NEC Voltage Drop Reference
Voltage Drop Chart
Maximum one-way run distance at the NEC-recommended 3% voltage drop, for copper and aluminum, by wire size and circuit. Plus the K-factor formula, the allowable-drop table for every common system voltage, and conductor circular mils.
Intry Verified. The circular mils, K-factors, and NEC 240.4 overcurrent limits behind this chart are the locked, re-checked engine constants; the maximum run distances are derived from them by the published K-factor formula (VD = 2·K·I·L / CM) on every build. Those figures are computed by a locked engine straight from the source it cites, not hand-typed, and that computation re-runs inside a deterministic test on every deploy. Source-checked. Locked. This is our own deterministic gate, not a third-party audit. What Intry Verified means
voltage_drop tool live, handing back the verdict with its cited source instead of a guess. Then keep the reading as one job in the Intry app.The Voltage Drop Formula (K-Factor Method)
The K-factor (or circular-mil) method is the standard field formula for estimating voltage drop on a conductor run:
Three-phase: VD = (1.732 × K × I × L) / CM
- K = resistivity constant: 12.9 for copper, 21.2 for aluminum (ohms-cmil/ft at 75°C).
- I = load current in amps.
- L = one-way length of the run in feet.
- CM = conductor cross-sectional area in circular mils (see the table below).
To reverse-solve for the minimum wire size, rearrange to CM = (2 × K × I × L) / VD_allowed, then pick the next standard conductor with at least that many circular mils.
Allowable Voltage Drop by System Voltage
The NEC recommends 3% maximum on a branch circuit and 5% total for feeder plus branch (NEC 210.19(A) and 215.2 informational notes). These are the voltages those percentages work out to, and the minimum voltage still present at the load at the 3% limit.
| System Voltage | 3% Drop | 5% Drop | Voltage at Load (3%) |
|---|---|---|---|
| 120V | 3.6 V | 6.0 V | 116.4 V |
| 208V | 6.2 V | 10.4 V | 201.8 V |
| 240V | 7.2 V | 12.0 V | 232.8 V |
| 277V | 8.3 V | 13.9 V | 268.7 V |
| 480V | 14.4 V | 24.0 V | 465.6 V |
Maximum Copper Wire Run at 3% Drop
The farthest one-way distance each copper wire size can run before exceeding 3% voltage drop, by circuit. Single-phase, computed as (CM × V × 0.03) / (2 × 12.9 × I) and floored to whole feet. Double these for the 5% limit. Cells marked “n/a” are code-impossible pairings under NEC 240.4: the circuit breaker exceeds the largest overcurrent device that conductor may sit on, so the wire is not permitted on that circuit at any run length. The small conductors are capped by 240.4(D) (#14 copper at 15A, #12 at 20A, #10 at 30A); the larger ones are capped by their 75°C ampacity and 240.4(B), so #8 copper (50A) and #6 (65A, one size up to 70A) cannot ride a 100A circuit.
| Wire Size | 20A / 120V | 20A / 240V | 30A / 240V | 50A / 240V | 100A / 240V |
|---|---|---|---|---|---|
| #14 | n/a | n/a | n/a | n/a | n/a |
| #12 | 45 ft | 91 ft | n/a | n/a | n/a |
| #10 | 72 ft | 144 ft | 96 ft | n/a | n/a |
| #8 | 115 ft | 230 ft | 153 ft | 92 ft | n/a |
| #6 | 183 ft | 366 ft | 244 ft | 146 ft | n/a |
| #4 | 291 ft | 582 ft | 388 ft | 232 ft | n/a |
| #2 | 462 ft | 925 ft | 617 ft | 370 ft | 185 ft |
| 1/0 | 736 ft | 1473 ft | 982 ft | 589 ft | 294 ft |
| 2/0 | 928 ft | 1857 ft | 1238 ft | 742 ft | 371 ft |
| 4/0 | 1476 ft | 2952 ft | 1968 ft | 1181 ft | 590 ft |
Maximum Aluminum Wire Run at 3% Drop
The same chart for aluminum conductors (K = 21.2). Aluminum drops more voltage than copper for a given size, so the distances are shorter. Aluminum is common on feeders and services where the cost per amp wins out. As on the copper chart, “n/a” cells are code- impossible under NEC 240.4: aluminum ampacity runs about one size behind copper, so #8 aluminum (40A) cannot ride a 50A or 100A circuit and #6 (50A) cannot ride 100A.
| Wire Size | 20A / 120V | 20A / 240V | 30A / 240V | 50A / 240V | 100A / 240V |
|---|---|---|---|---|---|
| #8 | 70 ft | 140 ft | 93 ft | n/a | n/a |
| #6 | 111 ft | 222 ft | 148 ft | 89 ft | n/a |
| #4 | 177 ft | 354 ft | 236 ft | 141 ft | n/a |
| #2 | 281 ft | 563 ft | 375 ft | 225 ft | n/a |
| 1/0 | 448 ft | 896 ft | 597 ft | 358 ft | 179 ft |
| 2/0 | 565 ft | 1130 ft | 753 ft | 452 ft | 226 ft |
| 4/0 | 898 ft | 1796 ft | 1197 ft | 718 ft | 359 ft |
| 250 kcmil | 1061 ft | 2122 ft | 1415 ft | 849 ft | 424 ft |
| 350 kcmil | 1485 ft | 2971 ft | 1981 ft | 1188 ft | 594 ft |
Calculate Your Exact Run
The charts assume single-phase at 3%. Enter your material, wire size, load, length, and voltage for an exact percent drop, or reverse-solve for the minimum wire size that meets 3%.
