Part 4 – Calculating Locked Rotor Current (LRC) for Motors

How to Determine the Value of Locked-Rotor Current (RLC) for Induction Motors?

What is Locked-Rotor Current (RLC)?

Locked-rotor current (LRA) is the maximum starting current drawn by an electric motor exceeding the motor’s nameplate FLA during starting or normal running operations. This happens when power is first applied, and the rotor is not yet turning or when it carries more load than the designed HP rating of the motor. This occurs because, at startup, the motor experiences zero back EMF (Electromotive Force), causing it to draw a high inrush current.

The high starting inrush current is known as locked rotor current (RLC) and must be considered while designing motor circuits, overload protection and counter the voltage drop issues.

LRA is typically 6 to 8 times the motor’s full-load current (FLC) for typical induction motors. It occurs only during startup and lasts for a short duration until the motor reaches its rated speed. The high inrush current impacts circuit protection design, requiring proper sizing of breakers, fuses, and overload devices. That’s why we need to install a starter with a motor.

Determining the Value of LRC

Locked-rotor current is determined using NEC Table 430.7(B) based on the motor’s marking letter and associated values. It is used for sizing motor starters and contactors to handle inrush current. The determined LRA value is also used to select appropriate overcurrent protection devices to prevent nuisance tripping.

Locked-rotor current markings and indicating codes are printed on the motor nameplate. The estimated value of LRC is calculated using Tables 430.7(B), with selection of disconnecting means as per 430.251(A), and 430.251(B) for different motor types.

The locked-rotor current for three phase motor can be calculated using the formula:

Locked Rotor Current (A) = Code Letter kVA/HP × 1000 ÷ (Voltage × √3)

Table 430.7(B) – Locked-Rotor Indicating Code Letters

The code letters “A” through “V” in Table 430.7(B) are used to estimate the locked-rotor current (LRC) for motors. Each letter corresponds to a specific multiplier factor that helps determine the motor’s inrush current.

For example, code letter “F” indicates a multiplier factor of 5.0–5.59. This means the motor will have 5.0–5.59 kVA/HP (kilovolt-amperes per horsepower) when locked.

To illustrate, consider a 7.5 HP, 460V, three-phase motor:

• Using the multiplier range, the LRC falls between 47A and 56.39A.

Now, let’s go through a solved example to determine the minimum and maximum locked-rotor current values for a three-phase induction motor.

Example: Calculating RLC Value of Motors

Calculate the Locked-rotor current for a 15 HP motor operated at 460V, three phase supply where the marking for RLC on its name plate is “D”.

1 – Determine the KVA rating per HP for RLC

Based on the Table 430.7(B), the locked-rotor current (RLC) for letter “D“:

RLC = 4.0-4.49 per kVA/HP.

2- Calculate the Estimated Value of RLC

Minimum RLC

Using the power formula of three phase circuits (Based on I = P ÷ V)

P = √3 × V × I × Cos Ф

Consider the value of power factor (Cos Ф) = 1; and √3 = 1.732

I_LRC = HP × kVA per HP ÷ (√3× V)

Putting the values:

I_LRC = (15HP × 4kVA) ÷ (1.732 × 460V)

I = 75.3 A

Maximum RLC

Using a max. multiplier of 5.59KVA for marking “F”,

I = (15HP × 4.49kVA) ÷ (1.732 × 460V)

I = 84.5 A.

3 – Determine the FLC (Optional)

The full load current for three phase motors can be determined using Table 430.250.

• FLC of 15HP at 460V = 21A

Based on the above calculations, a 15 HP, 460V, three-phase motor with a code letter “F” has a full-load current (FLC) of 21A. The minimum locked-rotor current (LRC) is 75.3A, and the maximum LRC is 84.4A.

Series Overview: Motor Circuit Calculations

• Part 1 – Motor Load Circuits: NEC Terms and Basic Terminologies
• Part 2 – NEC Requirements for Motor Circuits
• Part 3 – Understanding NEMA Motor Nameplate Data
• Part 4 – Calculating Locked Rotor Current (LRC) for Motors … (You are Here)
• Part 5 – Sizing Motor Branch Circuit Conductors
• Part 6 – Sizing Motor Feeder Conductors
• Part 7 – Sizing Motor Overcurrent Protection and OCPD’s Devices
• Part 8 – Sizing Motor Feeder Protection
• Part 9 – Sizing Motor Overload Protection
• Part 10 – Overcurrent Protection for Motor Control Circuits
• Part 11 – Sizing Disconnecting Means for Motor & Controller
• Part 12 – Sizing Motor Starter & Contactor – NEMA – NEC
• Part 13 – Sizing Direct Online (DOL) Starters/Contactors for Motors (*Bonus)
• Part 14 – Sizing Star-Delta Motor Starters/Contactors for Motors (*Bonus)

Resources & Tutorials:

• Cable Size Calculation for LT & HT Motors
• Three Phase Motor Power & Control Wiring Diagrams
• What is Motor Efficiency & How to improve it?
• Star Delta 3-phase Motor Automatic starter with Timer
• How to Run a Three-Phase Induction Motor on a Single-Phase Power Supply?
• How to Wire ST01 Timer with Relay & Contactor for 120V/240V Motors?
• How to Wire Twin Timer in Repeat Cycle & One-Shot Mode for 120V/240V Motors?
• How to Control a Single-Phase Motor from Multiple Locations?
• Automatic & Manual Control of Motor Using VFD & DOL Starter
• How to Reverse and Forward a Three-Phase Motor Using a VFD?
• How to Wire a VFD with Motor, PLC, Switches & External Devices
• Sequential Motor Control Circuit Using LOGO! V8 PLC
• Automatic Reverse Forward Motor Control Circuit Using Delta – DVP-14SS PLC
• Reverse Forward Motor Control Circuit Using PLC – ZEN Programming Relay
• Motor Protection – Types of Faults and Protection Devices