Voltage Drop Calculator
Calculate the voltage drop in volts and percent for a given wire size, load, run length and voltage, plus the longest run that stays inside the NEC 3% recommendation.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Voltage drop
- 5.93 V
- 4.94% of 120 V, over 3% but within the 5% feeder-plus-branch total
- Percent drop
- 4.94%
- 12 AWG copper carrying 15 A over 100 ft one way
- Voltage at the load
- 114.07 V
- Starting from 120 V at the panel
- Longest one-way run for 3% drop
- 61 ft
- At 15 A on 12 AWG copper; 101 ft for a 5% drop
- NEC recommendation
- 3% branch, 5% total
- Informational notes to 210.19 and 215.2. Planning estimate only: a licensed electrician and local code govern, and ampacity must be checked separately.
Voltage Drop formula
Voltage drop (V) = 2 x K x Amps x One-way distance (ft) / Circular mils (K = 12.9 copper, 21.2 aluminum; use 1.732 instead of 2 for three-phase)
Percent drop = Voltage drop / System voltage x 100
Voltage at the load = System voltage - Voltage drop
Longest run for 3% = 0.03 x System voltage x Circular mils / (2 x K x Amps)Worked example
A 30 A, 240 V single-phase load 150 ft from the panel on 10 AWG copper (10,380 circular mils): 2 x 12.9 x 30 x 150 = 116,100. Divide by 10,380 to get 11.18 V, which is 11.18 / 240 = 4.66%, leaving 228.8 V at the load. The longest run that holds 3% at 30 A is 0.03 x 240 x 10,380 / (2 x 12.9 x 30) = 74,736 / 774 = 96.6 ft, so this run needs 8 AWG.
A 100 A three-phase feeder at 480 V, 200 ft long in 2 AWG aluminum (66,360 circular mils): 1.732 x 21.2 x 100 x 200 = 734,368. Divide by 66,360 for 11.07 V, or 2.31% of 480 V. The same wire could run 0.03 x 480 x 66,360 / (1.732 x 21.2 x 100) = 955,584 / 3,671.84 = 260 ft before reaching 3%.
Quick reference
| One-way distance | Drop at 15 A | Percent at 15 A | Drop at 20 A | Percent at 20 A |
|---|---|---|---|---|
| 50 ft | 2.96 V | 2.47% | 3.95 V | 3.29% |
| 100 ft | 5.93 V | 4.94% | 7.90 V | 6.58% |
| 150 ft | 8.89 V | 7.41% | 11.85 V | 9.88% |
| 200 ft | 11.85 V | 9.88% | 15.80 V | 13.17% |
How voltage drop is calculated
Every conductor has resistance, and current through resistance loses voltage. The standard field formula multiplies the K constant (ohms per circular-mil foot, 12.9 for copper and 21.2 for aluminum at 75°C) by the current and the round-trip length, then divides by the wire's cross section in circular mils. The factor of 2 covers the outgoing and return conductors on a single-phase circuit; a balanced three-phase circuit uses 1.732 instead.
The result is volts lost between the panel and the load. Divide by the system voltage to get the percentage, which is what the NEC informational notes reference: 3% on a branch circuit and 5% for feeder and branch combined. This calculator answers the question in reverse from a wire size calculator: you already know the wire and want to see how much it drops.
What to do about a high drop
If the drop is over 3%, you have four levers: shorten the run, lower the current, raise the voltage or increase the wire size. Going from 12 AWG to 10 AWG cuts the drop by 37%, and to 8 AWG by 60%. On a 120 V circuit to a detached garage or shed it is often cheaper to run 240 V and split it at a subpanel than to pull very large wire.
The formula uses DC resistance at 75°C and ignores conductor reactance, power factor and temperature, which is accurate to within a few percent for residential wire sizes. The drop calculated here says nothing about whether the wire can safely carry the current; check ampacity, terminal ratings and conduit fill with a licensed electrician before pulling wire, and follow the NEC edition and amendments adopted in your jurisdiction.
Key facts
- 12 AWG copper has a resistance of about 1.93 ohms per 1,000 ft at 75°C (NEC Chapter 9 Table 8), so a 100 ft circuit, which has 200 ft of conductor, adds about 0.39 ohm to the loop.
- Voltage drop is proportional to both current and length: doubling the load or doubling the run doubles the drop, while moving up three wire sizes roughly halves it.
- Incandescent light output falls about 3.5% for every 1% of voltage drop, so a 5% drop cuts a lamp's light output by roughly 17%.
- NEMA MG-1 rates motors for plus or minus 10% of nameplate voltage; below that they draw more current, run hotter and lose torque, which is why long motor circuits are sized for drop rather than ampacity.
- At 240 V a circuit drops the same number of volts as at 120 V with the same wire, amps and length, but half the percentage, which is why long runs to outbuildings favor 240 V.
Frequently asked questions
How far can I run 12 gauge wire on a 20 amp circuit?
At 120 V and a full 20 A load, about 46 ft one way before the drop passes 3%; at 15 A the limit is 61 ft. At 240 V those distances double.
Is a 5% voltage drop acceptable?
The NEC recommends no more than 3% on a branch circuit and 5% for the feeder and branch circuit together. A 5% drop on a single branch circuit is outside the recommendation, and many local energy codes make the 3% figure mandatory.
Why does 240 V have less voltage drop than 120 V?
It does not drop fewer volts; the same wire, amps and length lose the same voltage. But the loss is a smaller share of 240 V, so the percent drop is half, and a 240 V load also draws half the amps for the same power, which halves the volts lost as well.
Does the voltage drop formula work for aluminum wire?
Yes, use K = 21.2 instead of 12.9. Aluminum has about 64% more resistance than copper of the same size, so it needs roughly two sizes larger for the same drop.
Sources and references
- NFPA 70, National Electrical Code (NFPA)
- Voltage Drop Calculations, IAEI Magazine (IAEI)
- Re3 Voltage Drop Calculator (Southwire)
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