Commercial Feeder Voltage Drop Calculator

A commercial feeder voltage-drop calculation estimates how much source voltage is lost in the feeder conductors between the supply and the downstream distribution equipment or load. The primary result is Estimated voltage drop in volts and percent, followed by the estimated end voltage and resistive conductor loss.

This calculation is useful during preliminary feeder design when comparing conductor bases, evaluating long raceway routes, reviewing voltage available at distribution equipment, or checking whether a motor, transformer, panelboard, or other load will receive adequate voltage. It is also useful when a feeder route changes because of structural obstructions, routing around equipment, or a longer-than-expected conduit path.

The calculation uses an explicit Conductor resistance (ohm/1000 ft) value. It does not select an AWG or kcmil conductor, determine ampacity, account for adjustment factors or correction factors, verify raceway fill, or approve an installation under the NEC.

Related feeder checks: Voltage Drop Calculator, Wire Size Calculator, and Feeder Load Calculator.

Feeder Voltage-Drop Inputs

Input	Electrical use
Feeder current (A)	The calculated or expected feeder current used for the voltage-drop estimate
One-way length (ft)	Distance from the source to the load or downstream equipment; the calculator applies the appropriate circuit-path multiplier
Conductor resistance (ohm/1000 ft)	Resistance basis of the proposed conductor, expressed in ohms per 1,000 feet
System voltage (V)	Nominal voltage used to express voltage drop as a percentage and calculate end voltage
Phase model	Selects single-phase or balanced three-phase voltage-drop arithmetic

The resistance input should represent the conductor basis actually being reviewed. Resistance varies with conductor material, conductor size, temperature, and whether the value is AC or DC resistance. A conductor’s ampacity and its resistance are related to size but are not interchangeable design values.

For a commercial feeder, the current entered should reflect the load condition being evaluated. That may be a calculated feeder load, a measured operating current, or a design current for a specific load condition. Where a feeder supplies motors, nonlinear loads, transformers, or mixed utilization equipment, voltage-drop review should use a load value that represents the condition of concern rather than an arbitrary nameplate total.

Balanced Three-Phase Calculation

For the Balanced three-phase phase model, the calculator uses:

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

\(\displaystyle V_D = \sqrt{3} \times I \times R_{one-way}\)

Where:

• V_D = Estimated voltage drop, in volts
• I = Feeder current, in amperes
• R_{one-way} = One-way conductor resistance used, in ohms
• sqrt{3} = 1.7321, the phase multiplier for balanced three-phase arithmetic

The percentage voltage drop is:

\(\displaystyle \%V_D = \left(\frac{V_D}{V_{system}}\right) \times 100\)

The calculator then reports estimated end voltage:

\(\displaystyle V_{end} = V_{system} - V_D\)

Estimated resistive loss is calculated as:

\(\displaystyle P_{loss} = 3I^2R_{one-way}\)

This is conductor I^2R loss for a balanced three-phase feeder using the entered resistance basis. It is not a complete feeder-loss model for installations with significant reactance, harmonic current, imbalance, or power-factor-related voltage-drop effects.

Calculation Example

A 480 V commercial feeder has the following inputs:

Field	Value
Feeder current	100 A
One-way length	250 ft
Conductor resistance	0.25 ohm/1000 ft
System voltage	480 V
Phase model	Balanced three-phase

First, the calculator converts the conductor resistance to the one-way feeder length:

\(\displaystyle R_{one-way} = \left(\frac{0.25 \Omega}{1000 ft}\right) \times 250 ft = 0.0625 \Omega\)

For balanced three-phase operation:

\(\displaystyle V_D = 1.7321 \times 100 A \times 0.0625 \Omega = 10.8253 V\)

The resulting voltage drop percentage is:

\(\displaystyle \%V_D = \left(\frac{10.8253}{480}\right) \times 100 = 2.2553\%\)

The estimated end voltage is:

\(\displaystyle 480 V - 10.8253 V = 469.1747 V\)

Estimated resistive loss is:

\(\displaystyle P_{loss} = 3 \times (100 A)^2 \times 0.0625 \Omega = 1875 W\)

The calculator result is therefore:

Result	Value
Estimated voltage drop	10.8253 V
Estimated voltage drop	2.2553%
Estimated end voltage	469.1747 V
One-way resistance used	0.0625 ohm
Estimated resistive loss	1875 W
Phase multiplier	1.7321 x

A 10.8253 V drop from a 480 V source leaves approximately 469.17 V at the load under the entered balanced resistive condition. The result can be compared against the voltage tolerance of connected equipment and used to compare alternative conductor resistance values or feeder lengths.

Feeder Design Use

Voltage drop is commonly reviewed alongside feeder ampacity, overcurrent protection, equipment terminal ratings, and conductor installation conditions. A larger conductor may reduce resistance and voltage drop, but that separate design decision can also affect raceway fill, pulling tension, bending space, lug range, equipment terminals, cost, and available conduit routing.

For long commercial feeders, the calculation can help identify when a route length creates an electrical penalty even though the selected conductor otherwise has adequate ampacity. It can also support comparisons such as:

• Reducing conductor resistance by evaluating a larger AWG or kcmil conductor basis
• Shortening a proposed raceway route
• Moving distribution equipment closer to the load
• Comparing a higher-voltage distribution arrangement
• Reviewing voltage at motor terminals during loaded operation
• Estimating feeder heat loss associated with conductor resistance

A voltage-drop result does not establish conductor ampacity. Ampacity must be determined separately using the applicable conductor insulation temperature rating, termination limitations, ambient-temperature correction factor, adjustment factor for current-carrying conductors, and the installation’s governing electrical requirements.

Field Verification

The estimate assumes the entered feeder current, one-way length, resistance value, and phase model represent the actual circuit. Field conditions can differ from this simplified resistive calculation.

Verify the following separately when finalizing a feeder installation:

• Actual conductor material, AWG or kcmil size, stranding, insulation type, and resistance basis
• Conductor operating temperature and whether the resistance value reflects that temperature
• AC impedance effects, especially on large conductors, long raceways, or installations where reactance is material
• Balanced versus unbalanced three-phase loading
• Power factor and motor starting conditions where terminal voltage performance is critical
• Parallel conductor arrangement and equal-length, equal-impedance installation details
• Feeder ampacity, overcurrent protection, neutral loading, equipment terminal rating, and raceway fill
• Local AHJ requirements, project specifications, and the applicable NEC edition

Use the calculated Estimated voltage drop and Estimated end voltage as an early electrical design check. Complete conductor selection and code compliance require a separate review of ampacity, terminations, installation conditions, and the final feeder configuration.