Voltage Drop Calculator — The 3% Rule, Properly
Voltage drop in volts and percent for any gauge, length, and load — plus the longest run that still lands inside the 3% recommendation. Every conductor has resistance, so every run gives up some voltage on the way to the load. This calculator returns the drop in volts and percent, the voltage that actually arrives, and the longest one-way run that keeps you inside your limit — for single-phase and three-phase, copper and aluminum, 120 V through 480 V.
Worth knowing: the 3% and 5% figures are Informational Notes in NEC 210.19(A) and 215.2(A) — explanatory guidance, not enforceable Code. Most specifications and many local amendments adopt them anyway, and a few articles (647.4(D), 695.7) do set hard limits. Treat 3% as the professional default.
Check the drop on your run
Voltage drop
Within the 3% recommendation.
Volts dropped
4.92 V
Volts at the load
235.1 V
See the breakdown
Uses the circular-mils approximation, which ignores conductor reactance. For large feeders, long runs in steel raceway, or power-factor-sensitive loads, use the impedance method with Chapter 9 Table 9.
The formula, explained in plain English
Voltage drop is Ohm's law applied to the wire itself. Current through the conductor's own resistance produces a voltage that never reaches the load.
One-way length, doubled
Enter the distance from panel to load, not the total conductor length. The formula's multiplier already accounts for the return path — entering round-trip footage doubles the answer.
Volts are absolute, percent is relative
The volts dropped don't change with system voltage. Only the percentage does. That single fact explains why 240 V and 480 V distribution exist.
Why aluminum drops more
K = 21.2 versus copper's 12.9 means about 64% more drop at the same gauge — which is why aluminum feeders are typically run two sizes larger than their copper equivalent.
What this method ignores
Reactance. For conductors above about 1/0 in steel raceway, or loads with poor power factor, the impedance method using Chapter 9 Table 9 gives a more accurate answer.
Worked examples
A pass, a bad fail, and the case that shows why voltage matters more than gauge on long runs.
Garage subpanel feeder — 6 AWG, 50 A, 100 ft
Copper · 240 V single-phase · 3% limit. The defaults above.
VD% = 4.92 ÷ 240 × 100 = 2.05%
V at load = 240 − 4.92 = 235.1 V
→ PASS · max compliant run = 146 ft
Result: comfortable at 2.05%, with 46 feet of headroom before the run would need 4 AWG.
Shed circuit — 12 AWG, 20 A, 150 ft at 120 V
Copper · 120 V single-phase · 3% limit.
VD% = 11.85 ÷ 120 × 100 = 9.88%
V at load = 108.1 V
→ FAIL · max compliant run = 45 ft
Result: 108 volts at the load will make motors buzz and struggle to start. The fix is 6 AWG, or feeding the shed at 240 V with a small subpanel.
Three-phase machine feed — 4/0, 100 A, 400 ft at 480 V
Copper · 480 V three-phase · 3% limit.
VD% = 4.22 ÷ 480 × 100 = 0.88%
→ PASS easily
Result: under 1% across 400 feet. The same 100 A load at 120 V single-phase on the same conductor drops 4.88 V — which is 4.06%, more than four times the percentage on identical copper. Higher voltage and three-phase working together is what makes long industrial feeders practical.
Maximum run length at 3% — 240 V copper
Longest one-way run in feet before drop passes 3%, single-phase at 240 V. Blank cells are combinations where Table 310.16 or the 240.4(D) small-conductor rule will not permit that current on that conductor at all. At 120 V, halve every number.
| Conductor | 15 A | 20 A | 30 A | 50 A | 100 A |
|---|---|---|---|---|---|
| 14 AWG | 76 ft | — | — | — | — |
| 12 AWG | 121 ft | 91 ft | — | — | — |
| 10 AWG | 193 ft | 144 ft | 96 ft | — | — |
| 8 AWG | 307 ft | 230 ft | 153 ft | 92 ft | — |
| 6 AWG | 488 ft | 366 ft | 244 ft | 146 ft | — |
| 4 AWG | 776 ft | 582 ft | 388 ft | 232 ft | — |
| 3 AWG | 979 ft | 734 ft | 489 ft | 293 ft | 146 ft |
| 2 AWG | 1234 ft | 925 ft | 617 ft | 370 ft | 185 ft |
| 1/0 AWG | 1964 ft | 1473 ft | 982 ft | 589 ft | 294 ft |
| 4/0 AWG | 3936 ft | 2952 ft | 1968 ft | 1181 ft | 590 ft |
Sources & standards: NEC (NFPA 70) 2023 — 210.19(A) and 215.2(A) Informational Notes on voltage drop, 90.5(C) on the status of Informational Notes, Chapter 9 Table 8 conductor properties, Chapter 9 Table 9 for the impedance method, Table 310.16 ampacities, 240.4(D) small-conductor protection. Local amendments override the model code.
Frequently asked questions
Common questions about voltage drop, the 3% rule, and long runs.
Is 3% voltage drop a code requirement or a recommendation?
A recommendation. The 3% branch-circuit and 5% combined feeder-plus-branch figures appear in Informational Notes to NEC 210.19(A) and 215.2(A), and Article 90.5(C) states that Informational Notes are explanatory and not enforceable as Code requirements. That said, many project specifications, engineering standards, and local amendments adopt them as mandatory — and some specific articles (like 647.4(D) for sensitive electronic equipment, or 695.7 for fire pumps) do impose hard voltage-drop limits. Treat 3% as the professional default, not as optional.
Why 2× for single-phase and 1.732× for three-phase?
Single-phase current flows out on the hot and back on the neutral, so it travels the run length twice — hence the factor of 2. In a balanced three-phase circuit the three phase currents are 120° apart and partly cancel in the return path, which works out to a factor of √3 ≈ 1.732 rather than 2. That is a big part of why three-phase distribution wins on long runs.
What is an acceptable voltage drop?
Aim for 3% or less on a branch circuit and 5% or less for the feeder and branch circuit combined. Motors are particularly sensitive: low voltage raises current draw, which raises heat and shortens winding life. Resistive loads like water heaters simply run slower and cost more per unit of output. LED drivers and electronics usually tolerate more drop but can flicker or reset near the limit.
Does voltage drop matter more on 120 V than 240 V?
Yes, dramatically. Drop in volts depends only on current, length, and conductor size — not on system voltage. But drop as a percentage is that figure divided by the system voltage. The same 6-volt drop is 5% at 120 V and only 2.5% at 240 V. That is why long outbuilding runs are almost always fed at 240 V and why commercial work uses 480 V.
How far can I run 12 AWG on a 20 amp circuit?
At 240 V single-phase, about 91 feet before 12 AWG copper passes 3%. At 120 V it is only about 45 feet at the full 20 A load. If the actual load is lower than the breaker rating, the compliant run gets longer in proportion — drop scales with current, not with breaker size. Enter your real load current above rather than the breaker rating.
Does upsizing wire for voltage drop change my ground wire size?
Yes. NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally when the ungrounded conductors are upsized — including when the reason is voltage drop. The ratio is by circular mils. The Ground Wire Size Calculator has a toggle that does this proportional upsize for you.
Should I use the load current or the breaker rating?
Use the actual load current. Voltage drop is a function of the current that really flows, so sizing to a breaker rating the circuit never reaches over-builds the conductor. The exception is a circuit you expect to load to capacity — an EV charger or an electric water heater running at nameplate — where load and rating are effectively the same number.
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