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Motor Starting Current / Code Letter Calculator

Convert NEC code letters to locked-rotor amps and get starter type recommendations

Free motor code letter to locked-rotor amperage calculator for electricians, controls technicians, and facility engineers who need to determine starting current from a motor nameplate code letter. Enter the motor horsepower, voltage, phase, and code letter (A through V), and the calculator converts the code letter to locked-rotor kVA per horsepower using NEC Table 430.7(B), computes the total locked-rotor kVA, and returns the locked-rotor amperage (LRA). The LRA is critical for sizing branch-circuit short-circuit protection per NEC 430.52, selecting the correct motor starter (NEMA size and type), and evaluating voltage dip on the supply bus during starting. A code letter G motor (the most common for standard NEMA Design B motors) draws 5.6 to 6.29 kVA per horsepower at locked rotor. On a 50 HP, 460V three-phase motor, that translates to roughly 362 to 406 locked-rotor amps, which is approximately 6 to 6.5 times the full-load current. Knowing this value before the motor starts prevents nuisance trips, helps size soft starters or VFDs when inrush is a problem, and allows proper coordination with upstream protective devices.

Pro Tip: If you are seeing nuisance trips on a motor circuit during starting, check the code letter before upsizing the breaker. A code letter H or J motor has significantly higher inrush than the common code G. NEC 430.52(C)(1) Exception No. 2 allows increasing the breaker size to 400% of FLA (inverse time) or 1700% (instantaneous trip) if the standard maximum is not sufficient for the starting current. If that still does not solve the problem, consider a soft starter or VFD, which limits inrush to 150 to 300 percent of FLA regardless of the code letter.

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Motor Starting Current / Code Letter Calculator
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How It Works

  1. Enter Motor Nameplate Data

    Enter the motor horsepower, rated voltage, and phase (single or three-phase). Then select the code letter from the motor nameplate. The code letter is a single letter (A through V) stamped or printed on the nameplate, usually near the design letter and service factor.

  2. Review Locked-Rotor Amperage

    The calculator multiplies the code letter kVA/HP range (from NEC Table 430.7(B)) by the motor horsepower to get locked-rotor kVA, then divides by the voltage (times 1.732 for three-phase) to get locked-rotor amps. Both the minimum and maximum LRA for that code letter range are displayed.

  3. Review Starter Recommendations

    Based on the LRA and motor HP, the calculator suggests appropriate starter types: full-voltage (across-the-line) for smaller motors, reduced-voltage starters (autotransformer, wye-delta, or part-winding) for medium loads, and soft starters or VFDs for large motors or sensitive supply buses.

Assumptions

  • Motor is a standard AC induction type (squirrel-cage or wound-rotor).
  • Code letter kVA/HP ranges are from NEC Table 430.7(B).
  • Voltage is the rated motor voltage at the motor terminals during starting.
  • Starting is across-the-line (full voltage) unless a reduced-voltage method is specified.

Limitations

  • Does not calculate actual starting time or acceleration torque curve.
  • Does not model voltage dip at the motor terminals caused by supply impedance during starting.
  • Does not account for reduced-voltage starting methods (the LRA shown is for full-voltage starting).
  • Does not cover DC motors or synchronous motors.

References

  • NEC (NFPA 70) Table 430.7(B) - Locked-Rotor Indicating Code Letters
  • NEC (NFPA 70) Tables 430.251(A) and 430.251(B) - Conversion Table of Locked-Rotor Currents
  • NEC (NFPA 70) Table 430.52 - Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices
  • NEMA MG 1 - Motors and Generators, Part 12 (Tests and Performance)

Frequently Asked Questions

The code letter indicates the locked-rotor kVA per horsepower that the motor draws when the shaft is stationary and full voltage is applied. NEC Table 430.7(B) assigns kVA/HP ranges to each letter. Code A is 0 to 3.14 kVA/HP (very low inrush), while code V is 22.4 and up (extremely high inrush). Most standard NEMA Design B squirrel-cage motors carry code letter F, G, or H, corresponding to roughly 5 to 7.1 kVA/HP. The code letter is used to calculate locked-rotor amps and to size short-circuit protection devices that must ride through the starting inrush without tripping.
At standstill, the rotor is not turning, so there is no back-EMF (counter electromotive force) to oppose the applied voltage. The motor winding impedance at standstill is mostly just the copper resistance and leakage reactance, which is much lower than the effective impedance during running. The result is that current at standstill is typically 5 to 8 times the full-load running current. As the rotor accelerates, back-EMF builds up and opposes the supply voltage, which progressively reduces the current until it reaches the normal running value at full speed.
NEC 430.52 sets maximum percentages of FLA for different protective device types, but Exception No. 2 allows increasing these values if the motor will not start at the standard maximum. The code letter determines whether you will need those exceptions. A code G motor at 250% FLA breaker sizing may start without issues, but a code J motor at the same breaker size may trip during starting because its inrush is 25 to 40 percent higher. Knowing the code letter before specifying the breaker prevents field callbacks and nuisance trip complaints.
The code letter (A through V) indicates locked-rotor kVA per horsepower and is used for electrical protection sizing. The design letter (A, B, C, or D per NEMA MG 1) describes the motor's torque-speed characteristics: Design A has normal starting torque and high LRA, Design B (most common) has normal starting torque and normal LRA, Design C has high starting torque for hard-to-start loads, and Design D has very high starting torque with high slip. A Design B motor typically carries a code letter in the F to H range. A Design D motor may carry a higher code letter due to its different rotor design.
Disclaimer: This calculator provides estimated locked-rotor amperage based on NEC Table 430.7(B) code letter ranges. Actual starting current depends on supply impedance, motor loading, and connected mechanical load. Starter selection should consider the specific application requirements and manufacturer recommendations.

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