Voltage Drop Calculator
Voltage drop on a single-phase or three-phase run, copper or aluminum, 14 AWG to 500 kcmil, from the resistance values of NEC Chapter 9 Table 8 or from the K method. The result comes in volts and in percent, with the formula and the figures it used.
The circuit
From the source to the load.
Between the two conductors, such as 120 or 240.
You sized one run. Mirage measures them all from the drawings.
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How it is calculated
- Table 8 method. Voltage drop = multiplier x resistance (ohms per 1,000 ft) x one-way length (ft) x current (A) / 1,000, with the direct-current resistance of the chosen size at 75°C from NEC Chapter 9, Table 8.
- K method. Voltage drop = multiplier x K x current (A) x one-way length (ft) / area (circular mils), with K = 12.9 for copper and 21.2 for aluminum.
- Multiplier. 2 for a single-phase two-wire circuit, the square root of 3 (about 1.732) for a balanced three-phase circuit, whose drop is then line to line.
- Percent. Voltage drop / system voltage x 100, compared with the 3 and 5 percent of the NEC's informational note.
Worked example
A 120 V single-phase circuit in 12 AWG stranded copper, 72 ft one way, carrying 16 A. With Table 8: 2 x 1.98 x 72 x 16 / 1,000 = 4.56 V, 3.80% of 120 V. With K: 2 x 12.9 x 16 x 72 / 6,530 = 4.55 V. Both are above 3 percent; 10 AWG brings it to 2.86 V, 2.38%.
The Table 8 values used here
| Size | Circular mils | Copper, uncoated | Aluminum |
|---|---|---|---|
| 14 AWG, solid | 4,110 | 3.07 | 5.06 |
| 14 AWG, stranded | 4,110 | 3.14 | 5.17 |
| 12 AWG, solid | 6,530 | 1.93 | 3.18 |
| 12 AWG, stranded | 6,530 | 1.98 | 3.25 |
| 10 AWG, solid | 10,380 | 1.21 | 2 |
| 10 AWG, stranded | 10,380 | 1.24 | 2.04 |
| 8 AWG, solid | 16,510 | 0.764 | 1.26 |
| 8 AWG, stranded | 16,510 | 0.778 | 1.28 |
| 6 AWG | 26,240 | 0.491 | 0.808 |
| 4 AWG | 41,740 | 0.308 | 0.508 |
| 3 AWG | 52,620 | 0.245 | 0.403 |
| 2 AWG | 66,360 | 0.194 | 0.319 |
| 1 AWG | 83,690 | 0.154 | 0.253 |
| 1/0 AWG | 105,600 | 0.122 | 0.201 |
| 2/0 AWG | 133,100 | 0.0967 | 0.159 |
| 3/0 AWG | 167,800 | 0.0766 | 0.126 |
| 4/0 AWG | 211,600 | 0.0608 | 0.1 |
| 250 kcmil | 250,000 | 0.0515 | 0.0847 |
| 300 kcmil | 300,000 | 0.0429 | 0.0707 |
| 350 kcmil | 350,000 | 0.0367 | 0.0605 |
| 400 kcmil | 400,000 | 0.0321 | 0.0529 |
| 500 kcmil | 500,000 | 0.0258 | 0.0424 |
Limits and assumptions
- It ignores reactance and power factor and uses direct-current resistance. For AC runs with large conductors, or where the result is close to a limit, use the AC resistance and reactance of NEC Chapter 9, Table 9.
- Resistance is taken at 75°C, uncoated copper or aluminum, one conductor per phase, Class B stranding or solid as listed.
- Three-phase assumes a balanced load. Unbalanced loads and neutral current are not modelled.
- It does not size conductors: ampacity, terminal ratings, conduit fill and the equipment grounding conductor are separate checks.
- The 3 and 5 percent figures are an informational note in the NEC, not a requirement. A project specification can set a limit of its own.
Sources
Checked on . The Table 8 values above were transcribed from the table and checked against two published reproductions of it; NFPA's panel reports for the 2023 and 2026 editions show no revision to the table.
- NFPA 70, National Electrical Code (NFPA's code development page), Chapter 9, Table 8, Conductor Properties: direct-current resistance at 75°C per 1,000 ft, uncoated copper and aluminum, and the area of each size in circular mils; Note 2, the temperature correction
- NFPA, NEC Code-Making Panel 2, first draft report for the 2026 edition (public input No. 281): quotes the 2023 text of 210.19 and its informational note on 3 and 5 percent, with the reference to 215.2(A)(2), Informational Note No. 2, for feeders
- NFPA, National Electrical Code Style Manual, 2023, section 2.1.10.2: informational notes are not written in mandatory language and do not contain requirements
- IAEI Magazine, Voltage Drop Calculations (2019): VD = 2 x K x I x L / CM for single-phase, 1.732 in place of 2 for three-phase; K = 12.9 for copper and 21.2 for aluminum at 75°C, from Table 8
- EC&M, Chapter 9 of the NEC, part 4 (2021): Table 8 gives DC resistance at 75°C; Table 9 gives AC resistance and reactance for 600 V cables
This calculator is not engineering, legal or compliance advice and does not replace the design engineer's calculation or a review by the authority having jurisdiction.
Questions
What voltage drop does the NEC allow?
The 3 and 5 percent figures are an informational note, not a requirement. In the 2023 edition, the note under 210.19 says branch-circuit conductors sized to prevent a drop over 3 percent at the farthest outlet, with no more than 5 percent on feeders and branch circuits together, provide reasonable efficiency of operation, and points to an informational note at 215.2(A)(2) for feeders. The NEC's style manual says informational notes do not contain requirements. A project specification can still set a limit of its own.
What is the K factor in the voltage drop formula?
The resistance of a conductor one circular mil in area and one foot long: 12.9 ohms for copper and 21.2 ohms for aluminum at 75°C, both derived from NEC Chapter 9 Table 8. Single-phase drop is 2 x K x current x one-way length / circular mils.
Should I use Table 8 or the K method?
Table 8 takes each size's own resistance; the K method applies one resistivity to every size. They differ slightly: a 12 AWG copper circuit of 72 ft at 16 A drops 4.56 V with Table 8 and 4.55 V with K = 12.9. Either is an approximation that leaves out reactance.
Why 2 for single-phase and 1.732 for three-phase?
On a single-phase two-wire circuit the current goes out on one conductor and back on the other, so the drop is counted over twice the one-way length. On a balanced three-phase circuit the multiplier is the square root of 3, about 1.732, and the result is a line-to-line drop, to compare with the line-to-line voltage.
Does the calculator account for power factor and reactance?
No. It uses DC resistance at 75°C. On AC circuits, especially with large conductors, reactance and the load's power factor change the effective impedance. NEC Chapter 9 Table 9 lists AC resistance and reactance for 600 V cables for a closer calculation.
What length should I enter?
The one-way length of the run, from the source to the load, in feet. The multiplier in the formula already accounts for the return path.
Does conductor temperature change the result?
Yes. Table 8 is at 75°C, and Note 2 to the table corrects for another temperature: R2 = R1 x (1 + alpha x (T2 - 75)), with alpha 0.00323 for copper and 0.00330 for aluminum. A conductor running cooler has less resistance, so the drop shown here is on the high side for it.
The lengths come from the drawings
A voltage drop check is only as good as the run length behind it. Mirage deploys agents that take off device counts, conduit runs and home runs from the electrical set, every quantity traced to the sheet it came from, for your estimators to review. They feed your estimate; they do not check a design against the NEC.