Help Sizing a UPS

[Saint Germain]

Hello all,

I am trying to size a 120 VAC UPS. Typically, all of my devices are 120V and I know either the VA or Watts so I would convert Watts to VA and add all of the VA's together to size the UPS. This time some of my devices are 24 VAC, which I will have a 120/24 VAC transformer for.

I am getting stuck with trying to calculate the total VA for the devices that are 24 VAC. The data sheet for these devices state 72 VA at 24 VAC. How can I find the VA for these devices @ 120VAC?

This is what I have calculated, but I need someone to please tell me where I am going wrong here.
24V - 72VA
1. Find current 72VA / 24V = 3.125A
2. Find resistance of device 24V / 3.125A = 7.68 Ohms
3. Find current @ 120VAC with resistance being constant 120V / 7.68 = 15.625A
* I think this is not correct..shouldn't the current be lower with a higher voltage??
4. Calculate VA @ 120V 120V * 15.625A = 1875VA

Thanks for your help,
Saint Germain

 

[nhee2]

Neglecting any xfmr losses, the VA at 24 vac will be the same at 120 VAC.

 

[GoldDigger]

Neglecting any xfmr losses, the VA at 24 vac will be the same at 120 VAC.

And as a practical matter the transformer will add very little to the watts, but will contribute a small percentage to the VA. The more the transformer is oversized the worse the idling losses will be.

 

[Saint Germain]

So the VA @ 120 VAC would also be 72 VA or a little higher? I was really over thinking this one!
Thank you!

 

[GoldDigger]

So the VA @ 120 VAC would also be 72 VA or a little higher? I was really over thinking this one!
Thank you!

The A decreases in the same ratio that the V increases, leaving the product the same.

 

[texie]

Just a word of caution about VA vs watts. Most small and less sophisticated UPS are advertised as, say, 1000 VA. But the fine print will usually tell you that the watts for this unit are, maybe, 600. Remember watts are real power and VA is apparent power. If you have equipment that is actually rated in watts, simply converting that to va will lead to an undersized UPS.
As an example, if you have a UPS with a rating of 1000 VA/600 watt and put a true 1000 watt load on it you will let the smoke out.

 

[Saint Germain]

Just a word of caution about VA vs watts. Most small and less sophisticated UPS are advertised as, say, 1000 VA. But the fine print will usually tell you that the watts for this unit are, maybe, 600. Remember watts are real power and VA is apparent power. If you have equipment that is actually rated in watts, simply converting that to va will lead to an undersized UPS.
As an example, if you have a UPS with a rating of 1000 VA/600 watt and put a true 1000 watt load on it you will let the smoke out.

Texie - So it sounds like a better way would be to convert my devices that are rated in VA to W and add up all of the W values to size the UPS, right?

 

[kingpb]

Also make sure you are looking at the output rating to serve your load, and then look at input rating to size your branch circuit.

Efficiency can play a huge role in the two, especially in less expensive UPS'.

 

[kingpb]

Texie - So it sounds like a better way would be to convert my devices that are rated in VA to W and add up all of the W values to size the UPS, right?

You should really know VA and pf, or watts and pf for each load; unless of course it's purely resistive, then pf=1, and watts = VA.

 

[texie]

Texie - So it sounds like a better way would be to convert my devices that are rated in VA to W and add up all of the W values to size the UPS, right?

Yes, but to be clear, that total value should not exceed the WATT rating of the UPS as opposed to the VA rating that is used to market small UPSs.
In other words, this is a conservative approach that will ensure that you never will overload the UPS as certainly not all your loads will not be unity PF.

 

[Besoeker]

Just a word of caution about VA vs watts. Most small and less sophisticated UPS are advertised as, say, 1000 VA. But the fine print will usually tell you that the watts for this unit are, maybe, 600. Remember watts are real power and VA is apparent power. If you have equipment that is actually rated in watts, simply converting that to va will lead to an undersized UPS.
As an example, if you have a UPS with a rating of 1000 VA/600 watt and put a true 1000 watt load on it you will let the smoke out.

Well made point, texie.