Soft Starter Bypass Calculator

A soft starter bypass calculator compares the motor full-load ampere value, running current, service-factor reference, and entered bypass contactor or bypass-path rating. The primary output is the service-factor reference current, followed by the available bypass rating margin and bypass utilization.

After the motor reaches speed and the soft starter transitions to bypass, the bypass contactor or bypass path carries the motor’s running current. The calculation provides a fast current comparison for equipment review, motor-control layout, feeder and branch-circuit load review, and early coordination of bypass components with the motor nameplate current.

The calculator does not select a soft starter, conductor, overload relay, protective device, or SCCR rating. Those decisions require the actual motor-control assembly details, conductor ampacity review, terminal ratings, available fault current, duty requirements, and applicable code review.

Calculation Inputs

Input	Electrical purpose
Motor FLA (A)	Motor full-load amps used as the base current value. This should be the intended motor full-load current reference for the comparison.
Running load (% FLA)	Expected steady operating load expressed as a percentage of Motor FLA. A motor operating at 90% of an 80 A FLA reference has an expected running current of 72 A.
Service factor reference (x)	Multiplier used to establish a comparison current above the expected running current. It is a reference value for this bypass-current comparison, not a substitute for a manufacturer’s stated bypass requirement.
Entered bypass rating (A)	Candidate bypass contactor or bypass-path ampere rating entered for comparison only.

Motor FLA is not necessarily the same as measured operating current. Actual current changes with shaft load, voltage, phase balance, motor condition, driven equipment, and operating conditions. The Running load input provides the expected load point used for the calculation rather than assuming the motor continuously operates at full-load current.

Bypass Current Formula

The calculation begins with expected running current:

\(\displaystyle \text{Running current} = \text{Motor FLA} \times \frac{\text{Running load}}{100}\)

The calculator then applies the Service factor reference:

\(\displaystyle \text{Service-factor reference current} = \text{Motor FLA} \times \text{Service factor reference}\)

Bypass rating margin is the difference between the entered bypass rating and the service-factor reference current:

\(\displaystyle \text{Bypass rating margin} = \text{Entered bypass rating} - \text{Service-factor reference current}\)

Bypass utilization expresses the comparison current as a percentage of the entered bypass rating:

\(\displaystyle \text{Bypass utilization} = \frac{\text{Service-factor reference current}} {\text{Entered bypass rating}} \times 100\%\)

A nonnegative margin and utilization at or below 100% produce the status “Entered bypass rating at or above reference.” A negative margin or utilization above 100% indicates that the entered bypass rating is below the calculator’s reference current.

Calculation Example

Use the following entered values:

Field	Value
Motor FLA	80 A
Running load	90% FLA
Service factor reference	1.15
Entered bypass rating	100 A

First, calculate expected running current:

\(\displaystyle 80\text{ A} \times 0.90 = 72\text{ A}\)

Then apply the Service factor reference:

\(\displaystyle 80\text{ A} \times 1.15 = 92\text{ A}\)

The Service-factor reference current is 92 A. The entered 100 A bypass rating is then compared with that reference:

\(\displaystyle 100\text{ A} - 92\text{ A} = 8\text{ A}\)

\(\displaystyle \frac{92\text{ A}}{100\text{ A}} \times 100 = 92\%\)

Result	Value
Running current	72 A
Service-factor reference current	92 A
Bypass rating margin	8 A
Bypass utilization	92%
Bypass note	Entered bypass rating at or above reference

The result shows that the entered 100 A bypass rating exceeds the calculator’s 92 A comparison current by 8 A. It does not establish that a 100 A bypass device is acceptable for a particular starter assembly.

Use in Motor-Control Review

A bypass rating comparison is normally one part of a larger motor-control review. The result can help identify whether an entered bypass contactor or bypass path is plainly undersized relative to the selected comparison basis before final component selection.

It can also support early design coordination when reviewing:

• Motor starter bucket layouts and motor-control-center lineups.
• Bypass contactor ampere ratings and manufacturer utilization categories.
• Motor branch-circuit conductor ampacity and terminal limitations.
• Raceway fill and conduit routing where motor branch circuits are being grouped.
• Feeder load calculations serving multiple motor controllers.
• Voltage-drop review for long motor branch circuits and feeders.
• Thermal conditions affecting conductors, enclosures, terminals, and control equipment.

A favorable bypass result does not determine AWG or kcmil conductor size. Conductor selection may require ampacity adjustment factors for current-carrying conductors, ambient-temperature correction factors, insulation temperature ratings, termination temperature limits, equipment listing requirements, and voltage-drop evaluation.

Equipment and Code Limits

The calculation compares current values only. It does not account for soft-starter manufacturer instructions, bypass contactor duty classification, motor starting duty, acceleration time, jogging or plugging duty, repeated starts, enclosure temperature rise, overload protection, short-circuit and ground-fault protection, or the assembly SCCR.

The Service factor reference (x) is an entered comparison multiplier. It should not be treated as the motor nameplate service factor, an overload setting instruction, or a universal bypass-contactor sizing rule unless the applicable manufacturer documentation specifically establishes that basis.

Final bypass-path selection should be verified against the listed soft starter or motor controller, bypass component ratings, motor data, conductor and terminal ampacity, available fault current, protective-device coordination, and project requirements. The AHJ and the adopted electrical code govern the installation decision.