UPS output/input

[Lady Engineer]

I need to know if I'm sizing the output and input correctly of a UPS.

I have a 750KVA UPS at 480V to be installed. I would like to size the input and output feeder properly.

I used 750/.831= 905.52A, then I multiplied that by 1.25 then by 1.20 for battery efficency.

Ok it comes to this 905.52A x 1.25 x 1.20 = 1298A so I say 1200A (without battery efficency) output to the Distrib. panel and input feeder of 1600A for main and by pass.

Please tell me if I'm in error...Please??

 

[rcwilson]

When power returns, the UPS input has to recharge the battery and still supply the full 750 KVA load. The 1600 A may be enough to do that but it is best to check the UPS manufacturer's recommended circuit sizing. The answer depends on the battery recharge rate and battery size.

 

[steve66]

Yes, the manufacturer usually lists recommended wire sizes in either the owners manual, or the installation manual. You can often download this manual online.

I would suggest checking that. I never seem to have any luck figuring out how they calculate the wire sizes.

Steve

 

[markstg]

Prior to finalizing the design you need to obtain the manufacturer's certified drawings for the UPS. That's a monster. I got bit once on a 480V In 480/277v out UPS, and the manufacturer needed a 4 wire input to the UPS as they used 277V for internal voltage reference or some such thing. I had only brought 3W + ground.

 

[StephenSDH]

Some UPSs have a seperate feeder for recharging the batteries.

 

[ron]

Without knowing the manufacturers data, your selection of 1200A on the output and 1600A on the input is good.

 

[sbspower]

A good rule of thumb for a 750kW would be 1600A on the main input (1200A on the bypass input if it is a dual input UPS) and 1200A on the output. If you detail what UPS you are using, i can confirm the manufacturer recommendations.

 

[Lady Engineer]

Thank you....that's what we put on the last one line diagram. It's been a while since I did one, and those size always confuse me.

I now have the manufacturer's recommendations.