Voltage Drop Calculator

The Voltage Drop Calculator helps you determine the voltage loss in electrical wires and cables based on wire size, material, length, current, and phase type. Proper voltage drop calculation is essential for safe and efficient electrical installations, ensuring that connected devices receive adequate voltage to operate correctly.

What is Voltage Drop?

Voltage drop is the reduction in electrical potential (voltage) that occurs as current flows through a wire. Every wire has some resistance or impedance, which causes a portion of the applied voltage to be "lost" as heat. The longer the wire and the higher the current, the greater the voltage drop.

Excessive voltage drop can cause lights to dim or flicker, motors to overheat and fail, and sensitive electronic equipment to malfunction. The National Electrical Code (NEC) recommends that voltage drop should not exceed 5% at the farthest outlet under full load conditions.

How to Calculate Voltage Drop

The voltage drop depends on several factors:

Wire Material: Copper has lower resistance than aluminum, resulting in less voltage drop.
Wire Size: Larger diameter wires (lower AWG number or higher kcmil) have less resistance.
Wire Length: Longer wires produce greater voltage drop (measured one-way).
Load Current: Higher current increases voltage drop proportionally.
Phase Type: DC and single-phase AC use a factor of 2 (round trip), while three-phase uses a factor of √3.
Power Factor: For AC circuits, the power factor affects the effective impedance.

Voltage Drop Formula

For DC circuits: V_drop = 2 × I × R × L

For AC single-phase: V_drop = 2 × I × (R×cosθ + X×sinθ) × L

For AC three-phase: V_drop = √3 × I × (R×cosθ + X×sinθ) × L

Where I is the load current in amperes, R is the wire resistance per unit length, X is the wire reactance per unit length, L is the one-way length, and θ is the phase angle derived from the power factor.

How to Use This Calculator

Simply adjust the input parameters on the left panel. Select the wire material (copper or aluminum), choose the appropriate wire size from the AWG/kcmil range, enter the one-way distance, load current, and source voltage. Select the correct phase type (DC, AC single-phase, or AC three-phase) and enter the power factor for AC circuits. The voltage drop and percentage will update automatically.

Frequently Asked Questions

What is an acceptable voltage drop percentage?

The NEC recommends a maximum voltage drop of 5% at the farthest outlet under full load. For sensitive equipment or critical circuits, a drop of 3% or less is often preferred. Our calculator highlights results in green (good), yellow (borderline), and red (exceeds 5%).

Should I use copper or aluminum wire?

Copper wire has about 60% lower resistance than aluminum, resulting in less voltage drop for the same wire size. However, aluminum is lighter and less expensive. For long runs where voltage drop is a concern, copper is generally preferred despite its higher cost.

What is the difference between DC, AC single-phase, and AC three-phase voltage drop?

DC circuits use the simplest formula with V_drop = 2 × I × R × L. AC single-phase uses the same round-trip factor of 2 but includes reactance. AC three-phase uses a factor of √3 (about 1.732) because the current is distributed across three conductors, making three-phase systems more efficient for power transmission.

What does AWG mean?

AWG stands for American Wire Gauge, a standardized wire sizing system used primarily in North America. Lower AWG numbers indicate thicker wires with less resistance. For example, 4/0 AWG (pronounced "four aught") is much thicker than 14 AWG. Wire sizes above 4/0 are measured in kcmil (thousand circular mils).

Related Electrical Calculators

For more electrical calculations, try the Electricity Calculator for power consumption analysis, the Amps To Watts Calculator and Amps to Volts Calculator for current and voltage conversions, the Resistance Converter for unit conversions, and the Electric Field Calculator for field strength calculations.

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