Free NEC Calculator
Voltage Drop Calculator
Work out voltage drop, minimum wire size, or maximum one-way distance for single-phase and three-phase circuits, copper or aluminum. Every result is checked against the 3% branch circuit, 3% feeder, and 5% combined figures referenced in NEC 210.19 and 215.2. Free, no signup, and it runs entirely in your browser.
Circuit parameters
Results update as you type. This calculator covers voltage drop only — it is not an ampacity, termination-temperature, or overcurrent-protection check. Always verify conductor sizing against the full Code and your local amendments.
The voltage drop formula
This calculator uses the circular-mil (K) method, the approximation most electricians work with in the field. It treats the conductor as pure resistance and ignores reactance, which is accurate enough for ordinary building wiring and slightly conservative for most runs.
| Term | Meaning | Value used here |
|---|---|---|
| VD | Voltage dropped across the conductors | Volts |
| M | Phase multiplier | 2 single phase · 1.732 three phase (√3) |
| K | Resistivity constant, ohms per circular mil-foot | 12.9 copper · 21.2 aluminum |
| I | Load current | Amperes |
| L | One-way circuit length | Feet |
| PF | Power factor | 1.0 to 0.80 |
| CM | Conductor area in circular mils | NEC Chapter 9, Table 8 |
Why K is 12.9 and 21.2
K is the resistance of one circular mil-foot of the conductor material. The values above are the common AC figures at roughly 75 °C, the temperature most building conductors are assumed to be operating at. You will also see the DC values of 10.8 for copper and 17.4 for aluminum, taken at 20 °C. The DC values produce a smaller calculated drop, so the AC values are the conservative choice and are what this calculator and the Electrician Pro X app both use.
Single phase versus three phase
The multiplier M is where the two systems differ. A single-phase circuit carries current out on one conductor and back on the other, so the conductor length that drops voltage is twice the one-way distance: M = 2. A balanced three-phase load uses the line-to-line relationship instead, so M = √3 = 1.732. That is why a three-phase circuit shows about 87% of the drop a single-phase circuit would show at the same current, distance, and conductor size.
Circular mils, not square inches
Conductor area here is in circular mils, from NEC Chapter 9, Table 8. A 12 AWG conductor is 6,530 circular mils, 1/0 AWG is 105,600, and a 250 kcmil conductor is 250,000 — kcmil means thousands of circular mils, which is why the larger sizes read as round numbers. Because drop is inversely proportional to area, going up two AWG sizes roughly cuts the drop in half.
Where the NEC addresses voltage drop
This is the part that trips people up on inspections and in design reviews: for ordinary branch circuits and feeders, the familiar 3% and 5% numbers are recommendations, not requirements. They live in informational notes, and NEC 90.5 explains that informational notes are explanatory material and are not enforceable as Code requirements. There are, however, several places where voltage drop is mandatory language.
| Section | What it covers | Status |
|---|---|---|
| 90.5 | Explains that informational notes are explanatory and not enforceable | Rule about the Code itself |
| 210.19(A) | Branch-circuit conductor sizing, with the informational note that carries the 3% branch / 5% combined figures | Informational note |
| 215.2(A) | Feeder conductor sizing, with the informational note that carries the 3% feeder / 5% combined figures | Informational note |
| 250.122(B) | Equipment grounding conductors must be increased proportionally when ungrounded conductors are upsized | Mandatory |
| 310.14(A)(1) | Ampacity selection, with an informational note pointing back to 210.19 and 215.2 for voltage drop | Informational note |
| 647.4(D) | Sensitive electronic equipment on separately derived 120/240 V technical power systems | Mandatory |
| 695.7 | Fire pump motor circuit voltage drop at starting and at running load | Mandatory |
The full text of these sections is copyrighted by the NFPA and is not reproduced here. Look them up in your copy of the Code, or read NFPA 70 free online at nfpa.org/free-access.
How the 3% / 3% / 5% split works
The two notes are read together. A feeder is recommended to stay within 3%, a branch circuit within 3%, and the two of them combined within 5% measured from the service to the furthest outlet. So you cannot spend 3% on the feeder and another 3% on the branch circuit and call it compliant with the recommendation — that is 6%. In practice most designers budget roughly 2% for the feeder and 3% for the branch circuit, or the reverse, and keep the total at or under 5%.
The checks in the calculator above evaluate the single run you entered against each of those three thresholds independently. If you are checking a feeder and a branch circuit, run each one separately and add the percentages to see the combined figure.
What else the Code does require
Voltage drop is only one of the checks a conductor has to pass, and it is never the one that makes a conductor legal on its own:
- Ampacity under NEC 310.16, with the ambient temperature correction and conductor bundling adjustment factors that apply to the installation.
- Termination temperature rating under NEC 110.14(C), which commonly limits you to the 60 °C or 75 °C column regardless of the insulation rating.
