Crest factor is the ratio of the instantaneous peak amplitude of a waveform, to its root mean square RMS value. The peak amplitude refers to the instantaneous peak current that may be required by a load, whereas the RMS is the average load current under normal conditions.

The crest factor specifies the properties of an electrical system such as the purity of a signal or waveform, and the capability of a system such as a power supply to output a particular current or voltage.

The ratio is also referred to as peak-to-RMS ratio is given by:

Crest Factor (peak-to-RMS ratio) = (peak value)/(RMS value).

The crest factor indicates the extreme peaks of a waveform. For a purely DC system with a resistive load, the value should be 1:1 which is also the minimum. A sinusoidal AC waveform with a resistive load has a CF of 1.414. However, the waveform may be distorted when supplying reactive loads with no power factor correction.

Some IT equipment and other loads with power factor corrected supplies have a crest factor of about 1.414, whereas those with no correction such as stackable hubs and personal computers have factors of 2 or 3.

For applications requiring a pure sine wave, the supply is desired to have a crest factor of 1.414 or the closest possible. Distortions caused by interactions between supply and the load may affect the relationship between the peak and RMS values and result into a higher value.

The crest factor of a computer load depends on the power source feeding it and may vary from one ac receptacle to another. It is important to note that the crest factor arises due to the interaction between the AC source and the load and that the crest factor required by the load will depend on the AC supply waveform.

Crest factor of a source shows the possible and safe output peak currents it can handle above rated current. Since the supplies can provide higher outputs, they should also have fail safe circuits that should shut down and cut off power in case the load continues to draw more of this high current.

The source must be capable of supplying the peak current desired by the load; otherwise the source voltage becomes distorted by the excess peak current. Most power sources manufacturers usually provide the crest factor, or peak repetitive current data, to help consumers or designers match their loads to the suitable sources.