Starting motors with large loads causes voltage drop which is often evidenced by flickering lights. This flicker is objectionable only when the magnitude and frequency of the voltage drop exceed certain thresholds.
This threshold of objection is shown on a Voltage-Flicker curve. If the magnitude of the voltage drop and the frequency of occurrence lie below the threshold of perception, people generally do not notice any flicker.
Example: A five horsepower three phase motor is supplied by a 208 Volt feeder which also supplies 120 Volt lighting circuits. Assume that the feeder resistance = 0.06 ohms. For a 5 horsepower motor, the Full Load Amps (FLA) = 16 amps. The motor starting current = 16 amps x 6 = 96 amps. V drop = starting current x feeder resistance = 96 amps x 0.06 ohms = 6 Volts.
The 6 Volt drop along the feeder is equal to 5% of the voltage on the 120 V lighting circuit, and it causes a noticeable flicker. If the motor is started once every hour, then the representative point, on the flicker curve is in the objectionable range. To correct this problem, supply the lighting circuits from a separate feeder, or reduce the voltage drop along the feeder. A reduced voltage starter often presents a very cost effective solution.