The UPS is rated in kVA or for smaller models VA, this is basically the product of the UPS rate output voltage and current. However due to the nature of the electrically complex load that uninterruptible power supplies are required to support manufacturers often derate the UPS. It is often stated for example that a 20kVA UPS has a power factor of 0.8, i.e. 20kVA at 0.8PF, this will give the true output rating in kW and we have the following equation for a typical 20kVA 3 phase input, 3 phase output UPS: –
20000 x 0.8 (kW) /3 (power per phase) / 230 (output voltage) = output current per phase which in this case would be 23A maximum.
Uninterruptible power supplies with ratings of 10kVA and below the output power factor is often stated as being 0.7. Therefore a 10kVA single phase input; single phase output UPS at 0.7PF would have a maximum output current capacity of 30A. Manufacturers have many different ways of calculating there power factor rating, some state 0.9, 0.95,0.99 etc. only by carefully comparing their methodology can the true factor be calculated. The figures themselves shouldn’t be taken on face value without further...
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