Voltage Drop Calculator — NEC Wire Size Estimator
Calculate the exact voltage drop percentage and final load voltage over long branch circuits or feeders. Built to the National Electrical Code standards.
The National Electrical Code (NEC) Section 210.19(A) recommends maintaining a maximum voltage drop of 3% on branch circuits, and exactly 5% total across both feeder and branch circuits combined to guarantee optimal appliance efficiency.
Mathematical Voltage Drop Equations
This calculator implements standard branch-circuit calculations using Ohm's Law and conductor resistivity metrics ($K$ factors):
Voltage Drop (V_drop) = (2 × K × I × L) ÷ CM
Three-Phase Formulas:
Voltage Drop (V_drop) = (1.732 × K × I × L) ÷ CM
Where:
- K (Resistivity Factor): Constant representing resistance of one circular-mil foot of conductor. Copper is valued at exactly 12.9 ohms, while aluminum is valued at 21.2 ohms at 75°C standard temperature.
- I: Current load in amperes passing through the wire.
- L: One-way distance of the wire run from power source to load (in feet).
- CM: Circular mil area of the chosen wire size.
AWG Circular Mil Size Reference Chart
| Wire Gauge (AWG) | Area (Circular Mils) | Resistance per 1k Ft (Cu) | Max Ampacity (75°C) |
|---|---|---|---|
| 14 AWG | 4,110 | 3.07 ohms | 15A |
| 12 AWG | 6,530 | 1.93 ohms | 20A |
| 10 AWG | 10,380 | 1.24 ohms | 30A |
| 8 AWG | 16,510 | 0.778 ohms | 50A |
| 6 AWG | 26,240 | 0.491 ohms | 65A |
| 4 AWG | 41,740 | 0.308 ohms | 85A |
| 2 AWG | 66,360 | 0.194 ohms | 115A |
The Mechanics of Voltage Drop in Long Wire Runs
When electricity travels through a wire, the copper or aluminum conductor naturally provides a small amount of resistance. Over short distances, this resistance is negligible. However, as the length of the wire increases, this resistance compounds, causing the voltage at the end of the line to drop. If voltage drops too low, motors will overheat, lights will dim, and sensitive electronics will fail to operate.
NEC Recommendations for Voltage Drop
While the National Electrical Code (NEC) generally treats voltage drop as a "Fine Print Note" rather than a strict safety mandate, NEC 210.19(A) Informational Note 4 strongly recommends a maximum voltage drop of 3% for the branch circuit, and a total maximum drop of 5% for both the feeder and the branch circuit combined. Adhering to these limits ensures operational efficiency and prevents premature equipment failure.
Variables That Affect Voltage Drop
- Conductor Material: Copper is an excellent conductor and experiences less voltage drop than aluminum of the exact same gauge.
- Wire Gauge (Circular Mils): A thicker wire has a larger cross-sectional area, meaning lower resistance and less voltage drop.
- Current (Amps): The higher the load pulling through the wire, the more severe the voltage drop will be.
- Distance: The total length of the run (there and back) is the primary multiplier for resistance.
Upsizing Wire to Solve Voltage Drop
If your calculation shows a 6% voltage drop on a 120V circuit running out to a detached garage 150 feet away, you cannot simply increase the breaker size to solve the problem. The only physical solution is to decrease the resistance of the wire. This is done by upsizing the wire gauge. For example, moving from a 12 AWG wire to a 10 AWG wire drastically increases the circular mils of the conductor, dropping the resistance and bringing the voltage back within the 3% safe operating margin.