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Voltage Drop Calculator

Voltage drop in electrical wiring is calculated using the formula VD = (2 x K x I x L) / CM, where K is the resistivity constant for the conductor material, I is the current in amps, L is the one-way wire length in feet, and CM is the circular mil area of the wire gauge. The NEC recommends keeping voltage drop at or below 3% for branch circuits. Enter your wiring details below to check whether a given wire run meets this guideline.

Quick Answer

A 100-foot run of 12 AWG copper wire carrying 15 amps at 120 volts has a voltage drop of approximately 5.93 volts (4.94%), which exceeds the 3% recommendation.

Common Examples

Input Result
120V, 15A, 50 ft, 12 AWG copper VD: 2.97V (2.47%)
120V, 20A, 75 ft, 10 AWG copper VD: 3.73V (3.11%)
240V, 30A, 100 ft, 8 AWG copper VD: 4.69V (1.95%)
120V, 15A, 100 ft, 12 AWG aluminum VD: 9.73V (8.11%)

How It Works

This calculator uses the standard voltage drop formula for single-phase circuits:

VD = (2 x K x I x L) / CM

Where:

  • VD = voltage drop in volts
  • K = resistivity constant of the conductor material in ohm-cmil/ft. For copper, K = 12.9. For aluminum, K = 21.2.
  • I = load current in amperes
  • L = one-way length of the wire run in feet (the formula multiplies by 2 to account for the full circuit, out and back)
  • CM = circular mil area of the wire, determined by the AWG gauge

The percent voltage drop is calculated as (VD / source voltage) x 100.

The voltage at the load is the source voltage minus the voltage drop.

Wire Gauge and Circular Mils

Each AWG (American Wire Gauge) size corresponds to a specific cross-sectional area measured in circular mils. Larger gauge numbers indicate smaller wire. For example, 14 AWG has 4,110 circular mils, while 4/0 AWG has 211,600 circular mils. Larger wire (lower gauge number or higher “ought” designation) has more circular mils and therefore less resistance, resulting in lower voltage drop.

The 3% Rule

The National Electrical Code (NEC) recommends that voltage drop on branch circuits not exceed 3%, and that the total voltage drop from the service entrance to the farthest outlet not exceed 5%. Exceeding these thresholds can cause equipment to underperform, lights to dim, and motors to overheat. This calculator flags any result that exceeds the 3% branch circuit recommendation.

Worked Example

For a 120V circuit with 15A load, 50 feet of 12 AWG copper wire: K = 12.9 (copper). CM = 6,530 (12 AWG). VD = (2 x 12.9 x 15 x 50) / 6,530 = 19,350 / 6,530 ≈ 2.96 volts. Percent drop = (2.96 / 120) x 100 ≈ 2.47%. Voltage at load = 120 - 2.96 = 117.04V. This is within the 3% recommendation. If the same run were 100 feet instead of 50, the voltage drop would double to approximately 5.93V (4.94%), exceeding the 3% threshold. In that case, upgrading to 10 AWG wire would bring the drop to approximately 3.73V (3.11%), or 8 AWG to approximately 2.34V (1.95%).

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Frequently Asked Questions

What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage as electrical current flows through a wire due to the wire's resistance. Excessive voltage drop means less voltage reaches the device at the end of the circuit. This can cause lights to dim, motors to run hot or inefficiently, and sensitive electronics to malfunction. Keeping voltage drop within recommended limits ensures equipment operates safely and efficiently.
What is the 3% rule for voltage drop?
The National Electrical Code (NEC) recommends that voltage drop on any individual branch circuit not exceed 3% of the source voltage. For the total circuit from the service panel to the farthest outlet, the recommended maximum is 5%. These are recommendations, not strict code requirements, but following them helps ensure reliable performance.
How do I reduce voltage drop on a long wire run?
There are three main strategies: use a larger wire gauge (which has more circular mils and lower resistance), shorten the wire run if possible, or increase the source voltage (for example, running a 240V circuit instead of 120V for applicable equipment). For very long runs, such as those to detached garages or outbuildings, larger wire gauges are often necessary.
What is the difference between copper and aluminum wire?
Copper has a lower resistivity (K = 12.9) than aluminum (K = 21.2), meaning copper wire produces less voltage drop for the same gauge and length. However, aluminum wire is lighter and less expensive. When using aluminum, a larger wire gauge is typically needed to achieve the same voltage drop performance as copper.
Does this calculator work for three-phase circuits?
This calculator is designed for single-phase circuits, which are the most common type in residential wiring. Three-phase circuits use a different formula with a factor of the square root of 3 (approximately 1.732) instead of 2. For three-phase calculations, the formula is VD = (1.732 x K x I x L) / CM.