UPS and PSU current consumption estimation

Thread Starter


Joined Mar 29, 2018
Dear all,

I have to design a test bench which includes a UPS and I am not sure how to calculate the power consumption (first time for me to work with a UPS). I also have some questions about the main PSU power consumption.

For now the power input of the bench (3 phases 400V) is designed with a current rating of 16A (considering the sizing of the wires / connectors / terminals). My main concern is to check whether 16A as input rating current is enough for this bench.
There is a main PSU 5kW (20V-250A) which is using 3 phases, all the other equipment are using 1 phase.

The main PSU 5kW datasheet gives that the maximum input current at full load is 12A. I guess the calculation is \( 5kW\div 400V \) but then the result should be 12.5A, and not 12A, is it because the real power is not exactly 5kW?
If I consider that the output of the PSU will never be higher than 20V with 25A (500W), then is it correct to assume that the input current will never be higher than \( 500W\div 400V=1.25A \) ?

The UPS (SRT3000RMXLI) has an input rating current of 16A (it's only 1 phase input UPS).
I don't know in which condition this input current would really reach 16A. If I consider the UPS output (load) current is maximum 10A. Then the UPS input current should be 10A + the current to charge the batteries? Is it correct? How to estimate what would be this charging current and therefore the maximum input current?

My main concern are these two equipment because they have to share the same phase. All the other equipment which are not connected to the UPS output (like monitor, USB hub, etc...) will be divided on phase 2 and 3 to share the load between the 3 phases.

In all this description I am not considering any inrush current, but is it ok to ignore it? I consider that inrush current is too short to really damage the wires / connectors and the MCB is 25A.

Thank you in advance!


Joined Apr 5, 2008

The current of the PSU will be slighly higher as the mentioned 1.25 A.
You will have to take the efficiency of the PSU into account.
What PSU are you using?
A SMPS will be likely more efficient as a linear one.


Thread Starter


Joined Mar 29, 2018
Hello Bertus,

Thank you for your feedback.

The PSU is TDK GEN-20-250-3P400. According to the datasheet the efficiency is 86%, therefore the input current would be \( 1.25A \div 0.86 = 1.45A \) , wouldn't it?
Where is going the power lost? Heat dissipation?