Voltage Drop Calculator

Enter the conductor metal, AWG size, current, one-way run length and system voltage to get the voltage drop in both volts and percent — and the voltage actually delivered at the load. Use it to keep a circuit inside the common 3% guideline.

Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.

Calculator

Voltage drop7.90 V
Voltage drop6.58 %
Voltage at the load112.1 V

Formula

Single-phase: VD = (2 × K × I × L) / CM. Three-phase: VD = (1.732 × K × I × L) / CM. The percentage is VD ÷ Vsource × 100. K is the resistivity constant (copper 12.9, aluminum 21.2 circular-mil·Ω/ft at ~75°C), I is the current in amps, L is the one-way length in feet, and CM is the conductor area in circular mils. The factor of 2 accounts for the round trip (out and back) on a single-phase run.

Worked example

A 12 AWG copper conductor (6,530 circular mils) carrying 20 A over a 100 ft one-way single-phase run: VD = (2 × 12.9 × 20 × 100) / 6,530 = 51,600 / 6,530 = 7.90 V. On a 120 V circuit that is 7.90 / 120 = 6.6%, well over the 3% guideline, so the load sees only about 112.1 V — step up to 10 AWG (4.97 V, 4.1%) or 8 AWG to bring it in range.

Voltage drop is the quiet failure mode of a circuit: everything is wired correctly and the breaker never trips, yet a motor runs hot or lights dim because too much voltage was lost in the conductor before it reached the load. The figure that matters is the percentage, not the raw volts — four volts lost is trivial on a 240 V feeder but crippling on a 12 V battery run. Distance is the usual culprit, since drop rises directly with the one-way length, so long runs to a shed, a pump or a trailer are where you check this first. If the percentage lands above your 3% target, step up one gauge and recompute rather than living with the loss.

Reference table: Voltage Drop Chart per AWG (20 A, 100 ft, 120 V)

Frequently asked questions

How do I calculate voltage drop?
Multiply the resistivity constant K (12.9 for copper, 21.2 for aluminum) by the current and the one-way length, then by 2 for single-phase or 1.732 for three-phase, and divide by the conductor area in circular mils. Divide that result by the source voltage to get the percentage.
What is the 3% voltage drop rule?
The NEC recommends (in informational notes, not as a hard requirement) keeping voltage drop to 3% on a branch circuit or feeder, and 5% total across feeder plus branch. Excessive drop wastes energy and can cause motors, lights and electronics to misbehave.
How do I calculate voltage drop on a 12V circuit?
Use the same formula with 12 in the denominator for the percentage. Low-voltage DC runs carry high current for the same wattage, so the 3% target is reached at much shorter distances — that is why van and RV wiring needs surprisingly large conductors. See the dedicated 12V DC wire size tool.
Does length mean one-way or round-trip?
Enter the one-way distance from the source to the load. The factor of 2 in the single-phase formula already accounts for the return conductor, so you do not double the length yourself.
Why is aluminum voltage drop higher than copper?
Aluminum has a higher resistivity, so its K factor (21.2) is about 1.64 times copper's (12.9). For the same gauge and run, aluminum drops about 64% more voltage, which is why aluminum conductors are typically sized one or two gauges larger.

Source: NEC Chapter 9, Table 8 (conductor properties); resistivity constant K for copper and aluminum · All sources

Related calculators