Voltage Drop

Calculating Voltage Lost Over a Wire Run

Every wire has some resistance, so some voltage is always lost between the source and whatever it’s powering — voltage drop is how much. Enter the wire’s material and gauge, the current it carries, its one-way length, and the source voltage, and this calculator shows exactly how many volts (and what percentage) are lost over that run.

This matters most on longer wire runs — a garage or shed sub-panel, an extension circuit, or a solar array’s home run back to an inverter — where too much voltage drop can make lights dim, motors run hotter and less efficiently, and other equipment misbehave even though it’s still technically receiving power.

The Formula

$$\text{Voltage Drop} = \frac{2 \times \vA{K} \times \vB{\text{Current}} \times \vC{\text{One-Way Length}}}{\vD{\text{Circular Mils}}}$$

• K is a resistivity constant specific to the wire material (copper or aluminum) — a long- standing, stable physical property, not something that changes over time.
• Circular Mils is the wire gauge’s cross-sectional area, looked up from the standard AWG (American Wire Gauge) reference table.
• The factor of 2 accounts for the round trip the current actually makes — out to the load through one conductor and back through the other — even though you only enter the one-way distance.

Worked Example

A 12 AWG copper wire carrying 20 amps over a 50-foot one-way run on a 120-volt circuit:

1. 12 AWG copper has a cross-sectional area of 6,530 circular mils, and copper’s K constant is 12.9.
2. Voltage drop: $\frac{2 \times 12.9 \times 20 \times 50}{6,530} \approx 3.95 \text{ volts}$.
3. As a percentage of the source: $3.95 \div 120 \times 100 \approx 3.29\%$ — just over the commonly-cited 3% guideline for a branch circuit, meaning a thicker gauge or a shorter run would be worth considering.

Key Factors to Consider

• A lower AWG number means a THICKER wire, which is a common source of confusion. Wire gauge numbering runs in the opposite direction from intuition — a 10 AWG wire is physically thicker and has less resistance than a 14 AWG wire, so upsizing a wire to reduce voltage drop means choosing a SMALLER gauge number, not a larger one.
• Copper and aluminum wire have meaningfully different resistivity, which changes voltage drop for the same gauge and length. Aluminum has notably higher resistance than copper, so an aluminum run needs a thicker gauge than a copper run to achieve the same voltage drop over an identical distance — always confirm which material is actually being used before choosing a gauge based on a voltage-drop calculation.
• Excessive voltage drop causes real, cumulative problems beyond just dimmer lights. Motors running on undervoltage draw more current to compensate, which generates extra heat and reduces their lifespan — this is exactly why voltage drop matters as a genuine equipment-longevity and safety concern, not just a minor efficiency nitpick.
• The commonly-cited 3% (branch circuit) and 5% (combined feeder-and-branch) voltage drop guidelines come from the National Electrical Code’s own recommendations, not a hard legal limit in most jurisdictions. These percentages are widely-followed best-practice targets for ensuring equipment receives adequate voltage — always check local electrical code requirements for anything beyond a DIY reference estimate, especially for a permitted electrical installation.

Common Mistakes

• Entering the round-trip wire length instead of the one-way distance. This calculator’s formula already doubles the length internally to account for the return conductor — entering an already-doubled length overstates the voltage drop by 2x.
• Confusing which direction AWG numbers run. Assuming a bigger gauge NUMBER means a bigger, lower-resistance wire is backwards — a 10 AWG wire is thicker (and drops less voltage) than a 14 AWG wire, not the other way around.
• Using copper’s resistivity constant for an aluminum run, or vice versa. The two materials have meaningfully different resistance, so plugging in the wrong material’s constant produces a voltage-drop estimate that doesn’t match the wire actually being installed.

Useful to Know

• Sizing a wire run for a solar array’s home run to an inverter? Solar Panel System Size Calculator and Solar Panel Savings Calculator help plan the system this wire will actually carry power for.
• Want to double-check the current and resistance relationship directly with Ohm’s Law? Ohm's Law Calculator solves for voltage, current, or resistance from the other two.