Voltage Drop Calculator

Calculate voltage drop in electrical circuits. Determine proper wire size and ensure NEC compliance.

System Voltage (V)

Phase Type

Wire Material

Wire Size (AWG)

Load Current (Amps)

One-Way Length (feet)

Allowable Voltage Drop (%)

Wire Temperature Rating

Voltage Drop

Load Voltage

NEC Compliance

Frequently Asked Questions

What is voltage drop?

Voltage drop is the decrease in electrical voltage as electricity travels through a wire due to the wire's inherent resistance. Longer wires and smaller gauges cause higher voltage drops. Excessive voltage drop can cause equipment malfunction, overheating, and energy loss.

What is the NEC allowable voltage drop?

The National Electrical Code (NEC) recommends:

Branch circuits: Maximum 3% voltage drop
Feeders + branch circuits combined: Maximum 5% voltage drop
These are recommendations, not mandatory requirements, for most installations

How do you calculate voltage drop?

The voltage drop formula is:

Single-Phase: Vd = 2 × K × I × L / CM
Three-Phase: Vd = 1.732 × K × I × L / CM
DC: Vd = 2 × K × I × L / CM
Where K = resistivity (12.9 for copper, 21.2 for aluminum), I = current in amps, L = one-way length in feet, CM = circular mils

What wire size do I need to reduce voltage drop?

To reduce voltage drop, use larger gauge wire (smaller AWG number). For example, upgrading from 14 AWG to 12 AWG reduces voltage drop by approximately 30%. For long runs, consider increasing wire size by 2-3 gauges to stay within recommended limits.

Does voltage drop affect LED lights?

Yes, LED lights are particularly sensitive to voltage drop. LEDs may flicker, dim, or fail to start with voltage drops as low as 5-10%. For LED lighting circuits, 3% is recommended, and for long runs, consider 2% or less.

What is the formula for voltage drop?

The basic voltage drop formula is: Vd = I × R, where I is current in amps and R is the total resistance of the wire (both directions). For practical calculations, use the formulas above with appropriate constants for your system.

What causes voltage drop?

Voltage drop is caused by:

Long wire distances
Small wire gauge (high resistance)
High current loads
Poor connections
Aluminum wire (higher resistance than copper)

Is voltage drop the same for AC and DC?

The calculation method is similar, but AC circuits may have additional factors like power factor and skin effect. For most residential applications (under 100 amps), DC calculations are sufficient. For long AC runs, consider power factor in your calculations.