Motor Circuit Conductor Size Calculator

A motor branch circuit conductor must have sufficient ampacity for the reviewed motor load and the applicable motor-sizing method. This Motor Circuit Conductor Size Calculator calculates the Entered required ampacity from the entered motor full-load current and the entered ampacity factor, then compares that requirement with the Selected conductor ampacity entered for the conductor record.

The primary result is the ampacity basis needed for the motor conductor review:

\(\displaystyle \text{Entered required ampacity} = \frac{\text{Motor FLA} \times \text{Entered ampacity factor}}{100}\)

The worksheet does not select an AWG or kcmil conductor, look up a conductor ampacity table, or determine a final code-compliant motor circuit conductor. It records the reviewed ampacity arithmetic and the margin represented by the selected conductor record.

Motor FLA and Required Ampacity

Motor FLA is the reviewed motor full-load current in amperes. It is the current basis used by the worksheet before applying the Entered ampacity factor.

The Entered ampacity factor is a percentage entered from the reviewed motor method. For example, an entered factor of 125 applies 125 percent of Motor FLA:

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

The resulting Entered required ampacity is the minimum ampacity value used for comparison with the selected conductor record. It can support the documentation stage of motor branch-circuit conductor sizing, feeder load review, equipment schedule review, or a motor control center conductor check.

It does not establish the correct factor by itself. The project’s reviewed motor method must determine the factor entered into the worksheet.

Worksheet Inputs

Input	Electrical purpose
Motor FLA (A)	The reviewed motor full-load current used as the ampacity basis.
Entered ampacity factor (%)	The percentage factor applied to Motor FLA under the reviewed motor method.
Selected conductor ampacity (A)	The ampacity represented by the selected conductor record for comparison against the calculated requirement.
Source reference	A project record identifying the source for the FLA, factor, and conductor ampacity values.

The Source reference does not alter the calculation. It provides a traceable record for the input basis, such as a motor schedule reference, reviewed design record, equipment submittal reference, or conductor schedule entry.

Ampacity Comparison and Margin

After calculating Entered required ampacity, the worksheet compares it with Selected conductor ampacity.

\(\displaystyle \text{Ampacity margin} = \text{Selected conductor ampacity} - \text{Entered required ampacity}\)

\(\displaystyle \text{Ampacity margin percent} = \frac{\text{Ampacity margin}} {\text{Entered required ampacity}} \times 100\)

A positive margin means the entered conductor ampacity is greater than the entered required ampacity. A zero margin means the two values are equal. A negative margin indicates that the entered conductor record is below the calculated ampacity requirement and requires review before the record is used for a motor circuit decision.

The percentage margin expresses the same difference relative to the calculated requirement. It is not an adjustment factor, correction factor, voltage-drop percentage, or spare-capacity determination.

Calculation Example

Use the following entered values:

Field	Entered value
Motor FLA	28 A
Entered ampacity factor	125%
Selected conductor ampacity	40 A
Source reference	motor-record-a

Step 1: Calculate entered required ampacity

\(\displaystyle \text{Entered required ampacity} = 28\text{ A} \times \frac{125}{100}\)

\(\displaystyle \text{Entered required ampacity} = 35\text{ A}\)

\(\displaystyle \text{Ampacity margin} = 40\text{ A} - 35\text{ A}\)

\(\displaystyle \text{Ampacity margin} = 5\text{ A}\)

\(\displaystyle \text{Ampacity margin percent} = \frac{5\text{ A}}{35\text{ A}} \times 100\)

\(\displaystyle \text{Ampacity margin percent} = 14.2857\%\)

For this record, the worksheet result is:

• Entered required ampacity: 35 A
• Selected conductor ampacity: 40 A
• Ampacity margin: 5 A
• Ampacity margin: 14.2857%
• Record comparison: Source recorded

The result documents that the entered 40 A conductor ampacity exceeds the calculated 35 A requirement by 5 A, based solely on the entered values.

Conductor Selection Review

The calculated required ampacity is an input to conductor selection, not a conductor-size answer. A field or design review may still need to establish the actual conductor material, AWG or kcmil size, insulation temperature rating, conductor ampacity, and terminal rating.

A conductor record can require separate verification for conditions including:

• Applicable conductor ampacity table values and terminal limitations
• Ambient-temperature correction factors
• Adjustment factors for current-carrying conductors in a raceway or cable
• Raceway fill and physical conductor installation requirements
• Motor branch-circuit, feeder, or combination-load rules
• Equipment terminal ratings and manufacturer instructions
• Available fault current, overcurrent protective device coordination, and motor protection
• Conductor length and voltage-drop review
• Local amendments, project specifications, and AHJ requirements

For example, a conductor ampacity entered as 40 A may represent a selected conductor record under a specific temperature-column and installation basis. The worksheet does not test whether that entered ampacity remains valid after ambient correction, conductor bundling, terminal limitations, or other installation conditions.

Field Verification

Use the calculated ampacity basis with the project’s reviewed conductor-selection process. Confirm that the Selected conductor ampacity reflects the actual installation conditions and that the Source reference identifies the record supporting the Motor FLA, entered factor, and conductor ampacity.

The worksheet boundary is limited to arithmetic: it does not provide a conductor table, conductor size, temperature adjustment, correction factor, voltage-drop calculation, raceway-fill calculation, or final motor design result. Final conductor and motor-circuit decisions remain subject to the applicable electrical code, equipment documentation, engineered design, and AHJ acceptance.