Voltage Drop Calculator & Wire Size Check

About this tool

This voltage drop calculator works out how many volts a run of copper or aluminum wire loses between the panel and the load, as a voltage and as a percentage of the source. It also searches every size from 18 AWG to 500 kcmil and suggests the smallest wire that keeps the drop within a target, 3% by default. It handles single-phase, DC and three-phase circuits, with distance in feet or meters. The wire resistance is calculated from first principles rather than taken from a table, and everything runs in your browser.

How to use the voltage drop calculator

1. Enter the source voltage, such as 120, 240, 208 or 480, and choose single-phase/DC or three-phase.
2. Enter the load current in amps and the one-way distance from the source to the load. The tool adds the return path itself, so do not double the length.
3. Pick copper or aluminum and a wire size. Leave the power factor at 1 and the temperature at 75 °C unless you have a reason to change them.
4. Read the results. They update as you type. The suggested size is the smallest one that meets your target percentage, and Use suggested size switches the calculation to it. The table below the results shows every size at the same settings.

The voltage drop formula

Current flowing through a wire's resistance loses voltage by Ohm's law, V = I × R. A single-phase or DC circuit has two current-carrying conductors, out and back, so the drop covers twice the one-way length. In a balanced three-phase circuit, the line-to-line drop uses √3 in place of 2. These are the standard forms used in Mike Holt's voltage drop guide:

Single-phase / DC:  VD = 2  × L × I × R ÷ 1000
Three-phase:        VD = √3 × L × I × R ÷ 1000
Percent drop      = VD ÷ source voltage × 100

L = one-way length (ft), I = current (A),
R = resistance of one conductor (Ω per 1000 ft)

When you enter a power factor below 1, the tool multiplies R by it. That is the resistance half of the usual effective impedance, R cos θ + X sin θ. The reactance half (X) is ignored, so with no reactance a lower power factor gives a smaller drop. A power factor of 1 is the safer choice for small wires. For conductors larger than about 1/0, reactance becomes significant and this estimate will read low.

How the wire resistance is calculated

The American Wire Gauge defines 36 AWG as 0.0050 inch and 4/0 as 0.4600 inch, with 38 sizes in between on a geometric scale (NBS Handbook 100, Copper Wire Tables). That gives the diameter of any gauge n as d = 0.005 × 92^((36 − n) ÷ 39) inches, where 1/0 to 4/0 use n = 0 to −3. The area in circular mils is the diameter in thousandths of an inch, squared. kcmil sizes are already areas: 250 kcmil is 250,000 circular mils.

The same handbook gives the International Annealed Copper Standard resistivity as 1.7241 µΩ·cm, or 10.371 Ω·cmil/ft, at 20 °C, with a temperature coefficient of 0.00393 per °C. Aluminum conductor grade is modelled at 61% of copper's conductivity, about 17.00 Ω·cmil/ft at 20 °C, with a coefficient of 0.00403 per °C. Resistance rises with temperature as ρ(T) = ρ20 × (1 + α × (T − 20)), so at 75 °C copper is about 12.613 Ω·cmil/ft and aluminum about 20.77. For comparison, the constants commonly used in hand calculations for stranded wire are 12.9 for copper and 21.2 for aluminum (EC&M), about 2% higher than these solid-conductor figures. Expect real stranded wire to drop slightly more than this tool shows.

Worked example

The calculator loads with a 120 V single-phase circuit feeding a 16 A load at 100 feet, in 12 AWG copper at 75 °C:

12 AWG diameter  = 0.005 × 92^(24/39) = 0.0808 in  →  6,530 cmil
R (75 °C)        = 12.613 ÷ 6,530 × 1000 = 1.9315 Ω per 1000 ft
VD               = 2 × 100 × 16 × 1.9315 ÷ 1000 = 6.18 V
Percent drop     = 6.18 ÷ 120 × 100 = 5.15%
Voltage at load  = 120 − 6.18 = 113.82 V

That is above the 3% target. 10 AWG would still drop 3.24%, so the tool suggests 8 AWG, which drops 2.44 V (2.04%). The wires in the 12 AWG case turn about 98.9 W into heat (drop × current). For a three-phase case, a 30 A load at 150 feet on a 208 V system in 8 AWG copper drops 5.95 V, or 2.86%.

The 3% and 5% guideline

Informational notes in the National Electrical Code, attached to section 210.19 for branch circuits and 215.2 for feeders, suggest sizing conductors so that the drop is no more than 3% on a branch circuit or feeder and 5% for the feeder and branch circuit combined. They describe this as giving reasonable efficiency of operation. Informational notes are explanatory and not enforceable, as both Mike Holt's voltage drop guide and EC&M point out, so these figures are recommendations rather than code requirements. Equipment must still run within its rated voltage, and some installations and local rules set their own limits. The target box lets you use any percentage you need.

Common mistakes

• Entering the round-trip length. Use the one-way distance; the formula already accounts for the return conductor.
• Choosing a size by voltage drop alone. A wire that passes the 3% check can still be too small for the current. Ampacity, terminal temperature ratings and code rules are separate checks.
• Using breaker size when the real load is lower, or the reverse. Use the current the load will actually draw, or the design load if you are sizing for the full circuit.
• Forgetting that low-voltage circuits are far more sensitive. The same 6 V drop is 5% at 120 V but 50% at 12 V.
• Treating these results as exact for large conductors. Above about 1/0, AC reactance adds noticeably to the drop.

Use cases

• Checking a long run to a detached garage, shed, well pump or outdoor lighting before buying wire.
• Comparing copper and aluminum for a feeder, side by side in the size table.
• Estimating drop on 12 V or 24 V DC runs for solar, RV, marine or low-voltage lighting.
• Studying for electrical exams and checking hand calculations.

For the basic relationships between volts, amps, ohms and watts, use the Ohm's Law Calculator. To convert feet, meters or other lengths, the Unit Converter helps, and the Celsius to Fahrenheit converter translates conductor temperature ratings.