Preliminary numbers for estimating and for checking a quote. The equipment's instructions and the local authority having jurisdiction come first. A licensed electrician sizes and installs the wire.

Maintained by BuildSolver · Updated October 17, 2026

Calculator · Electrical

Voltage drop calculator

A conductor that is big enough for the current can still lose too much voltage on a long run. A 16 A load at 120 V on 12 AWG copper over 100 ft drops 5.27 percent, and even 10 AWG drops 3.31 percent. It takes 8 AWG to come in under the 3 percent suggested in an Informational Note of the NEC. This page works the drop by the K method for single and three phase, finds the smallest size for a target, and the longest run for a size. For an air conditioner or heat pump circuit, the AC wire size calculator adds terminal ratings and derating to the same drop.

Pick single or three phase, the voltage, copper or aluminum, and enter the load current.
Enter a conductor size and the one-way length for the drop, a length and a target percent for the smallest size, or a size and a target for the longest run.
The result shows the volts and percent dropped, the voltage left at the load, and the Table 310.16 ampacity of the size so you can check it against the current.

Example result for the default inputs

6 AWG copper, single phase, 240 V, copper, 44 A, 160 ft one way.

Voltage drop

6.92 V

Percent of supply

2.88 %

Voltage at the load

233.1 V

Calculation details
Formula	2 x 12.9 x 44 A x 160 ft / 26,240 cmil = 6.92 V
3 percent guideline, Informational Note	Within 3 percent. The 3 percent figure is an Informational Note, not a requirement.
Table 310.16 ampacity of 6 AWG copper	55 A at 60°C (140°F), 65 A at 75°C (167°F), before corrections. Ampacity and the breaker are separate checks from the drop.

For estimation purposes only. Consult a licensed engineer for final designs.

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How the drop is worked

Voltage drop is the current times the resistance of the conductors between the panel and the load. The K method puts the resistance in a form you can work by hand from the conductor's circular mils, so the same formula covers every size in the table.

Single phase: VD = 2 x K x I x L / CMThe 2 counts the outgoing and the return conductor, so L is the one-way length. Mike Holt's example: No. 6 copper, 44 A, 160 ft at 240 V drops 6.92 V, 2.88 percent.
Three phase: VD = 1.732 x K x I x L / CMFor a balanced three-phase load the factor is the square root of 3, and VD is line to line. Holt's example: No. 1 aluminum, 100 A, 80 ft at 208 V drops 3.51 V, 1.69 percent.
K: 12.9 for copper, 21.2 for aluminumThe direct-current resistance in ohms of a conductor 1,000 circular mils in area and 1,000 ft long at 75°C (167°F). Holt notes it suits AC circuits up to 1/0. Larger conductors add self-induction that this page does not include.
CM: circular mils from NEC Chapter 9, Table 86,530 for 12 AWG, 10,380 for 10 AWG, 26,240 for 6 AWG, 83,690 for 1 AWG and 211,600 for 4/0.
Percent: VD / circuit voltage x 100Line to line for three phase. The Informational Note under NEC 210.19 suggests about 3 percent on a branch circuit and about 5 percent for feeder and branch circuit together. It is a recommendation, not a requirement. Some energy codes do make a limit binding, such as the 5 percent combined cap in California's Title 24, Part 6, section 130.5(c).

Eight circuits through the same formula

The first two rows are the worked examples in Mike Holt's EC&M article, and the page gives the same answers. The others show what one size up, a higher voltage or aluminum does to the drop.

