Voltage Drop Calculator — Volts, Percent and the 3 % Length Limit

Background.

An outlet in a detached workshop reads 120 V on a meter with nothing plugged in, then 113.7 V once a heater is running. Nothing is faulty. The cable feeding it has resistance, the current flowing through that resistance uses up voltage on the way, and at 16 A over 100 feet of 12 AWG copper the loss comes to 6.336 V, or 5.28% of the supply. The same run turns just over 100 watts into heat inside the walls.

Voltage drop is the reason long runs get heavier wire than the current alone would require. The calculator takes the conductor size and metal, the load current, the one-way length, the system voltage and whether the circuit is single-phase, DC or three-phase, and returns the drop in volts and as a percentage, the voltage left at the load, the heat lost in the conductors and the one-way length at which this conductor and current would reach a 3% drop. The resistances are the DC values at 75 °C from NEC Chapter 9 Table 8, read from two independently published copies of the table that agree on every shared size.

The 3% and 5% figures that the result is compared against are often quoted as rules. In the NEC they are informational notes: 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 in the 2023 edition suggest about 3% for a branch circuit and 5% for feeder and branch together, and NFPA 70 §90.5 says informational notes are not enforceable as requirements. Designers aim for them anyway, and specifications or local amendments can make them binding, so check what your jurisdiction has adopted.

A voltage-drop figure is one check among several and does not choose a wire size. Ampacity decides the minimum conductor first, with corrections for ambient temperature, bundled conductors and continuous loads, and voltage drop can only push that choice larger. The arithmetic also uses DC resistance at unity power factor, which ignores the reactance that matters on large conductors. A licensed electrician or engineer must approve any real installation.

What is voltage drop calculator?

Voltage drop is the part of the supply voltage that is used up across the conductors instead of reaching the equipment. It is Ohm's law applied to the wire: the conductor has resistance R, the load current I flows through it, and I × R volts appear along the cable. On a single-phase or DC circuit the current goes out on one conductor and back on another, so both count and the drop is 2 × I × R, with R the resistance of one conductor over the run. On a balanced three-phase circuit the line-to-line drop is √3 × I × R, about 87% of the single-phase figure for the same wire, current and length. Quoting the drop as a percentage of the source voltage is what lets one guideline serve every system: 3% is 3.6 V on a 120 V circuit and 14.4 V on a 480 V one. The effects of too much drop depend on the load. Heaters and filament lamps simply give out less, motors lose torque and run hotter, and electronic power supplies draw more current as their input falls.

How to use this calculator.

1. 01Choose single-phase or DC for any two-conductor circuit, including 12 V and 24 V battery systems, or three-phase for a balanced three-phase load. Picking the wrong one changes the answer by about 15%.
2. 02Choose copper or aluminium and the conductor size from 14 AWG to 500 kcmil. Sizes 3, 1 and 3/0 AWG are not offered; use the next size either side to bracket them.
3. 03Enter the current the load actually draws, from its nameplate or a clamp meter, not the breaker rating. If you only know the breaker, entering it shows the worst case the circuit can carry without tripping.
4. 04Enter the one-way length from the source to the load, measured along the route the cable takes through walls, joists or conduit. Do not double it; the return conductor is already counted.
5. 05Enter the nominal source voltage, such as 120, 240, 208 or 480 V, using the line-to-line value for three-phase.
6. 06Read the percentage against the 3% and 5% guidance, then check the 3% length to see how much longer the run could be, or how much shorter it needs to be.

The formula.

V_drop = k × I × R_kft × L ⁄ 1,000, and drop % = V_drop ⁄ V_source × 100

k is 2 for single-phase and DC and √3 ≈ 1.732 for three-phase line-to-line. R_kft is the conductor's resistance in ohms per 1,000 feet from NEC Chapter 9 Table 8, L is the one-way length in feet and I the load current in amps. The table values are DC resistance at 75 °C, the temperature a conductor with 75 °C insulation reaches when carrying its full rated current. Where the table lists both solid and stranded copper, for 14 to 8 AWG, the stranded figure is used: it is slightly higher, 1.98 against 1.93 Ω per 1,000 ft for 12 AWG, so the drop comes out a little larger.

The heat lost in the wiring is figured separately, as I² × R for each conductor that carries current: two on a single-phase run, three on a three-phase one. On single-phase that equals the drop times the current. On three-phase it does not, because the drop carries a √3 while the heat sums over three conductors, so multiplying the line-to-line drop by the current would understate the heat by a factor of √3.

