Three Phase Power Configurations

There are a number of variations of three phase power configurations, but there are two primary configurations we are going to detail in this article - 3-phase Wye and 3-phase Delta.

3-Phase Wye

In a 3-Phase Wye configuration, you typically have 4 wires - 3 'live' conductors and 1 neutral. (A ground wire may also be present, but in non-fault conditions the ground conductor carries no current, so for our purposes it can be ignored).

Common 3-phase Wye configurations in North America are 208Y120 and 480Y277.  For a 208Y120 configuration, 208 represents the 208V phase-to-phase voltage and 120 represents the 120V phase-to-neutral voltage.  A common depiction of a 208Y120 3-phase Wye configuration is below:

One advantage of the 3-phase Wye configuration is that phase-to-neutral voltage is 120V on all three phases.  Any phase (and neutral) can therefore be easily used for a single phase load.  Phase-to-neutral voltage can be derived from the phase-to-phase voltage divided by the square root of 3, or 208V / 1.73 = 120V.

 

3-Phase Delta

In a 3-Phase Delta configuration, you typically have only 3 wires - the 3 'live' conductors and no neutral conductor.

In a standard Delta configuration with no neutral, voltage is always a phase-to-phase (line-to-line) measurement:

This means that there is no phase-to-neutral voltage available (as there is no neutral!), so a single phase cannot easily be utilized.

The primary advantage of this three phase delta configuration is a reduction in the number of conductors required, it is therefore typically used in long distance transmission and high other high power applications.

Split Phase Delta

In the case of a 'split phase' or  tapped Delta configuration, a neutral is added tapped at the midway point of one of the transformer secondary coils.  This allows for a phase-to-neutral voltage as well as the phase-to-phase voltages.  Because the neutral tap exists only on one phase however, it results in an unequal phase-to-neutral reference across the 3 phases.  2 phase-to-neutral references are equal and one is higher (referred to as the "high leg", "wild leg", or "dog leg").  An example of a 240V split phase delta (or high leg delta) is below:

NOTE: Many 3-phase loads only operates on phase-to-phase voltage, meaning they can use the power from a Wye or Delta configuration with no need for a neutral line. In that case, the equipment may not require the neutral, but it may still exist in the configuration, which can be important when using a power meter.

Samsara PM20s can be used to monitor three-phase power detailed above (as well as single-phase power) and support both Wye and Delta configurations.

Please see the PM Installation Guide for more details: https://www.samsara.com/pdf/docs/pm-install-guide.pdf