Number of UPS phases

Number of phases (one or three)?

• Desktop workstations and small servers usually take single-phase input power.

• Larger scale equipment such as server racks or RAID disk systems may be three-phase

If your entire load comprises single-phase equipment you can use a single phase output UPS. However if your load includes any three-phase equipment you must use a three-phase UPS; this can supply both three-phase and single-phase machines.

If a single phase load exceeds 20kVA in total, then it can be distributed over the output phases of a three-phase UPS. Ideally, although not essentially, the load should be distributed as evenly as possible between the three phases.

	A typical example of this would be:-   A call centre with 50 single-phase workstations drawing 500VA each represents a total single-phase load of 25kVA.   50 workstations cannot be distributed into a perfectly balanced three-phase load, but they can be divided into three subsets of 17, 17 and 16 workstations (8.5kVA, 8.5kVA and 8kVA) respectively. This could become an acceptably distributed load for a three-phase UPS.
Assume the call centre also has a three-phase 9kVA server array evenly distributed across its three phase feed - 3kVA per phase. Taking into account the above distributed single-phase load this means that the call centre imposes total loads of 11.5, 11.5 and 11kVA on each phase respectively.
To correctly size the UPS, take the largest single phase load and multiply by three. In this example, the largest load is 11.5kVA, which, multiplied by three gives a three phase load of 34.5kVA. This could be efficiently met by using a standard UPS configuration rated with a 40kVA three-phase output capacity.   If single phase loads are supplied by different phases and in the same vicinity then Regulation 514-10-01 of BS7671 must be enforced. This generally states that:

"Where terminals or other fixed live parts between which a nominal voltage exceeding 230 volts exists are housed in separate enclosures or items of equipment which, although separated, can be reached simultaneously by a person, a notice shall be secured in a position such that anyone, before gaining access to such live parts, is warned of the maximum voltage which exists between those parts."