Voltage Drop for Motor Circuits: Starting and Running
Essential guide to voltage drop calculations for motor feeders and branch circuits. Covers starting current, running current, and motor performance considerations.
Motor Circuits: A Special Case
Electric motors present unique voltage drop challenges that require special consideration beyond typical circuit design. Motors are sensitive to supply voltage—low voltage dramatically affects their performance, while the high starting currents (typically 6-8 times running current) create temporary but significant voltage drops that can affect other equipment on the same system.
This guide covers the critical aspects of voltage drop for motor circuits, including the difference between starting and running calculations, the impact of low voltage on motor performance, and best practices for motor feeder design.
Starting vs Running Current
Motors draw significantly higher current during startup than during normal operation. This inrush current, typically 6-8 times the full load running current, creates a temporary voltage drop that must be evaluated separately from the running voltage drop.
Starting Current
- • 6-8× full load current (typical)
- • Duration: few seconds to tens of seconds
- • May affect other loads on system
- • VFDs can reduce starting current
Running Current
- • Per NEC Table 430.250
- • Continuous duration
- • Primary design consideration
- • Affected by load conditions
Motor Performance vs Voltage
NEMA standards specify that motors should operate satisfactorily at ±10% of rated voltage. However, performance degrades within this range, with significant effects at the lower end.
| Voltage | Torque | Current | Temperature |
|---|---|---|---|
| 100% (Rated) | 100% | 100% | Normal |
| 95% | 90% | 105% | Slightly elevated |
| 90% | 81% | 111% | Elevated |
| 85% | 72% | 118% | Hot - potential failure |
Design Recommendations
- Limit running voltage drop to 3% — This ensures the motor receives at least 97% of nominal voltage during normal operation
- Verify starting voltage drop — Calculate the temporary drop during starting and verify it doesn't cause issues for the motor or other loads
- Consider soft starters or VFDs — These reduce starting current and associated voltage drop for large motors
- Evaluate the entire system — Consider both feeder and branch circuit voltage drop combined
Calculate Motor Circuit Voltage Drop
Use our calculator to verify your motor circuit designs meet NEC recommendations and ensure optimal motor performance.
Motor Circuit Calculator