Voltage Drop on Long Residential Runs, When to Upsize the Wire

Voltage drop is the calculation residential electricians skip most often and regret most predictably. The code makes it a recommendation rather than a hard requirement for most branch circuits and feeders, so it is easy to ignore – right up until a detached garage’s lights dim when the compressor kicks on, or a well pump runs hot and fails early. This is a practical guide to when a long run needs bigger wire.

The 3 percent guideline

The widely used target is 3 percent voltage drop on a branch circuit, 3 percent on a feeder, and no more than 5 percent total from the service to the farthest outlet. These figures appear as informational recommendations rather than mandatory limits in most residential branch-circuit cases, but they exist because equipment is designed to operate within a band around its nameplate voltage. Motors are the least forgiving: a motor fed low voltage draws more current to make the same power, runs hotter, and loses life. Lighting and electronics tolerate more, but flicker and premature failure show up at the edges.

Treat 3 percent as the line where you start paying attention, not as a number you have to hit on every short run. A 20-foot circuit will never come close. A 150-foot feeder to an outbuilding will blow past it on the minimum-sized conductor.

What drives the number

Voltage drop scales with three things: the length of the run, the current it carries, and the resistance of the conductor (which falls as the wire gets bigger). Double the distance and you double the drop. Double the load and you double the drop. Go up a wire size and the drop falls. That is the entire intuition – everything else is looking up the resistance per foot for the gauge and material and doing the arithmetic.

One nuance worth knowing: copper has lower resistance than aluminum at the same size, so an aluminum feeder needs to be a size or two larger than copper to hit the same drop. For long outbuilding feeders, aluminum is often the economical choice precisely because you are upsizing for voltage drop anyway, and the larger aluminum conductor costs less than the copper it replaces.

Worked example: a detached garage

Consider a 60 A subpanel feeding a detached garage 140 feet from the house. At the minimum copper conductor size for 60 A, a 140-foot run carrying a real working load can easily land in the 4 to 6 percent drop range – enough that the lights visibly dim when a table saw or compressor starts. Upsizing the feeder one or two trade sizes brings the drop back under 3 percent and makes the motor loads behave. The extra wire cost on a single feeder is modest; the alternative is a panel that browns out every time a tool starts.

Worked example: a well pump

Well pumps are the classic voltage-drop casualty because they combine a motor load with a long run – often a couple hundred feet from the pressure tank to the wellhead. A pump fed at low voltage draws extra current on every start, heats the windings, and fails years early. For pump circuits, voltage drop is not a comfort issue, it is a equipment-life issue, and upsizing the conductor is cheap insurance against an expensive pump replacement and a backhoe.

When upsizing is not the answer

Sometimes the better fix is to move the load closer to the source or to feed it at a higher voltage. A 240 V feeder carries the same power at half the current of a 120 V feeder, which cuts voltage drop dramatically – one reason long outbuilding feeders are run at 240 V and stepped down locally. Before reflexively jumping two wire sizes, check whether the circuit can be reconfigured. But for most residential outbuilding and pump situations, upsizing the conductor is the simplest correct answer.

Run the numbers before you pull

You do not want to discover a voltage-drop problem after the trench is backfilled. Our free voltage drop calculator takes the conductor size, material, length, load, and voltage and returns the drop percentage in seconds, so you can size the feeder correctly before you order wire. Pair it with the wire size calculator to confirm the conductor also satisfies ampacity, and you have both halves of the sizing problem covered.

Bottom line

Voltage drop matters on long runs and motor loads, and it is cheapest to fix before the wire is in the ground. Use 3 percent as your trigger to check, pay special attention to outbuilding feeders and well pumps, and remember that upsizing one or two sizes – or feeding at 240 V – usually solves it. The calculation takes thirty seconds; the callback to dig up an undersized feeder takes a day.

This article explains voltage drop in general terms. Always verify conductor sizing against the National Electrical Code edition adopted in your jurisdiction and the authority having jurisdiction.