Voltage drop in volts and percent for eight circuits
Circuit	Conductor	Drop, V	Drop, %
Holt example: 240 V, 44 A, 160 ft	6 AWG Cu	6.92	2.88
Holt example: 208 V three phase, 100 A, 80 ft	1 AWG Al	3.51	1.69
120 V, 16 A, 100 ft	12 AWG Cu	6.32	5.27
120 V, 16 A, 100 ft, one size up	10 AWG Cu	3.98	3.31
Same 120 V load at 240 V (8 A)	12 AWG Cu	3.16	1.32
277 V lighting, 16 A, 150 ft	12 AWG Cu	9.48	3.42
480 V three phase, 40 A, 300 ft	8 AWG Cu	16.24	3.38
Holt 240 V example in aluminum	4 AWG Al	7.15	2.98

Moving the same 120 V load to 240 V halves the current and doubles the voltage, so the percent falls to a quarter: 1.32 percent against 5.27 percent on the same 12 AWG. At a full 20 A on 120 V, 12 AWG copper stays within 3 percent up to about 45.5 ft one way, and 10 AWG up to about 72.4 ft.

Standards and data sources

Mike Holt, "Code Calculations", EC&M, February 1, 2000 - the single-phase and three-phase K formulas, the K values of 12.9 and 21.2 at 75°C (167°F), the limit of K to conductors up to 1/0, and the two worked examples this page reproduces.
NEC 2023, Chapter 9 Table 8 and the Informational Note under 210.19 - the circular mils of each conductor size, and the 3 percent and 5 percent voltage drop recommendation.
NEC 2023, Table 310.16 and 110.14(C) - the 60°C (140°F) and 75°C (167°F) ampacities shown next to each result, and the terminal temperature rule for using the 75°C (167°F) column.
California Energy Code, Title 24, Part 6, section 130.5(c), as summarized in the 2019 Nonresidential Compliance Manual, section 8.4 (Energy Code Ace) - a binding cap of 5 percent on the combined voltage drop of feeder and branch circuit conductors for the buildings that section covers, an example of an energy code making the limit a requirement.

Limitations of this calculator

This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
The drop is worked from the K constant at 75°C (167°F). A conductor running cooler has a little less resistance, so the real drop is a little lower. One running at 90°C (194°F) has more.
K holds for AC conductors up to 1/0. At 2/0 and larger the self-induction of the conductors adds to the drop, and this page leaves it out, so the real drop is somewhat higher than shown. Sizes stop at 4/0, and parallel conductor sets are not worked.
Three phase assumes a balanced load and gives the drop line to line. Single phase assumes the current flows out on one conductor and back on the other. A multiwire branch circuit with an unbalanced neutral, and the neutral current of a three-phase system, are not modeled.
The current is what you enter. Motor starting current is several times the running current and causes a brief dip that this page does not work. Power factor and conductor spacing are not modeled.
Ampacity, the breaker, the equipment grounding conductor and conduit fill are not sized here. The page shows the Table 310.16 values of each size before any correction, so a size picked for the drop can be checked against the current. Use the AC wire size calculator for terminal ratings, heat and conductor-count corrections.
The 3 percent and 5 percent figures are in Informational Notes of the NEC, under 210.19 for branch circuits and in Article 215 for feeders. They are a recommendation, not a requirement. Energy codes can set a binding limit: California's Title 24, Part 6, section 130.5(c) caps feeder plus branch circuit at 5 percent for the buildings it covers, and ASHRAE Standard 90.1 has its own requirement for commercial buildings. Check the energy code your jurisdiction adopted, and the equipment's rated voltage range.
The tables are the 2023 edition of the NEC. Your state or city may enforce another edition or amend it. Check the edition your AHJ uses.
Solar PV, DC systems and low-voltage wiring have rules of their own in other articles of the NEC and are not covered here.

