Electrical – Calculate VA of device 100-240v


Firstly I apologise for any mistakes this is deifnitely not my field of expertise. I also apologise if this is not the right place for this question.

I have been doing some reading regarding sizing a UPS and whilst most of the information is making sense I am unsure on how to calculate the VA of some units.

Take for example this Device (power info on the second page).I see the following: 100-240v/50-60Hz/6A

In order to determine the VA I understand this to be Volt x Amp. Currently I am using this pdf. This tells me that the VA subtotal for this unit would be 240 x 6 = 1440.

I then need to multiply the VA subtotal by 1.2 whihc would give me 1440 x 1.2 = 1728.

Presumably this would then mean that I need a unit with a minimum VA of 1728 in order to power the unit.

When you have a device with 100-240v presumably you use the highest value ie 240 for the calculation is this correct? Where would be the best place to find the required info in order to calculate the VA correctly ie the product data sheet? The transformer etc.

Also if anyone has a good guide/link to check I would be most appreciative, whilst I have found a few resources I am not sure if they are excatly what I am looking for.

This is the Power Module

I have clearly been confusing the input values with the output values.
From the power module link I see the following info:

  • 48V, 400W

From this I take that the amps can be determined by 400/48 = 8 approx.

Therefore I believe the output values of the unit to now be:

  • 48V, 8A, 400VA, 400W

I am unsure how to now determine how long a particular ups would last. I assume in order to do this according to this link would be to also know:

  • DC Bus, Invert Efficiency, Charger

Looking at the equipment datasheets I do not seem to have this info. But I do seem to be progressing.

Best Answer

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Figure 1. The power supply module.

The PSU is rated at 400 W at 48 V DC so your calculation of \$ I = \frac {P}{V} = \frac {400}{48} = 8.3 \ \text A \$ is correct.

The Technical Specifications page states

Maximum transfer efficiency: 93%(220 V, 100% load, 25°C)

so I would work with 90% for some safety. This means that the input power (at maximum load) would be \$ \frac {400}{0.9} = 444 \ \text W \$.

Your UPS should be rated in Wh so you divide this by 444 W to get the run-time in hours.

I notice that the output is -48 V (minus 48 V) so this looks like a telecomms PSU as these used a positive ground. I would look around for a 48 V DC UPS as these were once commonly used for PABX backup power and consist of a fairly simple 4 x 12 V SLA battery pack, a charger and some way of switching between your mains-powered 48 V and the battery.

Regarding input current: the device is using a "universal input voltage" switched mode power supply. At 100 V AC input the current drawn will be \$ P = \frac {P}{V} = \frac {444}{100} = 4.4 \ \text A \$ and at 200 V AC input it will be 2.2 A.

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