Motor Feeder Voltage Drop Calculator

A motor feeder must deliver adequate terminal voltage both while motors are running and during the highest expected motor-starting event. The Motor Feeder Voltage Drop Calculator calculates the feeder voltage drop, end voltage, and approximate running conductor loss from the known feeder resistance and load current.

The primary result is the start-event end voltage. It represents the approximate voltage remaining at the feeder load end after one motor’s normal running current is replaced by its starting current. That value helps screen whether a selected feeder conductor, route length, and voltage system may need further review before final conductor sizing, raceway layout, motor-starting analysis, or equipment coordination.

For a feeder serving multiple motors, normal running voltage drop can appear acceptable while the voltage depression during across-the-line motor starting becomes excessive. The calculation separates those two operating conditions:

• Running feeder voltage drop uses the full Aggregate running current
• Start-event voltage drop uses a revised feeder current that substitutes Starting motor starting current for Starting motor running current

The calculation is useful during preliminary feeder conductor selection, especially where long raceway runs, large motor loads, 480 V distribution, or high-inrush motor starting conditions are present.

Feeder Current During Motor Starting

The feeder does not normally carry every motor’s starting current at once. This calculation models one defined event: one motor begins starting while the other feeder loads remain at their aggregate running condition.

Aggregate running current is the total feeder current before the motor starts. It includes the running current of the motor identified by Starting motor running current.

Starting motor starting current replaces that motor’s running current for the start-event calculation. The calculator determines the revised feeder current as:

\(\displaystyle \text{Start-event current} = \text{Aggregate running current} - \text{Starting motor running current} + \text{Starting motor starting current}\)

This approach prevents the selected motor’s running current from being counted twice.

For example, a feeder may carry 96 A during normal operation, including a 24 A motor. If that motor draws 144 A while starting, the feeder current during the modeled event is:

\(\displaystyle 96\text{ A} - 24\text{ A} + 144\text{ A} = 216\text{ A}\)

The calculator reports this as Start-event feeder current.

Resistance and Three-Phase Voltage Drop

One-way length is the physical feeder route length from the source to the motor load or downstream motor-control equipment. Enter the installed or planned route length in feet, not an assumed round-trip length.

Conductor resistance is entered in ohm/1000 ft. It must match the actual feeder conductor material, AWG or kcmil size, and intended temperature basis. Resistance changes with conductor temperature, so a resistance value based on a different temperature condition can materially change the voltage-drop estimate.

The calculator first determines one-way conductor resistance:

\(\displaystyle \text{One-way resistance} = \frac{\text{One-way length} \times \text{Conductor resistance}}{1000}\)

Phase model selects the voltage-drop multiplier. With Balanced three-phase, the calculator uses a phase multiplier of 1.7321, which is approximately sqrt{3}.

The running calculation is:

\(\displaystyle \text{Running feeder voltage drop} = \text{Phase multiplier} \times \text{Aggregate running current} \times \text{One-way resistance}\)

The starting-event calculation is:

\(\displaystyle \text{Start-event voltage drop} = \text{Phase multiplier} \times \text{Start-event current} \times \text{One-way resistance}\)

End voltage is then:

\(\displaystyle \text{End voltage} = \text{System voltage} - \text{Voltage drop}\)

The voltage-drop percentage is the calculated voltage drop divided by System voltage, expressed as a percentage.

Calculation Example

The following values model a 480 V balanced three-phase motor feeder with a 200 ft one-way route.

Input	Value
Aggregate running current	96 A
Starting motor running current	24 A
Starting motor starting current	144 A
One-way length	200 ft
Conductor resistance	0.321 ohm/1000 ft
System voltage	480 V
Phase model	Balanced three-phase

One-Way Resistance

\(\displaystyle \frac{200\text{ ft} \times 0.321\text{ ohm/1000 ft}}{1000} = 0.0642\text{ ohm}\)

One-way resistance: 0.0642 ohm

Running Condition

Using the 1.7321 three-phase multiplier and 96 A aggregate running current:

\(\displaystyle 1.7321 \times 96\text{ A} \times 0.0642\text{ ohm} = 10.675\text{ V}\)

Running result	Value
Running feeder voltage drop	10.675 V
Running feeder voltage drop	2.224%
Running end voltage	469.325 V

The feeder delivers approximately 469.325 V at the load end during the modeled running condition.

Motor Starting Event

The starting motor replaces 24 A of running current with 144 A of starting current:

\(\displaystyle 96\text{ A} - 24\text{ A} + 144\text{ A} = 216\text{ A}\)

The resulting feeder voltage drop is:

\(\displaystyle 1.7321 \times 216\text{ A} \times 0.0642\text{ ohm} = 24.0187\text{ V}\)

Start-event result	Value
Start-event feeder current	216 A
Start-event voltage drop	24.0187 V
Start-event voltage drop	5.0039%
Start-event end voltage	455.9813 V

During the modeled starting event, the feeder-end voltage is approximately 455.9813 V.

The calculator also reports Approximate running loss of 1024.7977 W. This is a resistance-loss estimate based on the running condition and should not be treated as a complete feeder thermal analysis.

Applying the Result to Feeder Design

A voltage-drop calculation is commonly reviewed alongside feeder ampacity, overcurrent protection, equipment terminals, and raceway design. A conductor can satisfy ampacity requirements but still produce an undesirable motor-starting voltage drop on a long run.

Where the calculated running or start-event end voltage is low, typical design alternatives may include:

• Increasing conductor size from the selected AWG or kcmil size
• Selecting a conductor with lower resistance for the same route
• Shortening the feeder route or relocating distribution equipment
• Revising motor-starting equipment or the starting sequence
• Separating large motor loads onto another feeder
• Confirming the source voltage and upstream impedance with the serving electrical system design

Increasing conductor size changes resistance, but it also affects raceway fill, conductor pulling conditions, bending space, lug compatibility, termination limitations, cost, and available conduit capacity. A feeder redesign must therefore be coordinated with the full installation rather than based on voltage drop alone.

Field Verification

The entered Conductor resistance should represent the actual conductor selected for the installation. Confirm conductor material, AWG or kcmil size, insulation temperature basis, and installed conductor condition before using the result for a design decision.

The current inputs should also be tied to the actual motor and load arrangement:

• Aggregate running current should reflect the feeder current before the modeled start event
• Starting motor running current must be included within the aggregate running total
• Starting motor starting current should represent the expected starting current for that motor and starting method
• System voltage should match the voltage basis used for the feeder review
• Phase model must match the selected voltage-drop multiplier and system arrangement

Motor voltage at the terminals may differ from the calculated feeder-end voltage because the motor branch circuit, disconnect, motor starter, variable-frequency drive, soft starter, transformer, busway, and other downstream components can add impedance or voltage drop.

Calculation Boundary

This worksheet performs motor feeder voltage-drop arithmetic only. It does not evaluate load diversity, simultaneous motor starts, source impedance, transformer impedance, utility flicker, available voltage at the source, motor acceleration, protection coordination, conductor ampacity, adjustment factor, correction factor, current-carrying conductors, terminal rating, insulation temperature rating, raceway fill, conductor sizing, NEC compliance, AHJ requirements, or manufacturer approval.

Final feeder design requires a separate review of conductor ampacity, overcurrent protection, equipment ratings, motor-controller data, available source performance, installation conditions, and applicable code requirements.