How to Size Wire for Long Runs Without Getting Burned by Voltage Drop
By the Super Simple Digital Tools Team · Updated June 2026
Voltage drop is the quiet failure mode of electrical work. A circuit can pass inspection for ampacity, power up fine on the bench, and still leave a pump stalling or a string of landscape lights fading toward the end of the run. The reason is distance: resistance accumulates over every foot of conductor, so the voltage at the far end is always lower than what the breaker delivers. A voltage drop calculator lets you see that loss as a number before it becomes a callback.
Start by gathering five facts: the conductor material, the wire size you intend to use, the one-way length of the run, the load current in amps, and whether the system is DC, single-phase, or three-phase. Current is where people slip up. For a continuous load, electricians often size for 125% of the rated amps, and for motors and pumps the brief startup surge can be several times the running current. Plug in the running amps first, then sanity-check the startup case, because a wire that is fine at steady state can still sag badly during inrush.
Read the result as a percentage, not just a raw voltage. A 120-volt branch circuit losing 4 volts is at 3.3%, already over the recommended branch-circuit guideline. The calculator turns the resistivity formula into that percentage instantly, so you can compare wire sizes in seconds. If the figure is comfortably under 3%, you have headroom. If it is close to or over the limit, step up one wire size and recalculate; each larger gauge roughly cuts the drop in proportion to the added cross-sectional area.
Material choice changes the math. Aluminum is cheaper and lighter, which makes it attractive on long feeders, but its higher resistivity means a given aluminum size drops more voltage than the same copper size. The practical workaround is to run aluminum one or two sizes larger than the copper you would otherwise use, and the calculator makes that trade-off easy to test side by side. For very long or very high-current runs, also weigh the energy wasted as heat, since that loss recurs every hour the circuit is loaded.
Finally, treat the output as a starting point, not a final design. The formula assumes a steady load at a typical conductor temperature and ignores power factor, harmonics, hot ambient conditions, and motor inrush, all of which push real drop higher. Use the number to choose a candidate wire size, then confirm it against ampacity tables, conduit fill, and your local code before buying cable. Done in that order, the calculator saves you from the expensive mistake of pulling an undersized run twice.
- Enter the one-way distance only; the tool applies the 2x (single-phase/DC) or root-3 (three-phase) factor for you.
- For motors and well pumps, check the calculation against startup surge current, not just running amps, to avoid hard starts and tripping.
- Aim under 3% on branch circuits and 5% feeder-plus-branch total, then upsize one wire gauge if you are near the limit.
- If switching to aluminum to save cost, run it one to two sizes larger than the equivalent copper to match the voltage drop.