The 3% length turns the equation around: L = 0.03 × V_source × 1,000 ⁄ (k × R_kft × I). Every term in the drop is linear, so doubling the current or the length doubles the volts lost, while doubling the area of the conductor roughly halves them. The supply voltage works differently. For the same current, a higher voltage leaves the volts lost unchanged and shrinks the percentage. For the same load power, the current also falls, so doubling the voltage cuts the percentage to a quarter.

Three simplifications are built in. Power factor is taken as 1. Inductive reactance, which NEC Chapter 9 Table 9 covers, is left out; it is negligible on small branch-circuit conductors but can make the AC drop on large conductors, particularly in steel conduit, noticeably higher than this figure. And the 75 °C resistance overstates the drop for a lightly loaded cable running cooler. Any run whose drop would equal or exceed the supply voltage is refused, since nothing would reach the load.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NFPA 70, National Electrical Code, Chapter 9, Table 8 'Conductor Properties' — the source of every resistance and area used on this page. The table gives DC resistance at 75 °C (167 °F) in ohms per 1000 ft for uncoated copper (solid and stranded) and for stranded aluminium, together with each size's circular-mil area. The values were taken from this published copy of the 2014 edition of the table: 14 AWG 4,110 cmil / 3.14 Ω-kFT Cu stranded / 5.06 Al; 12 AWG 6,530 / 1.98 / 3.18; 10 AWG 10,380 / 1.24 / 2.00; 8 AWG 16,510 / 0.778 / 1.26; 6 AWG 26,240 / 0.491 / 0.808; 4 AWG 41,740 / 0.308 / 0.508; 2 AWG 66,360 / 0.194 / 0.319; 1/0 105,600 / 0.122 / 0.201; 2/0 133,100 / 0.0967 / 0.159; 4/0 211,600 / 0.0608 / 0.100; 250 kcmil 0.0515 / 0.0847; 350 kcmil 0.0367 / 0.0605; 500 kcmil 0.0258 / 0.0424. The edition retrieved is stated because NFPA 70 itself is copyrighted; confirm against the edition your jurisdiction has adopted. Retrieved 2026-07-29.
2. [2]Voltage Lab, 'NEC Chapter 9 Table 8 Explained: Conductor Properties Guide' — an independent second reproduction of the same table, using 'NEC 2023 as the reference baseline' and advising readers to 'verify the final conductor values against the official NEC edition adopted in your jurisdiction'. Consulted specifically to cross-check the numbers taken from the first source rather than to supply them. It agrees on every value shared between the two: 14 AWG 4,110 cmil / 3.14 Ω-kFT, 12 AWG 6,530 / 1.98, 10 AWG 10,380 / 1.24, 8 AWG 16,510 / 0.778, 6 AWG 26,240 / 0.491, 4 AWG 41,740 / 0.308, 2 AWG 66,360 / 0.194, 1/0 105,600 / 0.122, 4/0 211,600 / 0.0608, 250 kcmil 250,000 / 0.0515, 500 kcmil 500,000 / 0.0258 — eleven sizes, from a copy referenced to a different NEC edition, with no disagreement. Retrieved 2026-07-29.
3. [3]OrbitalJump, 'Voltage Drop Limits in NEC 210.19: Recommendation vs. Requirement and When It Controls Wire Size' — source of the informational-note wording and section numbering used on this page, quoting NEC 210.19(A) Informational Note No. 4 (2023 edition) on conductors 'sized to prevent a voltage drop exceeding 3 percent at the farthest outlet … and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent', the parallel language in 215.2(A)(1) Informational Note No. 2 for feeders, and NFPA 70 §90.5: 'Informational notes are informational only and are not enforceable as requirements.' Used as a secondary source because the primary text is behind NFPA's registration wall. Retrieved 2026-07-29.
4. [4]NFPA 70, National Electrical Code — the authoritative text for Chapter 9 Table 8, 210.19(A), 215.2(A)(1) and §90.5. NFPA publishes free read-only access to its codes behind account registration. This document was NOT opened during the preparation of this page: every code figure quoted here is attributed to the two published reproductions cited above and to the secondary source for the informational notes, and the edition each of those referenced is stated rather than implied. Readers designing a real installation should read the adopted edition directly, together with any local amendments. Access: gated (registration required). Reference date 2026-07-29.
5. [5]OpenStax (Rice University), University Physics Volume 2, §15.4 'Power in an AC Circuit'. Source of the conductor-heating relation used for the wasted-power output: Equation 15.13 gives the average power in a resistance as 'P_ave = ½I_0V_0 = I_rms·V_rms = I_rms²R'. This page applies the I²R form to each current-carrying conductor separately — two on a single-phase run, three on a three-phase one — rather than multiplying the line-to-line drop by the current, which would be wrong for three-phase by a factor of √3. Retrieved 2026-07-29.