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Voltage Drop Calculator
Enter conductor material, wire size, load current, run length, voltage, and phase to instantly get voltage drop, percent drop, and voltage at the load. See at a glance whether your circuit meets the NEC 3% and 5% recommendations.
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Calculate Voltage Drop
Choose copper or aluminum, pick a wire size, and enter your load current, one-way run length, system voltage, and phase. Results update instantly and flag the NEC 3% / 5% status.
K = specific resistance in ohm-circular-mils per foot.
Larger circular-mil area means less voltage drop.
Used to compute percent drop and voltage at load.
Voltage drop results
Voltage drop
4.97 V
Percent drop
4.14%
Voltage at load
115.03 V
NEC status
3% – 5% (caution)
Above 3% branch but within the 5% total recommendation. Based on single-phase Copper, 10 AWG, 20 A over a 100 ft one-way run at 120 V.
K factor
12.9
Circular mils
10,380
Phase factor
2
One-way length
100 ft
What Is Voltage Drop and Why It Matters
Voltage drop is the loss of voltage that occurs as current travels through the resistance of a conductor. Every foot of wire has resistance, so by the time power reaches the load, the voltage is lower than what left the panel.
The NEC 3% and 5% recommendation
The National Electrical Code recommends keeping voltage drop under 3% on a branch circuit, and under 5% total across the feeder and branch circuit combined (NEC 210.19 and 215 informational notes). Excessive drop causes dim lights, nuisance motor tripping, overheating, and wasted energy.
These are recommendations for a quality installation rather than strict code mandates — but most engineers, inspectors, and project specifications treat the 3% / 5% limits as the design standard.
Increasing wire size reduces drop
Voltage drop is inversely proportional to conductor cross-section: the larger the circular-mil area, the lower the resistance and the smaller the drop. Going up one or two AWG sizes on a long run is the most common way to bring a circuit back within limits.
You can also shorten the run or raise the system voltage. The K-factor method used here: single-phase VD = (2 × K × I × L) / CM, three-phase VD = (1.732 × K × I × L) / CM, where K is 12.9 for copper and 21.2 for aluminum.
Copper AWG Circular Mils Reference
Cross-sectional area in circular mils for common copper conductor sizes. Larger circular-mil values carry more current with less voltage drop.
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Common Questions About Voltage Drop
How do you calculate voltage drop?
Voltage drop uses the circular-mils (K-factor) method. Single-phase: VD = (2 × K × I × L) / CM. Three-phase: VD = (1.732 × K × I × L) / CM. K is conductor resistivity (12.9 copper, 21.2 aluminum), I is load current in amps, L is one-way length in feet, and CM is the conductor area in circular mils. This calculator applies the right formula based on the phase you select.
What is the maximum allowable voltage drop per NEC?
The NEC recommends a maximum of 3% voltage drop on a branch circuit and 5% total across feeder plus branch circuit (210.19 and 215 informational notes). These are recommendations for efficiency and proper equipment operation, not strict code requirements — but most inspectors, engineers, and specs treat 3% / 5% as the standard for a quality install.
How do I reduce voltage drop?
Three practical fixes: increase the conductor size (a larger circular-mil area lowers resistance and drop), shorten the run (drop is proportional to one-way length), or raise the system voltage (the same load at higher voltage draws less current). Increasing wire size is the most common field fix on long runs.
What is the difference between single-phase and three-phase voltage drop?
It is the multiplier in the formula. Single-phase uses a factor of 2 because current flows out and back on two conductors. Three-phase uses 1.732 (square root of 3) due to the phase relationship between conductors. For the same current, length, and wire size, a three-phase circuit has a lower voltage drop.
Why does wire length affect voltage drop?
Voltage drop comes from conductor resistance, and resistance is directly proportional to length. The longer the wire, the more total resistance and the more voltage lost. Doubling the one-way length doubles the drop — which is why long runs to detached buildings, well pumps, and remote equipment often need oversized conductors.
What K factor do you use for copper and aluminum?
The standard K factor is 12.9 for copper and 21.2 for aluminum, in ohm-circular-mils per foot at about 75°C. Aluminum has a higher K because it is more resistive, which is why aluminum must be sized larger than copper for the same load and voltage drop. This calculator lets you switch between the two.
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Book a Free DemoVoltage drop results use the standard K-factor circular-mils method (K = 12.9 copper, 21.2 aluminum) and are estimates for planning only. The NEC 3% branch and 5% total figures are informational recommendations, not hard code requirements. Verify conductor sizing, temperature ratings, and ampacity against the current NEC and actual equipment data before installation.