The Voltage Drop Calculator will calculate the voltage drop across a circuit for long wire runs based on voltage, current, phases, conductor, wire size, and circuit distance. It will also calculate the voltage at the load, and the percent voltage drop.

## Voltage Drop Calculator

Enter the information below to calculate voltage drop across a circuit.

Voltage Drop -
Percentage Drop -

Voltage - Enter the voltage at the source of the circuit. Single-phase voltages are usually 115V or 120V, while three-phase voltages are typically 208V, 230V or 480V.

Amperes - Enter the maximum current in amps that will flow through the circuit. For motors, it is recommended to multiply the nameplate FLA by 1.25 for wire sizing.

Conductor - Choose the material used as a conductor in the wire. Common conductors are copper and aluminum.

Phases - Select the number of phases in the circuit. This is typically single-phase or three-phase. For single-phase circuits, three wires are required. For three-phase circuits, four wires are required. One of these wires is a ground wire which can be sized down. To calculate ground wire size, use the Ground Wire Size Calculator.

Wire Size - Choose the size wire in the circuit. Units for wire size are AWG or kcmil.

Distance - Enter the one-way length of the wires in the circuit in feet.

Note: The results of this calculator are based on conductor temperatures of 75°C.

Source: NFPA 70, National Electrical Code, Chapter 9, Table 8

### How to Calculate Voltage Drop

Voltage drop is calculated using the most universal of all electrical laws: Ohm's Law. This states that the voltage potential across the conductor is equal to the current flowing through the conductor multiplied by the total resistance of the conductor. In other words, $Vd = I x R$. A simple formula was derived from Ohm's law to calculate the voltage drop across a conductor. This formula can help you determine voltage drop across a circuit, as well as the size wire gauge you will need for your circuit based on the maximum desired voltage drop. The National Electrical Code states that the voltage drop of a feeder circuit must not exceed 5%, and the voltage drop of a branch circuit must not exceed 3%.

##### Single-Phase Circuits

Voltage drop is calculated for single-phase circuits as follows:

 Vd = 2 x K x L x I Cm

Vd = Voltage Drop

I = Current in Conductor (Amps)

L = One-way Length of Circuit (Feet)

Cm = Cross Section Area of Condcutor (Circular Mils)

K = Resistance in ohms of 1 circular mil foot of conductor.
Note: K = 12.9 for Copper conductors at 75°C (167°F), and K = 21.2 for Aluminum conductors at 75°C (167°F).

##### Three-Phase Circuits

Voltage drop is calculated for three-phase circuits as follows:

 Vd = 1.73 x K x L x I Cm

Vd = Voltage Drop

I = Current in Conductor (Amps)

L = One-way Length of Circuit (Feet)

Cm = Cross Section Area of Condcutor (Circular Mils)

K = Resistance in ohms of 1 circular mil foot of conductor.
Note: K = 12.9 for Copper conductors at 75°C (167°F), and K = 21.2 for Aluminum conductors at 75°C (167°F).

To calculate the maximum distance of a circuit based on percent voltage drop, use the Circuit Distance Calculator.

To calculate wire size for a circuit, use the Wire Size Calculator or the Advanced Wire Size Calculator. To calculate wire ampacity for a circuit, use the Wire Ampacity Calculator or the Advanced Wire Ampacity Calculator.