Common mistakes with voltage drop

Entering the round-trip length. The 2 in the single-phase formula already counts the return conductor, so the length is one way. Doubling it again doubles the answer: Holt's 240 V example would read 13.84 V instead of 6.92 V.
Using the single-phase factor on a three-phase circuit. A balanced three-phase load uses 1.732, not 2. On Holt's 208 V example the factor 2 gives 4.05 V against the correct 3.51 V, about 15 percent high.
Calling 3 percent an NEC limit. The 3 percent and 5 percent figures are in an Informational Note under NEC 210.19. In the NEC they are a recommendation for reasonable operation. A binding limit can come from elsewhere: the equipment's rated voltage range, a local amendment, or an energy code. California's Title 24, Part 6, section 130.5(c) caps feeder plus branch circuit at 5 percent for the buildings it covers, and ASHRAE Standard 90.1, which many states adopt for commercial buildings, has a voltage drop requirement of its own.
Sizing for the drop and forgetting ampacity. A long run can pass the drop on a size that is too small for the current, and a short run can carry a current the size cannot. Check both. The result shows the Table 310.16 values next to each size for that reason.
Using the copper constant for aluminum. Aluminum's K is 21.2 against 12.9 for copper, about 64 percent higher. The 240 V example on 6 AWG aluminum drops 11.38 V, 4.74 percent, where copper drops 2.88 percent.
Taking the percent of the wrong voltage. The same load at a higher voltage draws less current and loses a smaller share. A 16 A load at 120 V on 12 AWG over 100 ft drops 5.27 percent. Rewired for 240 V it draws 8 A and drops 1.32 percent.

Common questions about voltage drop

How do you calculate voltage drop?

For single phase, VD = 2 x K x I x L / CM, where K is 12.9 for copper or 21.2 for aluminum, I is the current, L the one-way length in feet and CM the circular mils of the conductor from NEC Chapter 9 Table 8. For a balanced three-phase load, replace the 2 with 1.732. Mike Holt's example of No. 6 copper carrying 44 A for 160 ft at 240 V gives 6.92 V, which is 2.88 percent.

What is the maximum voltage drop allowed by the NEC?

The NEC does not set a maximum for ordinary branch circuits and feeders. An Informational Note under 210.19 suggests about 3 percent on a branch circuit and about 5 percent for the feeder and branch circuit together, for reasonable efficiency of operation. Informational Notes are not requirements. An energy code can be: California's Title 24, Part 6, section 130.5(c) caps feeder plus branch circuit at 5 percent for the buildings it covers, and ASHRAE Standard 90.1, adopted by many states for commercial buildings, has its own voltage drop requirement. The equipment's rated voltage range and its installation instructions also apply.

What size wire do I need for 100 feet at 120 volts?

It depends on the current. For a 16 A load at 120 V over 100 ft one way, 12 AWG copper drops 5.27 percent and 10 AWG drops 3.31 percent. The smallest copper size within 3 percent is 8 AWG. The conductor must also carry the current under Table 310.16, which is a separate check.

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

At a full 20 A on 120 V, 12 AWG copper stays within 3 percent up to about 45.5 ft one way, and 10 AWG up to about 72.4 ft. At a smaller load the distance grows in proportion. These are voltage drop figures only. The 3 percent figure is an NEC recommendation, not an NEC requirement.

Why is the three-phase formula different?

In a balanced three-phase circuit the current that leaves on one conductor returns on the other two, and the drop between two lines works out to the square root of 3 times the drop in one conductor. So the factor in front of K is 1.732 instead of the 2 used for single phase, and the result is the line-to-line drop.

Is the length one way or round trip?

One way, from the panel to the load. The single-phase formula multiplies by 2 to count the return conductor, and the three-phase formula uses 1.732. Entering the round-trip length doubles the result.

Does aluminum wire have more voltage drop than copper?

Yes, at the same size. Aluminum's K is 21.2 against 12.9 for copper. For the same drop, aluminum needs a larger size. Holt's 240 V example on 6 AWG gives 2.88 percent in copper and 4.74 percent in aluminum.

Does voltage drop change with the load?

Yes, in direct proportion. Half the current gives half the drop on the same conductor and length. That is why the page asks for the load current, and why entering the breaker rating instead gives a more cautious answer.

Does this replace an electrician?

No. This page is for estimating and for checking numbers on a quote. A licensed electrician sizes and installs the circuit under a permit, and the local authority having jurisdiction inspects it.