The Six-Times Problem: Motors, Inrush and Sag

Every motor circuit hides a multiplier. A compressor that runs at 14 A starts at 70–90 A for a half-second — and for that half-second, your carefully calculated 2% voltage drop becomes 12–18%. That is the flicker in the lights, the chirp from the UPS, and occasionally the reason the motor never finishes starting at all.

Why inrush is so large

A stationary induction motor is essentially a transformer with a shorted secondary — locked-rotor current is typically 6× the running current (the code letter on the nameplate gives the exact ratio). Current falls as the rotor spins up, which usually takes under a second on a healthy supply.

When sag becomes failure

Starting torque scales with the square of voltage. A 15% sag delivers only 72% of rated starting torque — marginal for a compressor starting against head pressure or a loaded conveyor. The failure mode is a hum, a thermal trip, and a motor that cooks its start winding trying. Contactors are also victims: many drop out below ~80% coil voltage, chattering during deep sags.

Design rules that work

• Size motor feeders for ≤2% running drop — the start sag lands near 12%, which most motors and contactors tolerate.
• Check the start case explicitly for long runs: multiply running amps by 6 in the calculator and confirm the sag stays under ~15%.
• Hard-starting loads at distance (well pumps, irrigation) justify one size beyond the running calculation — the well pump preset bakes this thinking in.
• Repeated dimming of house lights from one appliance is a free diagnostic: that circuit, or a shared neutral path, is dropping real volts.