Industrial Feeder Capacity Calculator

An industrial feeder capacity calculation converts a connected load in volt-amperes into an estimated line current, then compares that current with an entered feeder rating. The primary result is Estimated feeder current, which supports an early load review before conductor sizing, raceway planning, voltage-drop evaluation, or equipment coordination begins.

For a balanced three-phase 480 V feeder, the calculation divides adjusted VA by the three-phase voltage relationship. The resulting amperes can be compared with a preliminary feeder capacity to identify available margin or a possible overload condition.

This calculation is useful when reviewing connected plant loads such as process equipment, panelboards, distribution equipment, lighting, HVAC loads, and other utilization equipment supplied from a common feeder.

Feeder Load Inputs

The calculator applies the entered Demand factor directly to the entered Connected load.

Field	Electrical use
Feeder capacity (A)	Entered ampere rating used only for comparison with the calculated feeder current.
Connected load (VA)	Total apparent connected load assigned to the feeder before the entered demand factor is applied.
Demand factor (%)	Explicit percentage used to reduce connected VA to the adjusted feeder load.
System voltage (V)	Nominal supply voltage used to convert adjusted VA to current.
Phase model	Selects the voltage relationship used by the calculation. The balanced three-phase model uses a multiplier of 1.7321.

The Feeder capacity (A) field is not a conductor ampacity selection, breaker rating determination, or equipment nameplate rating verification. It is the comparison value used to calculate remaining capacity and utilization.

Connected VA and demand factor should represent the same defined load group. Applying a demand factor to a partial load list while comparing it with a feeder capacity intended for a larger distribution system can produce a misleading utilization result.

Balanced Three-Phase Feeder Current

With Phase model set to Balanced three phase, the calculator uses the square root of three relationship:

\(\displaystyle \text{Adjusted feeder load} = \text{Connected load} \times \frac{\text{Demand factor}}{100}\)

\(\displaystyle \text{Estimated feeder current} = \frac{\text{Adjusted feeder load}} {\text{Phase multiplier} \times \text{System voltage}}\)

For the balanced three-phase selection:

\(\displaystyle \text{Phase multiplier} = 1.7321\)

\(\displaystyle I = \frac{VA}{\sqrt{3} \times V}\)

The calculation assumes the load is balanced across the three phases. A calculated feeder current of 144 A does not establish that each phase conductor will carry 144 A in an actual installation. Significant phase imbalance, nonlinear loads, harmonic current, or separately connected line-to-neutral loads require a more detailed load review.

Capacity Comparison

The calculator produces four results:

Result	Meaning
Adjusted feeder load	Connected VA after the entered demand factor has been applied.
Estimated feeder current	Calculated line current based on adjusted VA, system voltage, and phase model.
Remaining capacity	Entered Feeder capacity (A) less the estimated feeder current.
Utilization	Estimated feeder current expressed as a percentage of entered feeder capacity.
Phase multiplier	Multiplier used by the selected phase model; 1.7321 for balanced three phase.

The utilization equation is:

\(\displaystyle \text{Utilization} = \frac{\text{Estimated feeder current}} {\text{Feeder capacity}} \times 100\)

A positive Remaining capacity indicates that the calculated current is below the entered comparison capacity. A negative result indicates that calculated current exceeds that entered value. Neither result alone determines whether a feeder is acceptable for installation or modification.

Calculation Example

Enter the following early feeder-screening values:

Field	Entered value
Feeder capacity (A)	225 A
Connected load (VA)	150,000 VA
Demand factor (%)	80%
System voltage (V)	480 V
Phase model	Balanced three phase

First, apply the demand factor:

\(\displaystyle 150{,}000\ VA \times \frac{80}{100} = 120{,}000\ VA\)

Adjusted feeder load = 120,000 VA

Next, calculate the estimated three-phase feeder current:

\(\displaystyle \frac{120{,}000} {1.7321 \times 480} = 144.3376\ A\)

Estimated feeder current = 144.3376 A

Then compare that current to the entered 225 A feeder capacity:

\(\displaystyle 225\ A - 144.3376\ A = 80.6624\ A\)

Remaining capacity = 80.6624 A

Finally, calculate utilization:

\(\displaystyle \frac{144.3376\ A}{225\ A} \times 100 = 64.15\%\)

Utilization = 64.15%

For preliminary planning, this result indicates that the entered 225 A capacity exceeds the estimated diversified load by approximately 80.66 A. It does not confirm the size of the feeder conductors, the suitability of a 225 A overcurrent protective device, or the available capacity of upstream service and distribution equipment.

Feeder Design Verification

Use the estimated current as a starting point for the next electrical design decisions:

• Select conductor AWG or kcmil based on the applicable ampacity requirements, conductor material, insulation temperature rating, terminal rating, and installation conditions.
• Apply any required adjustment factor for the number of current-carrying conductors and correction factor for ambient temperature.
• Review whether the feeder includes continuous loads, motor loads, or other loads that require treatment beyond the entered demand factor.
• Verify that the overcurrent protective device, disconnecting means, panelboard, switchboard, and other feeder equipment are suitable for the intended loading.
• Perform a voltage-drop review using feeder length, conductor impedance, load characteristics, and actual installation routing.
• Check raceway fill, conductor pulling conditions, conduit layout, bends, and available termination space after conductor size is established.
• Confirm load distribution and phase balance rather than relying solely on a balanced three-phase current estimate.
• Coordinate the final design with the applicable electrical code, project specifications, manufacturer instructions, utility requirements, and the AHJ.

The calculation boundary is capacity screening only. It does not perform short-circuit analysis, motor-starting analysis, conductor ampacity selection, protective-device sizing, equipment evaluation, or code-compliance determination.