🔋 Voltage Drop in Conductors

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Related standards: NEC

Checking whether the voltage drop along a specific conductor run stays within NEC-recommended limits.

This is the verification counterpart to Cable Sizing: given a conductor you've already chosen (by its resistance-per-1000-ft rating), how much voltage does it actually drop over a specific run at a specific current, and is that within the commonly used 3% recommended limit?

Key formula

R = (r/1000) × L
V_drop = 2 × I × R
%Drop = (V_drop / V_system) × 100

Variables

r

conductor resistance per 1000 ft (from a wire table, temperature- and material-dependent)

L

one-way run length

How to use the Voltage Drop calculator

Use this to find how much voltage is lost along a run of wire, in volts and as a percentage of the supply, and whether it stays within the usual 3 percent guideline. It is the check to make for long runs: a feeder to a detached garage, a pump at the far end of a property, a run of outdoor lighting.

You supply the current, the one-way length of the run and the conductor resistance per 1,000 feet, which comes from a wire table. The calculator doubles the length internally for the out-and-back path, so enter the one-way distance, not the round trip.

Current (A)

The current the circuit carries when fully loaded, in amperes. Use the actual load current, or the circuit's rated maximum if you want the worst case.

Length (ft)

The one-way distance from the source to the load along the cable route, in feet. It is the route length, which is usually longer than a straight line.

Resistance (Ω/1000ft)

The conductor resistance in ohms per 1,000 feet, from the wire table for the size and material. Copper #8 AWG is about 0.78 Ω per 1,000 ft at working temperature; aluminum of the same size is roughly 1.6 times higher.

System Voltage (V)

The system voltage the percentage is measured against: 120 or 240 V for typical single-phase circuits.

Worked example: a feeder to a detached workshop

A 240 V single-phase feeder runs 150 ft from the house panel to a detached workshop and must carry 30 A. It is wired in copper #8 AWG, about 0.778 Ω per 1,000 ft. Is the voltage drop acceptable?

Worked by hand:

1. Resistance of one conductor. 0.778 Ω per 1,000 ft × 150 ft ÷ 1,000 = 0.1167 Ω.
2. Voltage lost in the out-and-back path. Current flows out on one conductor and returns on the other, so the drop is 2 × I × R = 2 × 30 × 0.1167 = 7.00 V.
3. As a share of the supply. 7.00 ÷ 240 = 2.92%.
4. Compare with the limit. 2.92% is just under 3%, so the calculator reports the run as within limits.

It passes, but with almost no margin: any increase in current, length or conductor temperature pushes it over. Stepping up to #6 AWG (about 0.491 Ω per 1,000 ft) would cut the drop to 4.4 V, or 1.84%, and leave room. Remember this is a voltage check only; whether the wire can carry 30 A without overheating is a separate ampacity question.

Reading the result: how much drop is too much

The common guidance is to keep drop to about 3 percent on a branch circuit and about 5 percent from the service to the far end of the load, including the feeder. These are recommendations that codes cite for reasonable performance, not hard limits on every installation, and equipment can be more sensitive than the guideline assumes.

• Loads respond differently. A resistive heater loses power with the square of the voltage, so a 3 percent sag costs it about 6 percent of its output; motors draw more current when the voltage sags and can overheat or fail to start.
• Low-voltage circuits suffer most. The same wire that drops 3 percent at 240 V drops 20 times as much in proportion at 12 V. A 50 ft run of #12 copper carrying 10 A at 12 V loses about 2 V, or 16.5 percent, which is why low-voltage lighting and solar circuits need thick wire or short runs. (This calculator accepts system voltages from 100 V up; work small DC circuits with the same formula by hand.)
• Three-phase is kinder: for the same conductor and current the drop is about 87 percent of the single-phase figure (√3 ÷ 2), and this calculator is the single-phase form.
• The remedies, in order of cost: shorten the run, use a larger conductor, raise the voltage, or reduce the current.

Notes & limitations

• The NEC's 3% figure is a recommendation for reasonable operation, not a hard code requirement for every circuit — but exceeding it is a strong signal to upsize the conductor.
• This formula assumes single-phase (factor of 2 for the round trip); use the √3-based three-phase form for three-phase circuits instead.

Common mistakes

• Entering the round-trip length. The out-and-back factor of two is already included, so entering twice the distance doubles the drop.
• Using the resistance at the wrong unit. The input is ohms per 1,000 feet; a table that lists ohms per foot or per kilometre needs converting first.
• Using cold-wire resistance when the conductor will run warm under load. Resistance rises with temperature, which is why the tabulated working-temperature figure is the honest choice.
• Treating a passing voltage drop as proof the cable is safe. Drop and ampacity are separate checks, and a thin wire can pass one comfortably while overheating on the other.
• Checking only the feeder. The drop in the branch circuit that follows it adds to the total, and the combined figure is what the equipment sees.
• Forgetting a starting surge. A motor draws several times its running current for a moment, and the voltage at the motor terminals dips accordingly.

Frequently asked questions