Voltage Drop: the Formula, the 3% Rule, and Worked Examples
NEC 210.19 IN · Voltage drop · Updated 2026-08-07
Voltage drop is the voltage you lose between the panel and the load because every foot of conductor has resistance. Run a circuit too far on wire that is too small and lights dim, motors run hot, and electronics brown out. The NEC does not set a hard limit for most installations, but an informational note in NEC 210.19 recommends keeping branch-circuit voltage drop at or below 3 percent.
This guide gives you the single-phase formula, the constants for copper and aluminum, two fully worked examples with verified arithmetic, and maximum-run charts for 12, 10, and 8 AWG copper at 120 V and 240 V, plus the mistakes that cause most callbacks.
What voltage drop is and why it matters
Every conductor is a resistor. Push current through it and Ohm's law takes its cut: some of the source voltage is spent heating the wire instead of powering the load. The longer the run and the smaller the conductor, the more you lose. A motor rated for 115 V that only sees 108 V draws more current, runs hotter, and dies younger. LED drivers flicker, compressors struggle to start, and receptacles at the far end of a long circuit read low under load even though they read fine with nothing plugged in.
The nominal voltage matters more than most people expect. At 120 V, a 3 percent budget is only 3.6 volts. At 240 V, the same 3 percent gives you 7.2 volts to spend. Double the voltage also halves the current for the same wattage, so a 240 V circuit can run roughly four times as far as a 120 V circuit on the same wire while staying at the same percent drop. This is why long runs to outbuildings, pumps, and gate operators are usually designed at 240 V when the equipment allows it.
The 3 percent recommendation in NEC 210.19
The 3 percent figure comes from an informational note to NEC 210.19, not from mandatory code text. It recommends limiting voltage drop on a branch circuit to 3 percent, with the combined drop across feeder plus branch circuit held to about 5 percent, for reasonable efficiency of operation. Because it is informational, most inspectors cannot fail a residential installation on voltage drop alone. But do not treat that as permission to ignore it: many jurisdictions, energy codes, utility programs, and project specifications convert 3 percent into a hard requirement, and some specific NEC applications do impose binding voltage drop limits. The local AHJ and the actual adopted code book always govern.
The single-phase voltage drop formula
For single-phase circuits, the field-standard formula is VD = (2 x K x I x L) / CM. It approximates the round-trip resistive drop and is accurate enough for branch-circuit sizing work. Here is what each variable means:
- VD - volts dropped across the circuit conductors, out and back.
- K - the material constant: approximately 12.9 for copper and 21.2 for aluminum. It represents the resistance of a conductor one circular mil in cross section and one foot long.
- I - the load current in amps. Use the actual load, not the breaker rating.
- L - the one-way length in feet from the source to the load. The 2 in the formula already handles the return path.
- CM - the conductor's circular-mil area: 6,530 for 12 AWG, 10,380 for 10 AWG, 16,510 for 8 AWG.
To get percent drop, divide VD by the nominal voltage and multiply by 100. For three-phase circuits, the 2 is replaced by 1.732 (the square root of 3) because the phases share the return, but everything in this guide works the single-phase case, which covers almost all branch-circuit questions.
| Value | Number | Where it goes |
|---|---|---|
| K, copper | 12.9 | Numerator, material constant |
| K, aluminum | 21.2 | Numerator, material constant |
| 12 AWG | 6,530 CM | Denominator |
| 10 AWG | 10,380 CM | Denominator |
| 8 AWG | 16,510 CM | Denominator |
| 3% of 120 V | 3.6 V | Allowable drop, 120 V branch |
| 3% of 240 V | 7.2 V | Allowable drop, 240 V branch |
Worked example 1: 12 AWG at 85 feet on a 120 V circuit
- List the knowns: A 20 A receptacle circuit with an actual measured load of 16 A, an 85 ft one-way run, 12 AWG copper THHN (6,530 CM), 120 V nominal. For how that circuit gets loaded in the first place, see how many outlets fit on a 20 amp circuit.
- Run the formula: VD = (2 x 12.9 x 16 x 85) / 6,530 = 35,088 / 6,530 = 5.37 V.
- Convert to percent: 5.37 / 120 = 4.5 percent. That blows past the 3 percent recommendation, which allows only 3.6 V on a 120 V circuit. This run fails.
- Upsize and recheck: Move to 10 AWG (10,380 CM): VD = 35,088 / 10,380 = 3.38 V, which is 2.8 percent. It passes. The breaker stays at 20 A - upsizing for distance never changes the overcurrent device.
Worked example 2: a 240 V pump at 250 feet
A well pump draws 12 A at 240 V, and the pump house is 250 ft from the panel. Try 10 AWG copper first: VD = (2 x 12.9 x 12 x 250) / 10,380 = 77,400 / 10,380 = 7.46 V. That is 7.46 / 240 = 3.1 percent - just over the line. Close calls like this are exactly where you want the arithmetic, not a guess.
Step up to 8 AWG (16,510 CM): VD = 77,400 / 16,510 = 4.69 V, which is 2.0 percent. That passes with margin, which matters for motor loads because starting current is far higher than running current and sags hardest on marginal conductors.
For comparison, the same 250 ft run in 10 AWG aluminum (K = 21.2) drops (2 x 21.2 x 12 x 250) / 10,380 = 127,200 / 10,380 = 12.25 V, or 5.1 percent. Aluminum needs about 64 percent more circular mils than copper for the same drop (21.2 / 12.9 = 1.64), so aluminum runs almost always land one or two sizes larger.
