Estimate the voltage drop across a wire run based on its length, cross-sectional area, material, and current load.
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When an electric current passes through a conductive wire, the voltage loses part of its value due to the wire's own resistance, which is known as voltage drop, and it increases the longer the wire is, the higher the current, or the smaller the wire's cross-sectional area. This tool first calculates the wire's resistance from the relationship R = ρ × L ÷ A, where ρ is the resistivity of the metal used (copper or aluminum), L is the wire's length, and A is its cross-sectional area. The voltage drop is then calculated by multiplying this resistance by the current, doubling the result (×2) in circuits where the current travels out and back through two wires (as in most DC circuits and single-phase AC circuits). If you enter the source voltage, the tool also shows the drop as a percentage of the total voltage, which is useful for comparing against the limits commonly set in electrical codes, usually 3-5%, since excessive voltage drop can cause connected equipment to underperform or malfunction even when the supply voltage at the source is entirely correct.
Every real wire has some resistance, even good conductors like copper and aluminum, and that resistance means voltage genuinely gets lost — converted to heat — as current travels along the wire's length, which is exactly why the voltage a device actually receives at the end of a long wire run can be measurably lower than the voltage available at the source, even with a perfectly functioning power supply.
The wire resistance formula, R = ρL/A, shows exactly why long wire runs are the classic scenario where voltage drop becomes a genuine practical concern: resistance increases directly and proportionally with length, meaning a wire twice as long has exactly twice the resistance (and, at the same current, twice the voltage drop) of a shorter run using the identical wire gauge.
The same formula also shows the solution to a long-run voltage drop problem: resistance decreases as cross-sectional area increases, meaning a thicker wire (larger A) has proportionally lower resistance and therefore lower voltage drop for the identical length and current. This is exactly why electricians specify progressively thicker wire gauges for longer circuit runs supplying the same load, rather than using the thinnest wire that would technically carry the required current safely over a short distance.
Excessive voltage drop causes real, sometimes subtle problems at the receiving end of a long wire run: motors can run hotter and less efficiently, LED lighting can dim or flicker, and sensitive electronic equipment can malfunction or fail to start correctly — all while the voltage measured right at the power source looks completely normal, which is exactly why voltage drop calculations, not just source voltage checks, are essential when troubleshooting equipment that malfunctions specifically at the end of a long circuit run.
Electrical codes commonly set a maximum acceptable voltage drop, often in the 3-5% range of the nominal supply voltage, specifically to keep connected equipment operating within its designed voltage tolerance. Calculating expected voltage drop before installation — factoring in wire length, expected current, and wire gauge — lets electricians choose an adequately thick wire gauge upfront, avoiding a costly and disruptive wire upgrade after installation reveals a voltage drop problem that wasn't caught during planning.
In most DC and single-phase AC circuits, current must travel out through one conductor and return through another, so the total resistance the current passes through is twice the one-way wire resistance.
Many electrical codes recommend keeping voltage drop under about 3% for lighting circuits and under 5% for general branch circuits, though exact limits vary by local code and application.
Aluminum has a higher electrical resistivity than copper, so for the same length and cross-sectional area, an aluminum conductor produces a larger voltage drop for a given current.