Crest Factor

Crest factor is the ratio of the peak value of a waveform to its RMS (root-mean-square) value. For a perfect sine wave the crest factor is about 1.414, the square root of two. The metric matters because it captures how peaky a current draw is, which a simple RMS reading hides. Loads that pull short, sharp current pulses have a crest factor well above 1.414.

Why nonlinear loads have high crest factor

Switch-mode power supplies, the kind inside every modern ASIC miner, do not draw current smoothly across the AC cycle. They charge their input capacitors in brief bursts near the voltage peak, so the current waveform is a series of tall, narrow spikes. The resulting crest factor can reach 2 to 3 or more, meaning the instantaneous peak current is two to three times the RMS value.

Implications for sizing

High crest factor forces upstream equipment to handle peak currents far above the average. Generators, UPS units, and transformers must be derated or oversized so they are not overloaded by the peaks even when the RMS load looks modest. Ignoring crest factor when sizing a backup generator for a mining site is a common way to end up with nuisance trips and overheating.

Crest factor is closely tied to harmonics, since the same pulsed current that raises crest factor also injects harmonic distortion. Both belong to the broader study of power quality.