What is NEMA Motor Codes and Inrush Current?
The NEMA Locked Rotor kVA Code is a single letter stamped on every AC induction motor nameplate (F, G, H, J...) that encodes the worst-case starting apparent power of that motor design. It exists because motor starting current is not determined by the motor's continuous HP rating alone — two 50 HP motors from different manufacturers can have dramatically different inrush currents depending on their rotor design. The NEMA code letter standardizes this information so electrical engineers can size overcurrent protection, feeder conductors, transformers, and emergency generators correctly.
Mathematical Foundation
Total Startup Apparent Power
= Motor nameplate horsepower.
= NEMA kVA per HP multiplier for the motor's locked-rotor code letter. Code G = 5.9 kVA/HP, Code J = 7.5 kVA/HP, etc.
Locked Rotor Amps (3-Phase)
= Line-to-line 3-phase supply voltage (e.g., 480 V).
= The 3-phase power factor, approximately 1.7321. Divides the total apparent power across the three phases to yield per-phase current.
Laws & Principles
- Back-EMF and Inrush: A running AC induction motor generates its own Counter-EMF (CEMF) that opposes the supply voltage, limiting current to the rated FLA value. At the instant of starting, the rotor is stationary — no CEMF exists. The motor impedance drops to only its stator winding resistance plus leakage reactance, allowing 500–700% of FLA to flow for the first few electrical cycles (typically 5–10 cycles = ~100–200ms on 60Hz systems).
- NEC 430.52 — Overcurrent Protection Sizing: Motor branch circuit overcurrent protection must allow the LRA through during normal starting. NEC Table 430.52 permits inverse-time circuit breakers to be sized up to 250% of motor FLA. For large LRA motors that still trip standard breakers, instantaneous-trip breakers can be set up to 800% of FLA under NEC 430.52(C)(3).
- Generator Sizing for Motor Starting: When an emergency generator must start a motor across-the-line, the generator must supply LRA without collapsing its terminal voltage below 80% of nominal. As a rule of thumb, the generator kVA rating should be approximately 3–4× the motor kVA (HP × 0.746) for reliable across-the-line starting. VFDs and soft starters eliminate this requirement by ramping voltage.
- Soft Starters vs. VFDs: Soft starters reduce inrush to approximately 200–300% FLA by ramping up voltage during starting, then connect the motor directly to the line. VFDs maintain complete speed control throughout operation and produce near-zero inrush. The tradeoff: soft starters are cheaper; VFDs offer continuous energy savings from speed reduction.
- NEMA Code Letter Ranges: Code A (lowest, < 3.15 kVA/HP) indicates motors with high-resistance rotors designed for minimum inrush. Code M (highest, ≥ 11.2 kVA/HP) indicates motors optimized for high torque at start. Energy-efficient NEMA Premium motors tend to fall in codes F–H due to lower impedance windings that also produce higher inrush.
Step-by-Step Example Walkthrough
" A 50 HP, 480V, 3-phase chiller compressor motor with a NEMA Code G nameplate (5.9 kVA/HP). Electrician needs to size the branch circuit breaker. "
- 1. Total VA at locked rotor: 50 HP × 5.9 kVA/HP × 1,000 = 295,000 VA = 295 kVA
- 2. LRA = 295,000 / (480 × √3) = 295,000 / 831.4 = 354.8 A
- 3. Estimated FLA ≈ 50 × 746 / (480 × √3 × 0.85 × 0.92) ≈ 60.8 A
- 4. LRA/FLA ratio = 354.8 / 60.8 = 5.8× running current
- 5. NEC 430.52 max inverse-time breaker = 250% × FLA = 250% × 60.8 = 152 A → use 150 A breaker.
- 6. The 150 A breaker must allow 354 A inrush for <200ms without tripping — this is why inverse-time (thermal-magnetic) breakers are specified, not fast-acting fuses.
Final Result: LRA = 354.8 A. The circuit breaker must be sized to 150 A (250% FLA per NEC 430.52) using an inverse-time device that tolerates the momentary 354 A inrush. Wire ampacity is sized to 125% FLA = 76 A, requiring minimum 4 AWG copper conductors.