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Calcimator

Motor Starting Current Calculator

Calculate motor full load amps, starting inrush current, voltage drop during starting, and starting KVA demand.

Inputs

HP

NEC Table 430.250: 5 HP/230V = 15.2A; 50 HP/480V = 65A; 100 HP/480V = 124A

V
× FLA

NEC Code letter: Code G = 6.3–7.1 kVA/HP; DOL starts: 6–8× FLA; VFD: 1–1.5× FLA

phase

Results

Full Load Amps (FLA)

57.37 A

Starting Current (LRA)

344.23 A

Estimated Voltage Drop10.33%
Starting KVA Demand286.19 kVA
How to Use This Calculator
  1. Enter the Motor Horsepower from the nameplate and the Motor Voltage (e.g., 208, 480, or 4160 V).
  2. Enter the Motor Efficiency at full load — typically 0.88–0.96 for standard NEMA motors.
  3. Set the Starting Current Multiplier: direct-on-line (DOL) starting ≈ 6–8×, soft starter ≈ 2–4×, VFD ≈ 1–1.5× FLA.
  4. Select the Number of Phases (1 or 3).
  5. Read the Full Load Amps (FLA) to size cables and overload protection, and the Starting Current (LRA) to size upstream breakers and transformers.
  6. Check the Estimated Voltage Drop % during starting — values above 5% can trip sensitive controls; above 10% may prevent motor start.

How the result changes with Motor Voltage

Motor VoltageFull Load Amps (FLA)Starting Current (LRA)
1,47018.73 A112.4 A
4,8955.63 A33.76 A
9,0053.06 A18.35 A
12,4302.22 A13.29 A

What each input means

Motor Horsepower
Nameplate horsepower rating of the motor. NEC Table 430.250 (three-phase) and 430.248 (single-phase) list full-load ampere values by HP and voltage for motor circuit sizing.
Motor Voltage
Rated voltage of the motor (e.g., 208, 480, 4160V).
Motor Efficiency
Motor efficiency at full load. Typical: 0.88-0.96 for standard motors.
Starting Current Multiplier
Locked Rotor Ampere (LRA) ratio to Full Load Amps (FLA). NEC Table 430.7(B) provides locked-rotor indicating letter codes by HP. DOL = 6–8×, soft starter = 2–4×, VFD = 1–1.5×.
Number of Phases
Select 1 for single-phase or 3 for three-phase motor.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Motor Horsepower = 50, Motor Voltage = 480, Motor Efficiency = 0.92, Starting Current Multiplier = 6 = 5 input(s) provided
  2. Calculate Full Load Amps
    Full Load Amps
    57.37 = 57.37
  3. Calculate Starting Current
    Starting Current
    344.23 = 344.23
  4. Calculate Estimated Voltage Drop
    Estimated Voltage Drop = Math
    10.33 = 10.33
  5. Calculate Starting KVA Demand
    Starting KVA Demand
    286.19 = 286.19

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