Uninterruptible Power Supplies (UPS) are unique devices that require careful planning when making the initial power connections. Proper matching of the voltages and phasing to and from the UPS and Maintenance Bypass Switch (MBS) is crucial in preventing damage to equipment. UPS come in many different input and output configurations. Some of the more common are three phase delta, three phase wye, two phase, split phase, and single phase two wire. More often than not, the UPS input is setup in one configuration, and the UPS output is an entirely different configuration. Complicate this with all of the different voltage combinations (480/277VAC, 120/208VAC, 120/240VAC, 240VAC, 208VAC, 120VAC, etc.) and it quickly becomes evident why careful attention must be given to the wiring between the input sources, UPS, MBS, and Load.


Single phase power can be derived and supplied from several different source configurations. Some of the most common are illustrated to the right.


Three phase power is generally available in two basic configurations: WYE and DELTA

Three phase voltage configurations reduce the required size of the wiring and associated installation materials, making it more economical for long runs or larger loads. Three phase power is basically three single phase sources operating at 120 degrees of separation.

Three phase power delivers a more continuous and efficient flow of power. When one phase of a three phase system is passing through the zero crossing or delivering zero power, the other two phases are still supplying power. A good analogy of this would be three men rowing a boat in sequence. When the first man completes his stroke, his oars come out of the water and he is no longer providing power. At this same point in time, the second man is still completing his stroke and the third man has just begun his. This shows the continuous power flow (current) pushing the boat forward. Single phase power is the equivalent of having only one oarsman in the boat and each time he lifts his oars out of the water, the boat is without power until he begins his next stroke. Single phase delivers less power and results in a more irregular motion or flow.


Since a UPS' output can automatically or manually be transferred from one source to another, it is imperative that the two sources be of approximately the same voltage and phase angle. For example, one cannot transfer a single phase 120VAC power source derived from a three phase WYE configuration to a 120VAC single phase source coming from a split-phase configuration. While the line to neutral voltage measures the same, the phase relationship between the two sources is not. The phase displacement or separation of the three phase wye is 120 degrees, the split-phase is 90 degrees apart, resulting in a 30 degree phase shift. Initiating a transfer between two sources that are out of phase with each other is referred to as an "out of phase transfer". The result is typically disastrous with high circulating currents that clear fusing, breakers and can possibly damage the equipment and the load.

Since a UPS' output will synchronize (sync) with phase L1 of its bypass input source, the same phase relationship must be observed when connecting the UPS output and bypass source to the external Maintenance Bypass Switch (MBS). Maintaining this phase relationship will result in a transparent bumpless transfer between sources. Refer to the one-line drawing below to see this. The oneline also illustrates how a UPS synchronizes its output to the bypass source by using an internal sync sensing circuit.


The same holds true for three phase UPS systems. The UPS bypass input source must have a clockwise phase rotation, L1-L2-L3 or often referred to as A-B-C phase rotation. The UPS will sync with the bypass L1 input and generate an output with the same matching phase rotation. Sometimes a bypass isolation transformer is required to match the source voltage and phasing to that of the UPS output. The bypass transformer can step the source voltage up or down to match that of the UPS output. This is common when using a three phase source to feed a single phase bypass. Check out our other knowledgebase pages for more information on how to accomplish this.


During installation, the wiring can be easily checked for correct phasing by using a digital volt meter (DVM). Measure between the same phases of each source with the meter set to 'AC Volts'. The DVM should measure less than 10 VAC from L1 to L1. If it is a multiphase configuration, also measure from L2 to L2 and L3 to L3 to verify the voltage is less than 10 VAC for each phase. If you measure a voltage that is much higher than 10 VAC, it means that the phasing is incorrect. Pay particular attention to the wiring of the MBS because this is a manually operated device and if the phasing between the bypass source and UPS output feeding the MBS is incorrect, damage can result to the UPS, MBS, and load.