Every wire has resistance, and resistance turns part of your voltage into heat before it ever reaches the device at the end of the run. That lost voltage, called voltage drop, is the difference between what the source supplies and what the load actually receives. The Voltage Drop Calculator takes your source voltage, current, wire length, gauge and conductor material, and returns the voltage lost along the way, the drop as a percentage, and the voltage your equipment will actually see.
Why Voltage Drop Matters
Voltage drop is not a theoretical curiosity; it is the reason lights dim at the end of a long extension cord and why tools strain on a long feed. When the voltage reaching a load falls too low, motors draw more current to compensate, heat up and shorten their lives; electronics can malfunction or shut down; and lighting runs dimmer than rated. Electrical codes recognize the problem by setting recommended limits: the widely cited guideline is to keep total voltage drop to 3 percent or less for feeders and no more than 5 percent for branch circuits. The gauge on this calculator is built directly around those figures.
The deeper reason the percentage matters is that it is relative to the supply voltage. Losing 3 volts on a 120 volt circuit is a 2.5 percent drop and barely noticeable, but the same 3 volts on a 12 volt automotive circuit is 25 percent, which will cripple the load. That is why electricians think in percentages and why this calculator reports the drop both in volts and as a share of the source voltage.
How the Resistance Is Determined
The heart of the calculation is the resistance of the conductor, which depends on its material, its cross-sectional area and its length. Thicker wire has lower resistance, which is why wire is sized by gauge, a numbering system where lower numbers mean thicker wire and lower resistance per foot. The calculator embeds the published resistance per 1000 feet for each gauge of copper at 75 degrees Celsius, the values in NEC Chapter 9 Table 8, from 14 AWG at 2.525 ohms per thousand feet down to 4/0 AWG at about 0.0617 ohms per thousand feet.
Material matters just as much. Aluminum has roughly 1.6 times the resistance of copper for the same gauge, which is why an aluminum run must be one or two sizes thicker to match copper's performance. The conductor material selector applies exactly that factor, so switching from copper to aluminum immediately shows the larger drop and the need for a heavier gauge.
The Round-Trip Factor
One detail trips up almost everyone new to the calculation: the current has to travel out to the load and then return through the circuit, so both conductors contribute resistance. A single-phase two-wire circuit therefore uses twice the one-way length in the formula. The calculator handles this automatically, doubling the length you enter. If your run is 100 feet from the panel to the outlet, the current actually travels 200 feet of wire total, and that is the distance the resistance math must reflect.
The Formula in Action
The calculation is straightforward once the inputs are set. Total resistance is twice the one-way length in feet, times the resistance per 1000 feet, divided by 1000, with the aluminum factor applied when selected. Voltage drop in volts is that total resistance times the current. The drop as a percentage divides that by the source voltage and multiplies by 100, and the load voltage is the source minus the drop. Four outputs, four steps, and each one is derived from the same inputs, so they always agree with each other.
Take a concrete case: a 15 amp load on a 120 volt circuit fed by 100 feet of 12 AWG copper. The total resistance is 200 feet times 1.588 ohms per thousand feet divided by 1000, about 0.318 ohms. At 15 amps that is a drop of roughly 4.8 volts, which is 4.0 percent of 120 volts, leaving about 115.2 volts at the load. That sits just inside the 5 percent branch-circuit guideline. Bump the run to 200 feet and the drop doubles to about 9.5 volts, 7.9 percent, firmly in the high zone and a clear signal to step up to 10 AWG.
Reading the Percentages
The gauge translates the drop percentage into a practical verdict. Under 3 percent is excellent, the recommended ceiling for feeders and the target most installers aim for. Between 3 and 5 percent is acceptable for branch circuits serving the final loads. Between 5 and 8 percent the drop is high, and long-run equipment like motors will likely notice it. Above 8 percent the run is effectively oversized and should be redesigned with heavier wire or a shorter path. These bands match the guidance electricians actually use, so the colored gauge is a decision aid rather than decoration.
Low-Voltage Systems Are the Strictest Test
The calculator's value becomes most obvious on low-voltage systems. A 12 volt solar or automotive circuit with a meaningful run loses a large percentage of its voltage even on decent wire, because the fixed wire resistance is being divided against a small source number. A 10 foot run of 14 AWG carrying 10 amps at 12 volts drops about 0.5 volts, over 4 percent of the supply, before the load even turns on. Designers of solar panels, boat wiring and automotive accessories routinely use voltage drop math to avoid undersized cables, and this tool covers those cases with the same formula.
The Length Chart
The line chart plots the voltage drop percentage against the one-way wire length at your chosen gauge, current and voltage. It shows the drop growing linearly with length, which is why doubling the distance doubles the loss, and why the chart is a straight line rather than a curve. Reading the chart tells you at a glance how far you can run this wire before the drop crosses the 3 and 5 percent thresholds, which is often the first question a designer asks when laying out a circuit.
What the Calculator Does Not Model
Being honest about the limits keeps the estimate trustworthy. The calculation models a single-phase two-wire circuit and ignores power factor, which affects real-world reactive loads, and it uses the 75 degree Celsius table values without re-derating for ambient temperature or conductor bundling, both of which can raise resistance on a hot, crowded installation. It also does not address ampacity, the separate current-carrying limit of a wire, which is checked independently. For the great majority of residential and light commercial runs these simplifications leave the result comfortably accurate, and where conditions are extreme, a qualified electrician makes the final call.
A Sizing Decision
The practical use of the calculator is deciding whether your planned wire is big enough. If the drop percentage lands in the acceptable range, the run is fine as designed. If it lands in the high zone, step up one or two gauges and watch the number fall, then check whether the heavier wire still fits the conduit and budget. Because the gauge selector carries the resistance data, comparing 12 AWG against 10 AWG on the same run takes one click, and the drop percentage settles the argument with numbers instead of guesswork.
Common Wire Gauges and Their Roles
A few gauges cover most everyday wiring. Fourteen gauge is the classic lighting-circuit wire, twelve gauge handles general receptacles at 15 to 20 amps, and ten gauge is common for window air conditioners, dryers and long 30 amp runs. Eight gauge and larger move into the territory of subpanels, electric ranges and heavy feeders, where the resistance per thousand feet drops well under an ohm. Knowing which gauge you are using is the first step of the calculation, and the selector's resistance values make it easy to compare, for instance, whether stepping from 12 to 10 gauge brings a long 20 amp run back under the 3 percent target. When in doubt between two adjacent sizes, the heavier of the two is almost always the safer choice.
Assumptions to Remember
- Single-phase two-wire circuit, so the one-way length is doubled.
- Resistance values are NEC copper values at 75 °C; aluminum is 1.6× copper.
- Power factor and reactive losses are ignored.
- The drop percentage is relative to the source voltage.
- 3 percent for feeders and 5 percent for branch circuits are the practical limits used.
Voltage drop is the quiet tax every wire run pays, and the only way to know whether it is too high is to compute it. The Voltage Drop Calculator makes that computation instant, turning length, gauge, current and material into a clear percentage and a clear verdict, so the wire you install is the wire your load actually needs. Run the numbers before you pull the cable, and let the percentage be your guide to a circuit that works exactly as rated.
Disclaimer
Results are provided as estimates for informational purposes only and may be inaccurate. Always verify outcomes with a qualified professional before making financial or personal decisions based on these calculations.