Voltage Drop Calculator (NEC) for Copper & Aluminum

Enter the load in amps, the one-way length of the run, the voltage, the phase and the conductor. The calculator returns the volts lost in the wire, the percent drop, the voltage left at the load, and the smallest wire size that keeps the drop at or under your target (3% by default).

How to use the calculator

Pick copper or aluminum. Aluminum has about 64% more resistance for the same size, so it needs a larger conductor for the same drop.
Enter the load current in amps. Use the actual load you expect to run, not the breaker size. If you are sizing a feeder, use the calculated load from your load calculation.
Enter the one-way distance in feet. Measure from the panel to the load along the actual cable route, not in a straight line. The calculator already accounts for the current going out and back.
Choose the system voltage and phase. 120 V and 240 V single-phase cover most residential work. Use three-phase for 208 V, 240 V or 480 V commercial circuits.
Set your target. 3% is the common design target for a branch circuit; 5% is the common target for feeder plus branch circuit combined.

The result also shows the minimum conductor that meets the NEC ampacity for the load. You need the larger of the two: the size required for ampacity and the size required for voltage drop.

The voltage drop formula

For a single-phase circuit:

VD = (2 × K × I × L) ÷ CM

For a three-phase circuit:

VD = (1.732 × K × I × L) ÷ CM

VD is the voltage drop in volts.
K is the resistivity constant of the conductor: about 12.9 ohm-cmil/ft for copper and 21.2 for aluminum at typical operating temperature.
I is the load current in amps.
L is the one-way length in feet.
CM is the conductor area in circular mils, from NEC Chapter 9, Table 8 (for example, 12 AWG is 6,530 cmil and 10 AWG is 10,380 cmil).

The "2" in the single-phase formula is there because current travels to the load and back. In three-phase circuits the return current is shared between phases, so the multiplier is √3 (1.732).

Percent drop is VD divided by the source voltage, times 100. For a deeper explanation, including how to solve the formula for the wire size instead of the drop, see how to calculate voltage drop.

Worked example: a 20 A circuit, 100 feet away

A homeowner wants a 20 A, 120 V circuit to a detached shed 100 feet from the panel, and the load is a table saw that draws close to 20 A.

With 12 AWG copper: VD = (2 × 12.9 × 20 × 100) ÷ 6,530 = 7.90 V, which is 6.6% of 120 V. That is more than double the 3% target. The saw would see about 112 V while running at 20 A, and much less during startup, when it draws several times its running current.
With 10 AWG copper: VD = (2 × 12.9 × 20 × 100) ÷ 10,380 = 4.97 V, or 4.1%. Better, but still over 3%.
With 8 AWG copper: VD = (2 × 12.9 × 20 × 100) ÷ 16,510 = 3.13 V, or 2.6%. This meets the 3% target.

12 AWG is legal for the ampacity of a 20 A circuit, but at 100 feet it is the wrong wire for this load. This is the most common voltage drop mistake on long residential runs: the wire passes the ampacity check and fails on performance.

Voltage drop sizing tables

These tables show the smallest copper conductor that keeps a single-phase circuit at or under 3%, with the resulting percent drop in brackets. They start from the minimum size required for ampacity and then step up until the drop is within 3%.

120 V circuits (copper, starting from the 60°C ampacity column):

Load	50 ft	75 ft	100 ft	150 ft
15 A	12 AWG (2.5%)	10 AWG (2.3%)	8 AWG (2.0%)	8 AWG (2.9%)
20 A	10 AWG (2.1%)	8 AWG (2.0%)	8 AWG (2.6%)	6 AWG (2.5%)

240 V circuits (copper, starting from the 75°C ampacity column):

Load	50 ft	100 ft	150 ft	200 ft
20 A	12 AWG (1.6%)	10 AWG (2.1%)	8 AWG (2.0%)	8 AWG (2.6%)
30 A	10 AWG (1.6%)	8 AWG (2.0%)	8 AWG (2.9%)	6 AWG (2.5%)
40 A	8 AWG (1.3%)	8 AWG (2.6%)	6 AWG (2.5%)	4 AWG (2.1%)
50 A	8 AWG (1.6%)	6 AWG (2.0%)	4 AWG (1.9%)	4 AWG (2.6%)

The tables assume the load equals the circuit rating and use K = 12.9. Your actual drop is lower if the load runs below the breaker rating. Always confirm the terminations, insulation type and conditions of use for the ampacity side.

Is 3% voltage drop an NEC requirement?

For most circuits, no. The 3% and 5% figures come from informational notes in the NEC (at 210.19 for branch circuits and 215.2 for feeders). Informational notes explain good practice but are not enforceable requirements. Some specific cases do carry mandatory limits, such as sensitive electronic equipment under Article 647 and fire pumps under Article 695. Many engineers' specifications and some local amendments also make the limits mandatory, so check the project documents.

Even where it is not required, designing to 3% protects motors from overheating at low voltage, keeps LED drivers and electronics inside their input range, and avoids customer callbacks.

Common mistakes

Doubling the distance. The formula already doubles the length for single-phase. Enter the one-way distance.
Using the breaker size when the load is much smaller. Voltage drop depends on actual current. A 50 A EV charger circuit set to 40 A continuous should be calculated at 40 A.
Forgetting upstream drop. A branch circuit fed from a subpanel adds its drop to the feeder's drop. Keep the total under about 5%.
Mixing up aluminum and copper constants. Using the copper K-factor for an aluminum feeder understates the drop by roughly 40%.
Ignoring startup current. Motors draw several times their running current when starting. A circuit that is fine at running current can still sag enough to cause trouble at startup.

Frequently asked questions

What is an acceptable voltage drop?

The NEC informational notes recommend no more than 3% on a branch circuit and 5% total for the feeder and branch circuit together. On a 120 V circuit, 3% is 3.6 V. On a 240 V circuit it is 7.2 V.

How far can you run 12 gauge wire on a 20 amp circuit?

At a full 20 A load on 120 V, 12 AWG copper reaches 3% drop at about 45 feet (one way). At lighter loads it can go farther. For a 20 A circuit at 100 feet, plan on 8 AWG copper to stay under 3% at full load.

Does voltage drop depend on the breaker size?

No. It depends on the current actually flowing, the conductor length, the conductor material and its size. The breaker determines the minimum wire size for ampacity, not the voltage drop.

Does aluminum wire have more voltage drop than copper?

Yes. For the same size, aluminum has roughly 1.6 times the resistance of copper, so the drop is about 60% higher. That is why aluminum feeders are usually upsized by one or two sizes.

Why does my calculator give a different answer than another one?

Calculators use slightly different K-factors (some use DC resistance from Chapter 9 Table 8 at 75°C, others use AC impedance from Table 9, which accounts for power factor and raceway type). Differences of 5–10% are normal. For large commercial feeders, use the Table 9 impedance method.

Need to price the job, not just size the wire? SparkQuote reads your plans, counts devices and runs, and builds a priced bid with material and labor. See how electrical estimating works in SparkQuote.

Formulas and table values Last updated: October 6, 2026. Based on the 2023 NEC; check which edition your jurisdiction enforces. Electrical work should be done by qualified people.

Voltage Drop Calculator