Locked Rotor Current Calculator

A motor can draw several times its running current while its rotor is stationary during starting. The Locked Rotor Current Calculator estimates that initial current demand in amperes from the motor’s full-load current and a locked-rotor multiplier.

The resulting Estimated locked-rotor current is useful for an early review of motor branch-circuit equipment, feeder loading, starting voltage drop, motor starter capacity, raceway planning, and utility service limitations. It identifies the magnitude of current that may exist during a locked-rotor or across-the-line starting condition; it does not replace the motor nameplate or manufacturer starting data.

Updated August 22, 2026

Locked-Rotor Current

Locked-rotor current, often called locked-rotor amps or LRA, is the current associated with an energized motor that has not yet reached operating speed. At standstill, the motor has no normal running back electromotive force, so current can be substantially higher than the current measured at full load.

For preliminary motor calculations, the calculator multiplies the stated running-current value by a project-specific starting-current multiplier:

\(\displaystyle \text{Estimated locked-rotor current} = \text{Full-load current} \times \text{Locked-rotor multiplier}\)

The calculation is expressed in amperes. The multiplier is dimensionless and is entered as an x value.

Calculation Inputs

Field	Electrical use
Full-load current	The motor’s running current basis in amperes. Enter measured, nameplate, or calculated full-load current.
Locked-rotor multiplier	The selected multiplier used to estimate locked-rotor current. Enter a screening multiplier from nameplate, manufacturer, or project data.
Starting-current reference	An optional ampere value used for comparison. Enter 0 if no comparison value is being used.

The full-load current input may come from the motor nameplate, field measurement, or a separate motor calculation. For design work, record the source of that value because measured operating current, nameplate full-load current, and calculated full-load current can represent different conditions.

The locked-rotor multiplier should be based on the most applicable available project information. A generic multiplier can support early load screening, but it is not a substitute for the actual motor locked-rotor data, NEMA code letter information, or manufacturer-provided starting characteristics.

Calculation Example

A motor has a Full-load current of 28 A and a Locked-rotor multiplier of 6 x.

\(\displaystyle 28\text{ A} \times 6 = 168\text{ A}\)

Result field	Value
Estimated locked-rotor current	168 A
Full-load current reference	28 A
Locked-rotor multiplier used	6 x
Starting-current reference	140 A
Comparison margin	28 A
Comparison ratio	1.2 x

The estimated locked-rotor current is 168 A. This is the preliminary current value to carry into a motor-starting review. With a Starting-current reference of 140 A, the estimated current is 28 A above the reference and the comparison ratio is 1.2 x.

Motor Starting Review

Locked-rotor current affects equipment differently from normal running current. A motor branch circuit must support normal operation, while the starting event may drive a separate review of voltage drop, starting performance, protective-device coordination, and upstream capacity.

Common uses for the estimated value include:

• Reviewing whether motor starting current could produce excessive voltage drop at the motor terminals.
• Identifying a potentially high inrush contribution when evaluating feeder or service loading.
• Comparing a preliminary starting-current estimate against available starter, controller, generator, transformer, or utility information.
• Flagging installations where conductor length, conductor AWG or kcmil size, raceway route, and source impedance may affect motor acceleration.
• Establishing an early project value before obtaining final motor nameplate, starter, and utility data.

A 168 A starting estimate does not mean the motor branch-circuit conductors are selected as though they continuously carry 168 A. Conductor ampacity, overcurrent protection, disconnecting means, controller selection, terminal ratings, insulation temperature rating, adjustment factor, correction factor, and current-carrying conductor conditions require their own applicable design and code checks.

Field Verification

Verify the calculated estimate against the motor nameplate and manufacturer information before finalizing equipment or installation decisions. Confirm the actual motor voltage, phase, horsepower or kW rating, connection method, controller type, starting method, and driven-load requirements.

The calculator’s arithmetic is limited to the entered full-load current multiplied by the entered locked-rotor multiplier. Final electrical decisions must separately evaluate the motor nameplate, NEMA code letter where applicable, starter data, branch-circuit and feeder requirements, voltage-drop performance, available source capacity, utility requirements, and AHJ-approved installation conditions.

Related calculations: Motor Starting Current Calculator, NEMA Code Letter Calculator, Motor Voltage Drop Calculator.