Maximum one-way run at 3 percent: copper charts
These charts rearrange the formula to solve for distance: L = (allowable VD x CM) / (2 x K x I). All values use K = 12.9 for copper and are rounded down to the nearest foot so the chart never flatters a run. Enter with the actual load current, then read the longest one-way distance that still meets 3 percent.
| Copper size | 12 A load | 16 A load | 20 A load |
|---|---|---|---|
| 12 AWG | 75 ft | 56 ft | 45 ft |
| 10 AWG | 120 ft | 90 ft | 72 ft |
| 8 AWG | 191 ft | 143 ft | 115 ft |
| Copper size | 12 A load | 16 A load | 20 A load |
|---|---|---|---|
| 12 AWG | 151 ft | 113 ft | 91 ft |
| 10 AWG | 241 ft | 181 ft | 144 ft |
| 8 AWG | 383 ft | 287 ft | 230 ft |
Notice how fast 120 V circuits run out of room: a fully loaded 12 AWG circuit is out of budget past 45 ft one-way, and even at a more typical 16 A it clears only 56 ft. Many electricians plan around the 16 A column for 20 A circuits to leave headroom for continuous loading. Pair these distances with ampacity from the wire size chart - voltage drop and ampacity are separate checks and the conductor must pass both.
Flip the formula to solve for wire size
When the distance is fixed, solve for the circular mils you need instead: CM = (2 x K x I x L) / allowable VD. Say you need 200 ft one-way at 16 A on 120 V. CM = (2 x 12.9 x 16 x 200) / 3.6 = 82,560 / 3.6 = 22,933 CM. That is bigger than 8 AWG at 16,510 CM, so this run needs a conductor larger than 8 AWG - check the wire size chart for the next sizes up, and remember the terminations at the device may need pigtails when the wire outgrows the lugs.
Common voltage drop mistakes
- Doubling the length twice. L is the one-way distance. The 2 in the formula already accounts for the return path, so plugging in round-trip footage doubles your answer and makes good runs look like failures.
- Using the breaker rating as I. The formula wants actual load current. A 20 A breaker on a circuit drawing 10 A drops half as much voltage as the breaker-rating math suggests.
- Treating 3 percent as enforceable code everywhere - or ignoring it entirely. It is an informational note in most adoptions, but specs and local amendments can make it binding, and physics does not care either way.
- Forgetting aluminum's K. Using 12.9 for an aluminum feeder understates the drop by about 39 percent and the circuit will sag in service.
- Confusing voltage drop with derating. Upsizing for distance does not fix a bundling problem, and conduit derating does not fix a distance problem. Run both checks separately.
- Leaving the ground wire alone. When you upsize the circuit conductors for voltage drop, the NEC also requires the equipment grounding conductor to be increased proportionally - see the ground wire size chart for the baseline sizes from 250.122.
- Testing with no load. An unloaded receptacle at the end of 300 ft of 14 AWG reads a healthy 120 V. Voltage drop only appears when current flows, so always measure with the load running.
Field technique: verifying and fixing a drop problem
- Calculate before you pull: Run the numbers at design time, when upsizing costs one line on the material order. Fixing a failed run after the pull means demo, a re-pull, and possibly a larger raceway.
- Measure under load: Read voltage at the panel and at the load with the load running, and subtract. That difference is the real-world drop, including terminations and splices the formula cannot see. A loose splice can mimic an undersized-conductor problem.
- Pick the cheapest fix: In rough order of cost: upsize the conductors one or two sizes, shorten the route, feed the load at 240 V instead of 120 V where the equipment allows, or set a subpanel closer to the load so the long haul happens at feeder size.
- Recheck the raceway math: Bigger conductors change everything downstream: conduit fill is capped at 40 percent for three or more conductors, so verify the raceway against the conduit fill chart, keep total bends at or under 360 degrees between pull points, and recheck box fill at the terminations.
Run voltage drop checks in seconds. Conduit Fill & Bending Calc by Panda Taps puts a voltage drop calculator on your iPhone and iPad alongside conduit fill, bend marking with multipliers and shrink, box fill, and ampacity with derating. It is built for the field, works offline, and needs no account - punch in amps, length, and wire size at the truck and get the answer before the wire leaves the reel. Download Conduit Fill & Bending Calc on the App Store.
Frequently asked questions
Is the NEC 3 percent voltage drop rule mandatory?
In most adoptions it is an informational note to NEC 210.19, which is a recommendation rather than enforceable code text. Many jurisdictions, energy codes, and project specifications do turn it into a hard requirement. Confirm with your local AHJ and the adopted code edition before treating it either way.
What is K in the voltage drop formula?
K is the material resistance constant, approximately 12.9 for copper and 21.2 for aluminum. It represents the resistance of a conductor one circular mil in cross section and one foot long, which is why it pairs with circular mils and feet in the formula.
Do I use one-way or round-trip distance for L?
Use the one-way distance from the source to the load in feet. The 2 in the single-phase formula already accounts for current traveling out and back, so entering round-trip footage doubles the result incorrectly.
Does upsizing wire for voltage drop change the breaker size?
No. The overcurrent device is sized for the circuit and load, so a 20 amp circuit keeps its 20 amp breaker even if you pull 10 AWG or 8 AWG for distance. The small-conductor caps still apply: 14 AWG maxes at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A.