Free — no sign-up required.
Motor starting creates a large transient inrush current — typically 6 to 8 times the motor's full-load current — that causes a temporary voltage dip at the motor terminals and throughout the system. IEEE 399 (the Brown Book, "IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis") is the engineering reference for voltage flicker and motor starting analysis.
The voltage dip during across-the-line (DOL) motor starting is calculated using the motor locked-rotor current (LRC) and the source impedance. The simplified formula is: voltage dip (%) = (LRC / (LRC + source fault current)) × 100. For a 100 hp motor with 600A LRC connected to a bus with 5,000A available fault current, the voltage dip is approximately 600 / (600 + 5,000) = 10.7%. If this dip exceeds acceptable limits (typically 15-20% for industrial systems, 10% for systems with other sensitive loads), a reduced-voltage starter (soft starter or VFD) is required.
Generator-supplied systems present the most challenging motor starting conditions. A generator's available fault current is limited by its subtransient reactance (Xd''), typically 10-20% — much lower than utility-fed systems. A motor starting on a standalone generator can produce voltage dips exceeding 30-40%, which may prevent starting or disrupt other running equipment.
Acceptable voltage dip limits depend on the load type: lighting circuits tolerate less than 5% before flicker becomes visible; sensitive electronics require less than 3%; industrial motors typically tolerate 15-20% momentary dip during another motor's start.
SparkShift's motor starting calculator computes the voltage dip for across-the-line starts, reduced-voltage starts (autotransformer, solid-state soft starter, VFD), and generator-fed systems. Results include the minimum transformer kVA or generator kW rating to maintain voltage within acceptable limits.
Open the motor flc calculator. Results include NEC citations and can be saved to a project for PDF export.
During across-the-line starting, a motor draws 6-8 times its full-load current (locked-rotor current). This large inrush current creates a significant voltage drop across the source impedance, temporarily reducing the voltage at the motor terminals and connected buses.
Typical acceptable limits are 15-20% for industrial systems with motor loads, 10% for systems with sensitive electronics, and less than 5% for lighting circuits (to avoid visible flicker). IEEE 399 provides guidance on acceptable limits for various load types.
A variable frequency drive (VFD) ramps motor current up gradually from essentially zero, eliminating the locked-rotor inrush. The maximum current during a VFD start is typically 100-150% of FLA rather than 600-800%, reducing the voltage dip to near zero.
The generator must be large enough to maintain acceptable voltage during the motor starting inrush. Rule of thumb: generator kVA should be 2-3x the motor starting kVA for across-the-line starts. Use SparkShift's motor starting calculator for the exact dip calculation based on the generator's subtransient reactance.
Disclaimer: SparkShift is a calculation tool for electricians and engineers. It is not a substitute for a professional power systems engineering study. SparkShift is not approved by NCEES, IEEE, NFPA, or any professional organization. For stamped engineering documents, engage a licensed Professional Engineer in your jurisdiction.