Motor Starting Current
Estimates the current an induction motor draws in the first moments of a start, before it comes up to speed. Pick a motor from the NEC full-load current tables or enter its full-load amps. Then give the locked-rotor current as a multiple of full-load amps or from the nameplate code letter, and choose how the motor is started. Electricians and designers use it to check voltage dip, generator and transformer capacity, and the trip curves of breakers and fuses.
Equations
Istart = k·ILR ILR = m·FLA
ILR is the locked-rotor current: what the motor draws at full voltage with its shaft stopped. It is m times the full-load amps (FLA); m is about 6 for a typical motor. k is the share of that current the supply sees with the chosen starter. Across the line, k = 1. A wye-delta starter gives k = 1/3 [4]. An autotransformer at tap a gives k = a², so 0.64, 0.42 or 0.25 for the 80, 65 and 50 % taps [3].
The code letter gives the locked-rotor kVA per horsepower as a range [1], [2]. The page turns it into current:
ILR = 1000·(kVA/hp)·hp / (√3·V) three-phase, or / V single-phase
where V is the motor’s rated voltage. It shows the top and bottom of the letter’s range and bases the starting current on the top, the worst case. The full-load currents come from NEC Tables 430.250 and 430.248 [2].
Limits and common mistakes
- The NEC tables are for sizing conductors and protection, not the motor’s actual current. Use the nameplate FLA for a closer estimate of a particular motor.
- The first half cycle carries an asymmetrical peak well above ILR. Allow for it when you set instantaneous trips.
- Reduced-voltage starting also cuts the starting torque, by the same factor k. Make sure the load can still be started.
- A soft starter or a drive limits current by its own settings and isn’t covered here.
Worked example
A 50 hp, 460 V pump motor with code letter G will be started wye-delta. Estimate the line current during the start, to check the feeder.
- Choose Motor (NEC full-load current table) 460 V three-phase and Motor rating 50 hp — 65 A full-load.
- Choose Locked-rotor current From the nameplate code letter (NEC Table 430.7(B)) and Locked-rotor code letter G — 5.6 to 6.29 kVA/hp.
- Choose Starting method Wye-delta (1/3 of full-voltage current) and press Solve. The Solution shows:
Estimated starting current (Istart): 131.577 A
Full-load current (NEC Table 430.250, 50 hp, 460 V three-phase): 65 A
Locked-rotor current at full voltage (top of code G, 6.29 kVA/hp): 394.732 A
Locked-rotor current, bottom of code G (5.6 kVA/hp): 351.431 A
Line current while starting, fraction of full-voltage locked-rotor current: 0.333333
Check: 6.29 × 50 = 314.5 kVA, and 314,500 / (1.732 × 460) = 394.7 A, about 6.1 times the 65 A full-load current. A third of that is 131.6 A.
