Electrical Engineering · Worked example

Voltage Drop and Wire Sizing Explained: A Practical Design Method

Learn how voltage, current, distance, conductor area, material, temperature, and allowable drop interact when choosing a feeder conductor.

By Neutron Studios LLC8 minute readPublished 2026-08-11Reviewed 2026-08-11

Why this calculation matters

Wire sizing is not one calculation. A conductor can satisfy a voltage-drop target and still be unacceptable for ampacity, temperature, short-circuit withstand, mechanical strength, or terminal compatibility.

A useful early design method is to separate electrical loss from regulatory ampacity: first calculate resistance and drop, then apply the installation-specific current-carrying and protection requirements.

What you will calculate

Relate conductor area to resistance and voltage drop.
Understand why low-voltage systems are especially sensitive to feeder drop.
Compare percentage-drop and power-loss criteria.
Avoid using a voltage-drop result as a code-compliance claim.

Given values

24 V DC source
25 A load
8 m one-way copper run
Target maximum feeder drop: 3%
Candidate conductor area: 8.37 mm², approximately 8 AWG
Copper resistivity for the screening calculation: 0.01724 Ω·mm²/m at 20 °C

Governing equations

Conductor resistance

R = \frac{\rho L}{A}

Resistance is proportional to length and inversely proportional to conductor area.

DC loop drop

\Delta V = \frac{I\; \rho \left(2 L\right)}{A}

The factor of two accounts for the outbound and return conductors.

Percent voltage drop

Drop\; \% = \frac{100\; \Delta V}{V_{\mathrm{source}}}

A fixed absolute drop is more serious on a lower-voltage bus.

Minimum area from a drop target

A_{\mathrm{min}} = \frac{I\; \rho \left(2 L\right)}{\Delta V_{\mathrm{max}}}

This rearrangement gives a first-pass conductor area before selecting a standard size.

Worked solution

1. Convert the drop target to volts

A 3% target on a 24 V bus corresponds to only 0.72 V. This illustrates why low-voltage DC distribution often becomes conductor-heavy.

\Delta V_{\mathrm{max}} = 0.03 \times 24 = 0.72\; V

2. Estimate the minimum copper area

Using a 16 m loop length and the room-temperature copper resistivity gives a theoretical area of about 9.58 mm². That falls between common AWG sizes, so the next larger standard conductor should be evaluated.

A_{\mathrm{min}} \approx \frac{25 \times 0.01724 \times 16}{0.72} = 9.58\; \mathrm{mm}^{2}

3. Test the 8 AWG candidate

An 8.37 mm² conductor is smaller than the calculated 9.58 mm² target, so it should fail the 3% drop target. The calculation confirms roughly 0.824 V drop, or 3.43%.

\begin{gathered}R_{\mathrm{loop}} \approx \frac{0.01724 \times 16}{8.37} = 0.03296\; \Omega \\\Delta V \approx 0.824\; V\\\text{drop} \approx 3.43 \%\end{gathered}

4. Move to the next larger conductor

A 6 AWG conductor is approximately 13.3 mm². Repeating the same calculation gives roughly 0.519 V, or 2.16%, before temperature correction and connection losses.

\Delta V_{6} \mathrm{AWG} \approx \frac{25 \times 0.01724 \times 16}{13.3} = 0.519\; V \approx 2.16 \%

Result

Engineering interpretation

For the stated 24 V, 25 A, 8 m run, 8 AWG does not meet the 3% screening target while 6 AWG does at the reference temperature.

The choice of 6 AWG is still provisional until ampacity, installation temperature, bundling, terminal ratings, fault protection, and local code requirements are checked.

Sanity checks

At the same current and length, halving conductor area should approximately double voltage drop.
At the same power, increasing system voltage reduces current and therefore reduces feeder loss dramatically.
Temperature correction should never make copper resistance decrease as the conductor gets hotter.

Common mistakes

Selecting wire only from an ampacity table and ignoring voltage drop.
Selecting wire only from voltage drop and ignoring ampacity.
Mixing feet, meters, circular mils, and mm² without explicit conversion.
Forgetting that three-phase voltage-drop equations differ from two-wire DC or single-phase calculations.

References and model boundaries

Use current conductor-resistance data and the applicable electrical code for the installation.
Voltage-drop targets are design criteria; mandatory requirements depend on jurisdiction and application.

For safety-critical, regulated, production, or otherwise consequential work, independently verify the result using the governing standard, current manufacturer data, and qualified engineering review. See the site methodology and engineering disclaimer.