Intry VerifiedA worked default reading, traceable end to end: what it was calculated from, what it was run with, how it was checked, and who has final say.
Where this number comes fromIntry Verified
- Calculated from
- NEC 210.19 Note, 3% guideline
- Run with
- Wire#12 AWG
- Load20 A
- One-way length100 ft
- System240 V, 1-phase
- MaterialCopper
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit.
- Final say
- Your AHJ and local amendments have final say. Confirm before rough-in.
Intry Verified · NEC 2023 · Build AE2FC52 · 2026-08-21
Worked Example: 20A Circuit, 120V, 100 ft Copper
Given: A 20A branch circuit on 120V single-phase, #12 copper THHN, running 100 feet one-way to the farthest outlet.
Step 1: Apply the formula. VD = (2 × 12.9 × 20 × 100) / 6,530 = 7.90 V.
Step 2: Convert to percent. 7.90 / 120 × 100 = 6.58%. That is well over the 3% recommendation, and the load only sees 112.1V.
Fix: Upsize to #10 copper (10,380 CM): VD = (2 × 12.9 × 20 × 100) / 10,380 = 4.97V or 4.14%, within 5% but still over 3%. The copper chart above confirms #12 tops out near 45 ft at 20A/120V, so a 100 ft run needs #8 for full 3% compliance.
Frequently Asked Questions
How far can you run wire before voltage drop is a problem?
A #12 copper branch circuit at 20A/120V can run about 45 feet before it hits the 3% voltage-drop limit, and roughly 91 feet at 240V since the allowable drop doubles with the voltage. NEC does not mandate a limit, but the Informational Notes at 210.19(A) (branch circuits) and 215.2(A) (feeders) recommend keeping branch-circuit drop under 3% and combined feeder-plus-branch drop under 5% for reasonable efficiency. To size any run yourself, use the K-factor method: VD = (2 × K × I × L) / CM, where K is 12.9 for copper, I is amps, L is one-way length in feet, and CM is the conductor's circular mils (6,530 for #12), then solve for the length that keeps VD at or below 3% of your system voltage. See the chart on this page for max lengths at 3% by AWG, load, and voltage so you can pick the run without doing the math.
What is the NEC voltage drop formula?
For a single-phase circuit, voltage drop = (2 x K x I x L) / CM, where K is the conductor resistivity constant (12.9 for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way run length in feet, and CM is the conductor area in circular mils. For three-phase, replace the 2 with 1.732 (the square root of 3). Percent drop = voltage drop divided by system voltage, times 100.
What is an acceptable voltage drop?
The NEC recommends a maximum of 3% voltage drop on a branch circuit and 5% total for feeder plus branch circuits combined (NEC 210.19(A) and 215.2 informational notes). On a 120V circuit, 3% is 3.6V and 5% is 6.0V. These are recommendations, not enforceable requirements, but most inspectors and engineers treat 3% as the practical target. For sensitive electronics, aim for 2% or less.
How far can I run 12 gauge wire?
On a 20A 120V copper circuit at the 3% limit, #12 AWG copper reaches about 45 feet one-way. At 240V the same wire and load reaches about 91 feet, because doubling the voltage halves the percent drop. Aluminum #12 reaches roughly 27 feet at 120V. See the copper and aluminum charts on this page for every common wire size and circuit.
How much farther can I run wire at 240V versus 120V?
Twice as far. Percent voltage drop is inversely proportional to system voltage, so at the same wire size and current, a 240V circuit has half the percent drop of a 120V circuit. That means you can run the wire about twice the distance before hitting the 3% limit. This is why long runs to detached garages, well pumps, and EV chargers are almost always wired at 240V.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has a K-factor of 21.2 versus 12.9 for copper, so an aluminum conductor of the same size drops about 64% more voltage than copper. To match copper's run distance you generally go up about two wire sizes in aluminum. Aluminum is still common for feeders and services because it is cheaper per amp of capacity, but you must account for the higher drop on long runs.
Is the maximum run distance based on ampacity or voltage drop?
The charts on this page are limited by voltage drop only, at the 3% recommendation. Ampacity (NEC 310.16) is a separate, independent check: a wire can pass ampacity and still fail voltage drop on a long run, or vice versa. Always size the conductor to satisfy both. Use the wire size calculator for the ampacity side with derating and terminal temperature limits.
Related Calculators
Voltage Drop Calculator
Exact percent drop for any run, with reverse-solve for the minimum wire size at 3%.
Wire Size Calculator
The ampacity side: NEC 310.16 sizing with temperature derating and terminal limits.
Wire Ampacity Chart
What a conductor carries in your conditions at 60/75/90°C insulation with the 110.14(C) termination rule.
EV Charger Calculator
Size the circuit for a Level 2 charger, where long garage runs often fail on drop.
Distance checks out? Confirm the ampacity side.
Voltage drop and ampacity are independent checks. A wire that clears the run distance can still fail NEC 310.16. Run the wire size calculator with derating and terminal temperature limits so the whole circuit holds up.