Single and Three Phase System Considerations

We are so used to simply being able to plug-in items to the local wall sockets or power distribution unit outlets that we often don’t consider the electrical planning that has gone into a building design and power continuity plan. Yet without adequate planning, electrical circuits can become overloaded and in the event of a power cut, loads can suddenly be left without electrical power.

The Eco Power projects team includes power continuity specialists who are also NICEIC certified electrical engineers and contractors. The way they approach a building site survey is a multi-stage process that includes both load sizing and also electrical supply review, from the building incomer, electrical switchgear and distribution units to the room the loads to be protected are within. This not only builds up a complete picture of the electrical circuits on-site. It also helps to determine whether to offer a three phase or single phase UPS system.

UPS Systems – Input and Output Phases

In UPS terms there are three potential phase configurations available. This is because a 3 phase mains or generator supply actually consists of three single phase supplies (and a neutral) with a 120degree phase orientation between the them (see three-wire and four-wire 3 phase supplies for more information).

A 3 phase supply can deliver more electrical power than a single phase supply. A commonly used analogy for a 3 phase supply versus a single phase supply is that of a rowing team. In a three-phase crew, a greater constant power can be delivered as each rower can maintain contact with the water, whilst one is exiting and the other entering. The power output is greater than can be achieved from a single-phase crew.

For UPS systems the common phase configurations include:

The laws of physics and Ohms Law also come into play, meaning that cable sizes also increase in diameter as amperages rise. A 20kVA output is generally the largest single phase UPS system available. This is due to the output amperage and cable requirements. 20kVA=20,000VA / 230Vac = 86.9Amps.

Single phase may also be referred to as 1 phase or 1-phase and three phase as 3 phase or 3-phase.

In the world of UPS, it is common to see refer to a single phase UPS only by its kVA/kW rating i.e. 5kVA. However for a three phase UPS it is common to refer to the kVA/kW rating along with the number of phases i.e. 20kVA 3/1 or 100kVA 3/3.

3 Phase UPS Systems (3/3 and 3/1)

Most datacentres, commercial and industrial buildings will have a 3 phase electrical incomer that connects them via a local sub-distribution transformer to the National Grid. Three phase circuits may be required throughout the building to carry the large amounts of electrical power required for loads (critical, essential and non-essential) before the supply is split via distribution panels into single phase runs for individual areas, offices, comms and server rooms. This is a generalisation as some IT environment can include both single and three phase loads of course.

From a UPS systems perspective, if we are to connect to a three phase supply we require a UPS with a 3/x configuration. If the loads are three phase as well we require a 3/3 configuration. If the loads are single phase we need a 3/1 configuration.

Using a three phase UPS system can simplify a power continuity plan and allow a site to adopt a centralised power protection plan, where one large UPS is used to protect a complete building or critical circuits and operations within it. This is in contrast to a decentralised power continuity plan using a number of smaller UPS geographically dispersed to protect clusters of server, cabinets and loads throughout a facility.

Single Phase UPS Systems (1/1)

The wall sockets that we typically plug-into are single phase supplies rated at 230Vac 50Hz in the UK. Most small IT hardware systems and can be powered via a BS1363 UK style plug or IEC320 outlet. Typical examples would include file servers, switches, routers, hubs and telecoms systems.

Single phase UPS systems up to 2kVA can be supplied with a BS1363 style plug or with covered terminals for hardwired installation. At 3kVA the power required means that the UPS will be supplied as either a hardwired system or with a 16A blue commando plug. Above this from 4kVA to the largest single phase UPS system available (typically 20kVA) the UPS will required a hardwired installation and should also include an external UPS maintenance bypass switch.

UPS System Load Sizing

When sizing uninterruptible power supplies it is important to know the phase configuration required by both the mains supply and the loads, in addition to the overall load size. Power continuity specialists and electrical contractors will often state both a load size and phase configuration. An example would include ‘60kVA three phase’. This refers to a 60kVA load run from a three phase 400Vac, 50Hz supply. In terms of load sizing, this means that each phase (of the 3 phase electrical supply) will deliver up to 20kVA (or 86.9Amps at 230Vac). If the statement was 60kVA per phase then we would be looking at 3×60kVA per phase = 180kVA UPS load. The need for a 60kVA three phase UPS could be met with three single phase output 20kVA UPS. These would be 3/1 configured and installed one per phase. However, the overall capital, installation and energy efficiency costs just rose by a factor of 3 compared to a single 60kVA UPS system installation.

For more information on sizing single or 3 phase UPS systems please contact the Eco Power projects team.

We provide a free site survey and power continuity advice service across the UK.