 percent for each degree C above 20 C, so a hot harness drops more voltage than the table value. Use worst-case conductor temperature.

Is this the same as wire ampacity?

No. Ampacity is limited by heating, insulation rating, bundle size and altitude. A wire can satisfy voltage drop and still be overloaded thermally, so check both limits.

What allowed drop should I use?

Use the figure in your interface control document, power budget or customer specification. Many designs allow only a few percent in distribution wiring, but there is no universal value.

If harness definitions and wire lists live in PLM and ERP systems, Netray can discuss how to make that data searchable and traceable on your own infrastructure.

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