Rating plates are required by the electrical safety standards to show the required Applied Voltage, Frequency and the equipment current draw in Amps. So a typical rating plate may show something like this:
Voltage: 120-240VAC 50/60Hz
Current: 3A
Problem #1: The power consumption is misleading.
So what is the power consumption of this system? Easy isn’t it. We take the 240V and multiply this by the 3A to give us 720VA. So this is the minimum size of UPS – right? Well, actually you wouldn’t go far wrong, however you are actually specifying a UPS System bigger than it needs to be. This is because the maximum current is (probably) taken at 120V, not at 240. So the actual power consumption is 120×3 = 360VA. A big difference!
Problem # 2 – They are never accurate
Manufacturers are duty bound to quote the maximum current draw of their equipment, so that it can be connected to a suitably protected circuit. This means that the manufacturer is under no pressure to accurately describe the power consumption, as long as he doesn’t under specify it. Take the example above, the manufacturer will have measured the current draw and if it happened to be around 2A, he would be advised to add on a considerable margin of error and then round up. So long as he doesn’t exceed the circuit rating he is wanting to connect his equipment to, then he is quite at liberty to do this. So the actual VA consumption of this equipment could now be as low as 240VA, and we could be specifying a UPS of 720VA! Three times the required!
Problem #3: They rarely give a full load picture.
Take a rating plate that states:
Voltage: 230V 50/60Hz
Current: 5A
Well, we would work out the power by multiplying Volts x Amps so we get 1150VA for the minimum size of UPS – right? No, this could be wrong, because we have no information of power factor. Let us suppose this load is an electric heater. It has a power factor of unity, so the WATTS are actually 1150W. Most UPS have a Watts rating and a VA rating, and the Watts rating is usually around 0.7x the VA rating. So a 1150VA UPS would be overloaded for this application. You would need to specify the VA rating of the UPS as the Volts x Amps divided by 0.7, which will give us a UPS rating of 1643VA. This may be overkill, but we don’t know the power factor of the connected equipment, so we have to make this adjustment.
Conclusion
If you’ve got nothing else to work on, you have to use the rating plate, but you will more than likely be oversizing the UPS. This may not be too bad a thing, but you are more than often than not, specifying a UPS that is not really appropriate for it’s application. The best way is to actually measure the load consumption – this has some drawbacks as well, but with good practice you get the correct UPS for the application.