- Overcurrent protection under NEC 240.4, including the small-conductor rules in 240.4(D).
- Equipment grounding conductor size under NEC 250.122, including the proportional increase in 250.122(B) whenever you upsize for voltage drop.
Size for ampacity and terminations first, then run the voltage drop check, then upsize if the drop is too high — and remember to grow the equipment grounding conductor with it.
Worked example
A 20 A, 120 V single-phase branch circuit feeds a receptacle in a detached shop. The run is 100 feet one way in 12 AWG copper, at unity power factor. Does it meet the 3% recommendation?
M = 2 (single phase) · K = 12.9 (copper, AC) · I = 20 A · L = 100 ft · PF = 1.0 · CM = 6,530 (12 AWG, Chapter 9 Table 8)
VD = (2 × 12.9 × 20 × 100 × 1.0) ÷ 6,530
VD = 51,600 ÷ 6,530 = 7.90 volts
% drop = (7.90 ÷ 120) × 100 = 6.58%
Voltage at the load = 120 − 7.90 = 112.10 volts
At 6.58% this run is more than double the 3% branch-circuit recommendation and above the 5% combined figure as well. The load would see roughly 112 V instead of 120 V — enough to make motors run hot, dim incandescent lighting noticeably, and shorten the life of anything with a power supply in it.
Fixing it
Switch the calculator to Minimum wire size with a 3% target and it solves the formula backwards for the required area:
Target VD = 120 × 0.03 = 3.60 V
CM = (2 × 12.9 × 20 × 100 × 1.0) ÷ 3.60 = 14,333 circular mils
The smallest listed conductor that clears 14,333 circular mils is 8 AWG at 16,510 — 10 AWG only provides 10,380, which would still leave you at 4.14%. Re-running the drop at 8 AWG gives 3.13 volts, or 2.60%, with 116.87 V at the load. That passes.
Two things to carry forward. First, the circuit is still a 20 A circuit protected at 20 A — upsizing the conductor for voltage drop does not change the overcurrent device or the receptacle rating. Second, because the ungrounded conductors went from 6,530 to 16,510 circular mils, NEC 250.122(B) requires the equipment grounding conductor to be increased by that same ratio.
Frequently asked questions
Is 3% voltage drop a code requirement?
For ordinary branch circuits and feeders, no. The 3% figure appears in informational notes to NEC 210.19 and 215.2, and NEC 90.5 states that informational notes are explanatory and not enforceable as requirements. It is a design recommendation that most inspectors, engineers, and specifications treat as the working target.
Voltage drop is mandatory in specific places, including NEC 647.4 for sensitive electronic equipment and NEC 695.7 for fire pumps. A project specification, a local amendment, or an energy code can also make a voltage drop limit binding on your job even where the NEC does not.
Do I use one-way distance or total circuit length?
One-way distance, measured from the source to the load along the actual conductor route — not the straight-line distance between them. The formula already accounts for the return path: the multiplier of 2 for single phase covers the run out and back, and the 1.732 factor for three phase is the line-to-line √3 factor. If you enter the round-trip length you will roughly double the answer.
What K value should I use for copper and aluminum?
This calculator uses K = 12.9 for copper and K = 21.2 for aluminum, the common AC values at about 75 °C. Some references use the DC values of 10.8 and 17.4, which are based on resistance at 20 °C and give a smaller drop.
The AC values are the conservative choice for normal building wiring at operating temperature. For a long, heavily loaded run where the difference matters, calculate from the actual AC resistance and reactance values in NEC Chapter 9, Table 9 rather than the K approximation.
Why is my three-phase drop lower than single phase at the same amps?
Because the multiplier is different. Single phase uses 2, since current travels out on one conductor and back on the other. Three phase uses 1.732, the √3 factor for the line-to-line drop of a balanced load. At the same current, distance, and conductor, a three-phase drop is about 87% of the single-phase drop — and the percentage is smaller again, because three-phase system voltages are usually higher.
If I upsize conductors for voltage drop, do I have to upsize the ground?
Yes. NEC 250.122(B) requires equipment grounding conductors to be increased in size proportionally to the increase in circular mil area of the ungrounded conductors. If you go from 12 AWG to 8 AWG to fix a voltage drop problem, the equipment grounding conductor has to grow by the same circular mil ratio.
The overcurrent device does not change, and the conductor terminations under NEC 110.14(C) still govern the rest of the sizing.
Does this calculator replace an ampacity check?
No. Voltage drop and ampacity are two separate checks, and the conductor has to pass both. Size the conductor first for ampacity using NEC 310.16 with whatever correction and adjustment factors apply, and for terminations under NEC 110.14(C). Then run the voltage drop check and upsize if the drop is too high.
The minimum size reported above is the minimum for voltage drop only. It can easily come out smaller than the size ampacity requires, and in that case ampacity wins.
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