Motor Voltage Drop Calculator

A motor voltage-drop calculation estimates the voltage lost in the branch circuit or feeder conductors between the source and the motor terminals. The calculation produces separate values for running voltage drop and starting voltage drop, then shows the associated percentage drop and remaining voltage at the motor.

The result is used during conductor sizing and motor circuit review to determine whether a selected AWG or kcmil conductor provides acceptable voltage performance over the installed one-way length. Starting conditions require separate attention because locked-rotor or starting current can be several times higher than normal motor running current. A conductor that performs acceptably at running load may produce materially lower motor terminal voltage during acceleration.

For a three-phase motor circuit, the calculator uses the selected Phase model to apply the appropriate voltage-drop multiplier. The supplied workflow uses Balanced three-phase.

Circuit Inputs

Calculator field	Example value	Electrical use
Running current (A)	28 A	Expected motor current after the motor reaches normal operating speed
Starting current (A)	168 A	Expected starting or locked-rotor current
One-way length (ft)	150 ft	Distance from source to motor; the three-phase multiplier accounts for the circuit path used by the formula
System voltage (V)	480 V	Voltage basis for percentage-drop and remaining-voltage results
Phase model	Balanced three-phase	Selects the voltage-drop multiplier
Conductor resistance (ohm/1000 ft)	0.628	Verified resistance of the selected conductor at the applicable temperature basis

Conductor resistance (ohm/1000 ft) is the central conductor property in this calculation. It must correspond to the actual conductor selected for the circuit, including its AWG or kcmil size, conductor material, and temperature basis. The calculator does not select conductor size, establish ampacity, or apply temperature correction or adjustment factors.

Balanced Three-Phase Formula

For the Balanced three-phase phase model, the voltage-drop calculation is:

\(\displaystyle V_D = \sqrt{3} \times I \times R \times \frac{L}{1000}\)

Where:

• \(V_D\) = voltage drop in volts
• \(\sqrt{3}\) = balanced three-phase multiplier
• (I) = circuit current in amperes
• (R) = conductor resistance in ohm/1000 ft
• (L) = One-way length (ft)

The calculator applies the formula twice:

\(\displaystyle V_{D,\text{running}} = \sqrt{3} \times I_{\text{running}} \times R \times \frac{L}{1000}\)

\(\displaystyle V_{D,\text{starting}} = \sqrt{3} \times I_{\text{starting}} \times R \times \frac{L}{1000}\)

It then calculates percentage voltage drop and remaining voltage:

\(\displaystyle \text{Voltage drop \%} = \frac{V_D}{V_{\text{system}}} \times 100\)

\(\displaystyle \text{Remaining voltage} = V_{\text{system}} - V_D\)

Running and Starting Results

Using the stated circuit values:

• Running current (A): 28
• Starting current (A): 168
• One-way length (ft): 150
• System voltage (V): 480
• Phase model: Balanced three-phase
• Conductor resistance (ohm/1000 ft): 0.628

The running-voltage-drop calculation is:

\(\displaystyle \sqrt{3} \times 28 \times 0.628 \times \frac{150}{1000} = 4.5685\text{ V}\)

The starting-voltage-drop calculation is:

\(\displaystyle \sqrt{3} \times 168 \times 0.628 \times \frac{150}{1000} = 27.4107\text{ V}\)

Result	Value
Running voltage drop	4.5685 V
Running voltage drop	0.9518%
Running remaining voltage	475.4315 V
Starting voltage drop	27.4107 V
Starting voltage drop	5.7106%
Starting remaining voltage	452.5893 V

At 28 A, the circuit loses 4.5685 V and leaves an estimated 475.4315 V at the motor. At 168 A starting current, the same conductor path loses 27.4107 V, leaving an estimated 452.5893 V at the motor terminals.

The starting current is six times the running current in this example, so the starting voltage drop is also six times the running voltage drop. With conductor resistance and circuit length held constant, voltage drop changes directly with current.

Conductor and Motor Circuit Review

Use the calculated running and starting values alongside the actual motor circuit design:

• Compare alternate conductor sizes by entering the verified Conductor resistance (ohm/1000 ft) for each AWG or kcmil option.
• Review long branch circuits and feeders before raceway routing, especially where conduit layout increases the source-to-motor distance.
• Evaluate the starting result when reviewing motor acceleration, reduced-voltage starting equipment, motor controllers, or loads with substantial breakaway torque.
• Maintain a consistent resistance temperature basis when comparing conductor options or revising the design.
• Confirm that the selected conductor separately meets required ampacity after applicable adjustment factor and correction factor calculations.

Voltage-drop arithmetic does not establish conductor ampacity. Conductor selection also requires separate review of motor circuit requirements, overcurrent protection, equipment terminal rating, insulation temperature rating, current-carrying conductors, raceway fill, termination conditions, manufacturer instructions, and AHJ requirements.

Field Verification

The result represents resistive conductor voltage drop using the entered current, length, phase model, and conductor resistance. It does not include impedance components, source-voltage variation, transformer impedance, motor power factor, unbalanced loading, conductor splices, connection resistance, or voltage changes caused by other loads operating on the same system.

Verify the actual one-way route rather than relying on straight-line building dimensions. Use the installed circuit path through the raceway layout, including vertical risers and equipment routing. For existing equipment, measured source voltage and motor-terminal voltage during start-up provide the field check for whether the estimated starting voltage remains consistent with actual operating conditions.