Complete Guide to Motor Load Calculations
The Motor Load Calculator computes full load amperage (FLA), kVA, input power, and torque using nameplate data. Supports NEC 430.250 tables and custom efficiency/PF inputs.
What is Motor Full Load Current?
FLA is the current a motor draws at rated HP, voltage, and speed under full load. Used for:
- Wire sizing (NEC 430.22)
- Breaker selection (430.52)
- Overload protection (430.32)
- Panel load calculations
Motor Load Formulas
Single-Phase: I = (HP × 746) / (V × Eff × PF)
3-Phase: I = (HP × 746) / (V × Eff × PF × √3)
kVA = (V × I) / 1000 (1φ) or (V × I × √3) / 1000 (3φ)
Input Power (W) = HP × 746 / Eff
NEC Table 430.250 (3-Phase FLA Examples)
| HP | 230V | 460V | 575V |
|---|---|---|---|
| 5 | 13.6A | 6.8A | 5.4A |
| 10 | 26.4A | 13.2A | 10.6A |
| 25 | 64.0A | 32.0A | 25.6A |
| 50 | 126A | 63A | 50A |
Why Use Custom Efficiency & PF?
NEC tables assume ~84% efficiency and 0.85 PF. Premium motors (NEMA Premium) reach 90%+ efficiency. Accurate inputs ensure proper conductor and protection sizing.
Practical Examples
- 10 HP, 460V, 3-Phase, 92% Eff, 0.88 PF → FLA = 12.2A
- 3 HP, 230V, 1-Phase, 80% Eff, 0.75 PF → FLA = 18.6A
- 50 HP, 575V, 3-Phase → NEC FLA = 50A
Motor Sizing Tips
- Verify nameplate voltage matches supply (±10%).
- Use service factor (SF) for overloads (e.g., 1.15 SF = 15% extra).
- Apply demand factor for intermittent loads.
- Check starting current (5–8× FLA for DOL).
Common Mistakes
- Using kW instead of HP (1 HP = 0.746 kW).
- Forgetting √3 in 3-phase formula.
- Ignoring voltage drop (>3% affects torque).
- Using running current for breaker sizing.
Advanced Applications
- Soft Starters: Reduce inrush to 2–3× FLA
- VFDs: Improve PF, save energy
- Energy Audit: Measure actual vs. nameplate
- NEMA Design: B (general), C (high torque), D (high slip)
Standards & Codes
- NEC 430: Motor circuits
- NEMA MG-1: Motor performance
- IEEE 112: Efficiency testing
- IE3/IE4: IEC premium efficiency
Conclusion
Accurate motor load calculation ensures safety, efficiency, and code compliance. Our Motor Load Calculator supports both NEC tables and custom inputs. Pair with Breaker Size and Cable Sizing tools. Build smarter with 1000 Calculators.