Voltage Drop Calculator

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:

Phases:

Amperes:

Wire size:

Conductor:

Distance:

Voltage Drop	-
Voltage at Load	-
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.

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