Motor Branch Voltage Drop Calculator

A motor branch voltage drop calculation estimates how much voltage is lost in the branch-circuit conductors between the source and motor terminals. The result is used during conductor sizing and circuit review to determine whether the motor will receive adequate voltage during normal operation and, more critically, during starting or locked-rotor current.

The calculator produces both Running voltage drop and Starting voltage drop, along with the corresponding Running terminal voltage and Starting terminal voltage. It uses the entered conductor resistance rather than selecting a conductor from an ampacity table. That makes it useful when the installed or proposed conductor resistance, circuit length, motor current, system voltage, and phase arrangement are already known.

For motor work, starting voltage drop can govern the practical result even when normal running voltage drop is low. A conductor may supply a motor satisfactorily at running current but impose enough impedance during acceleration to reduce motor terminal voltage, starting torque, or the likelihood of a successful start under load.

Branch-Circuit Voltage Drop Inputs

The calculation uses the following entered electrical values.

Input	Electrical use
Running current (A)	Motor current used to calculate normal operating voltage drop and approximate running conductor loss
Starting current (A)	Motor starting or locked-rotor current used to calculate starting voltage drop and approximate starting conductor loss
One-way length (ft)	Distance from the supply point to the motor; the phase model applies the appropriate conductor-path multiplier
Conductor resistance (ohm/1000 ft)	AC/DC resistance basis entered for the selected conductor and temperature basis
System voltage (V)	Nominal voltage used to calculate terminal voltage and percentage voltage drop
Phase model	Selects the voltage-drop multiplier: 2 for single-phase two-conductor circuits or \sqrt{3} for balanced three-phase circuits

One-way length is not the total conductor path. The calculator derives the effective voltage-drop path through the Phase multiplier. For a single-phase two-conductor circuit, current travels out on one conductor and returns on the other, so the multiplier is 2. For a balanced three-phase circuit, the calculator uses \sqrt{3}, approximately 1.7321.

The entered Conductor resistance (ohm/1000 ft) should match the conductor actually under review, including the intended conductor material, AWG or kcmil size, and temperature basis. Resistance changes with conductor temperature, so a cold-resistance value may understate voltage drop in a loaded installation.

Voltage-Drop Calculation

The calculator first determines the resistance of one conductor over the entered one-way distance:

\(\displaystyle \text{one-way resistance} = \frac{\text{one-way length} \times \text{conductor resistance}}{1000}\)

It then selects the phase multiplier:

\text{phase multiplier} =
\begin{cases}
2 & \text{single-phase two-conductor} \\
\sqrt{3} & \text{balanced three-phase}
\end{cases}

Running and starting voltage drop are calculated separately:

\(\displaystyle \text{running voltage drop} = \text{phase multiplier} \times \text{running current} \times \text{one-way resistance}\)

\(\displaystyle \text{starting voltage drop} = \text{phase multiplier} \times \text{starting current} \times \text{one-way resistance}\)

The resulting motor terminal voltage is:

\(\displaystyle \text{terminal voltage} = \text{system voltage} - \text{voltage drop}\)

The calculator also reports each voltage drop as a percentage of System voltage:

\(\displaystyle \text{voltage drop percentage} = \frac{\text{voltage drop}}{\text{system voltage}} \times 100\)

Approximate running loss and Approximate starting loss estimate conductor heating associated with the entered current and resistance. These values help compare branch-circuit alternatives, but they do not represent the motor’s input power, output power, or actual operating efficiency.

Calculation Example

For a balanced three-phase motor branch circuit, enter:

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

The one-way conductor resistance is:

\(\displaystyle \frac{150 \times 0.628}{1000} = 0.0942\ \text{ohm}\)

For the selected phase model:

\(\displaystyle \text{phase multiplier} = \sqrt{3} = 1.7321\)

Running voltage drop is:

\(\displaystyle 1.7321 \times 28 \times 0.0942 = 4.5685\ \text{V}\)

The calculator reports:

• Running voltage drop: 4.5685 V
• Running voltage drop: 0.9518%
• Running terminal voltage: 475.4315 V

At starting current:

\(\displaystyle 1.7321 \times 168 \times 0.0942 = 27.4107\ \text{V}\)

The starting results are:

• Starting voltage drop: 27.4107 V
• Starting voltage drop: 5.7106%
• Starting terminal voltage: 452.5893 V

The same conductors that lose about 4.57 V at 28 A lose about 27.41 V at 168 A because starting current is six times the running current. The motor’s calculated terminal voltage falls from 475.4315 V while running to 452.5893 V during the stated starting condition.

Using the Result for Motor Circuits

Use the voltage-drop result when comparing practical branch-circuit options such as:

• Increasing conductor size from a smaller AWG or kcmil conductor to a larger conductor with lower resistance.
• Shortening the route between the motor controller, disconnect, transformer, panelboard, or motor.
• Reviewing whether the raceway route adds avoidable distance through a building or process area.
• Comparing starting methods or expected starting current where motor starting performance is a concern.
• Evaluating terminal voltage at a remote motor before finalizing a feeder and branch-circuit arrangement.
• Checking whether the voltage at the motor remains acceptable during acceleration, especially where the motor starts against load.

Lower conductor resistance reduces both running and starting voltage drop. Since the result is proportional to current, a motor with high locked-rotor current can have a materially different starting-voltage result than a motor with similar full-load running current.

Voltage drop is separate from conductor ampacity. A conductor selected for required ampacity may still need to be increased in size to reduce voltage drop. Conversely, a larger conductor selected for voltage-drop performance still requires separate verification of overcurrent protection, equipment terminations, conductor insulation temperature rating, adjustment factor, correction factor, raceway fill, grounding and bonding requirements, and installation constraints.

Field Verification

This calculation models branch-circuit conductor resistance using the entered length, current, voltage, and phase model. It does not include source impedance, transformer impedance, utility characteristics, motor-controller voltage drop, contact resistance, feeder voltage drop, conductor reactance, unbalanced loading, power factor effects, or changes in motor current during acceleration.

Use the result as motor branch-circuit arithmetic, then separately verify the complete installation. Confirm the actual conductor material and size, installed route length, termination locations, ambient conditions, number of current-carrying conductors, applicable ampacity adjustments and corrections, terminal rating, equipment listing instructions, motor data, overcurrent protection, disconnecting means, and AHJ requirements.

For a field investigation, voltage measured at the motor terminals during a start can differ from the calculated Starting terminal voltage because the real electrical system includes upstream conductors and source impedance not represented by this calculation.