Motor Feeder Calculator

A motor feeder serving more than one motor must account for the running current of all entered motors plus an added contribution based on the largest motor. The Motor Feeder Calculator produces a Feeder screen current from the entered motor schedule and the selected largest-motor factor.

That result is useful during early feeder load review, preliminary conductor sizing, raceway planning, voltage-drop evaluation, and coordination of a multi-motor lineup. It provides a transparent current value that can be carried into the next design step, where conductor ampacity, terminal ratings, insulation temperature rating, correction factors, adjustment factors, overcurrent protection, and equipment requirements are evaluated.

The worksheet does not select a feeder conductor, AWG size, kcmil size, breaker, fuse, disconnect, raceway, or final motor-system configuration.

Motor Schedule Inputs

Each motor is entered as a separate motor row. The calculation uses the numerical value in Running current (A) for every row.

Field	Electrical use
Motor label	Identifies the motor row in the schedule, such as “Motor A.” It does not affect the arithmetic.
Running current (A)	The entered operating current for that motor. Every motor-row current is included in Total running current.
Entered largest-motor factor (%)	The percentage applied to the largest entered motor running current.
Source reference	Records the source used for the motor schedule and factor, such as a motor schedule, design record, or project reference.

The Entered largest-motor factor (%) is entered by the user rather than selected by the calculator. It must match the method, project basis, and applicable electrical requirements being used for the installation. The calculator does not determine whether 125% or any other factor is appropriate.

A complete motor schedule should identify every motor served by the feeder. Omitting a motor reduces both the total running-current value and, potentially, the largest-motor value.

Feeder Screen Formula

The worksheet applies the following arithmetic:

\(\displaystyle \text{Total running current} = \sum \text{motor running currents}\)

\(\displaystyle \text{Largest motor contribution} = \text{largest running current} \times \frac{\text{Entered largest-motor factor}}{100}\)

\(\displaystyle \text{Feeder screen current} = \text{Total running current} + \text{largest motor contribution}\)

The Largest motor running current is the highest individual value entered under Running current (A). The largest-motor factor is then applied only to that one motor current.

Because the total already includes the full running current of every motor, including the largest motor, the calculation adds the factor-based contribution to the total rather than replacing the largest motor’s current.

Calculation Example

For one entered motor row:

Input or result	Value
Motor label	Motor A
Running current (A)	28 A
Entered largest-motor factor (%)	125%
Total running current	28 A
Largest motor running current	28 A

The largest-motor contribution is:

\(\displaystyle 28\text{ A} \times \frac{125}{100} = 35\text{ A}\)

The feeder screen current is:

\(\displaystyle 28\text{ A} + 35\text{ A} = \mathbf{63\text{ A}}\)

The worksheet result is therefore Feeder screen current: 63 A.

For a multi-motor feeder, the same sequence applies: sum all entered running currents, identify the single highest motor current, calculate its factor-based contribution, and add that contribution to the total.

Using the Result in Feeder Design

The Feeder screen current can be used as an input to later engineering or field review, including:

• Preliminary feeder conductor ampacity screening in AWG or kcmil.
• Raceway routing and preliminary conduit layout for a multi-motor feeder.
• Voltage-drop review using actual feeder length, conductor material, conductor size, circuit arrangement, and operating current.
• Identification of the feeder current basis to be checked against termination limitations, equipment ratings, and available conductor ampacity.
• Comparison of the motor schedule against panel, MCC, switchboard, or distribution equipment loading assumptions.
• Documentation of the motor-current arithmetic before final feeder and branch-circuit decisions are made.

The result is not a conductor ampacity result. A conductor selected from an ampacity table can require further evaluation for terminal rating, insulation temperature rating, ambient-temperature correction factor, adjustment factor for current-carrying conductors, installation method, raceway fill, and voltage drop. Those conditions are outside the entered fields and are not calculated here.

Field Verification

Verify the Running current (A) values against the project motor schedule or the selected design basis before relying on the calculation. Motor nameplate amperes, full-load current values, actual measured current, and design schedule values are not interchangeable unless the governing design method specifically permits their use.

Also verify that the motor list reflects the actual feeder scope. A feeder may supply motors with different duty cycles, starting methods, operating modes, or control arrangements. Those characteristics can affect final feeder, overcurrent, service, and motor-system decisions even though they do not change this worksheet’s arithmetic.

The worksheet boundary is limited to entered motor-current arithmetic: it makes no feeder-conductor, service, overcurrent protective device, demand, motor-starting, voltage-drop, raceway-fill, or final code-compliance determination. Final installation decisions require the applicable electrical code, equipment documentation, project specifications, and AHJ requirements.