UPS calculations / loading?

[shooterq]

WHAT IS THE FULL LOAD OF A 30 KVA UPS @ 480 V.AND THE SIZE OF THE OVERCURRENT DEVICE?
I HAVE BEEN USING A CHART THAT STATES A 480V. 30 KVA HAS AN OUTPUT OF 36 A. I SIZED THE BREAKER AT 125% OF 36A.WHICH GIVES YOU A 45 A.BREAKER.I'AM BEING TOLD THAT IT IS NOT A CODE REQUIRMENT BUT THE MANUFACTURE STATES YOU SHOULDN'T LOAD A UPS MORE THAN 80%. I'AM ASKING IF THE 80% IS ALREADY INCLUDED IN THE OUTPUT OF 36 A.?

 

[steve66]

Re: UPS calculations / loading?

The 80% is not included in the 36 amps. A 30kva ups can provide 30,000 Volt-Amps. If you divide this by 480 Volts, you get 62.5 amps. Assuming this is a 3-phase UPS, you would then divide the amps by the square root of 3 (or 1.73) to get the line amps. This gives you your 36 amps.

Circuit breakers are usually only rated for 80% of their trip value for a continuous load. That's one reason they are usually sized at 125%.

If I were you, I would go ahead and size the breaker as you have to allow the UPS to deliver its rated output continuously. (After all, if the breaker should trip, your UPS isn't doing its job of always keeping something powered).

To limit the UPS to 80% of its rating, add all the loads connected to it, and keep this below 24 KVA. Of course, getting the true loads of all the equipment plugged in to it won't be easy.

Steve

 

[brian john]

Re: UPS calculations / loading?

Shoot;

What is the pf rating of the UPS. Many are rated 30 kva 24 kw .8 pf.

 

[jerryb]

Re: UPS calculations / loading?

Be careful on what your are describing as the 36 amp or 30 KVA rating. Are you talking about the "input" or "output" of the UPS. Your numbers are correct for the output and that is what a UPS is rated for. But the input is always more current than the outbut because it has to have the capacity to charge the batteries and deliver the rated capacity all at the same time. The manufacturer should be telling you what the input overcurrent protection is to be rated at.

 

[steve66]

Re: UPS calculations / loading?

Jerry & Brian:

Good points I hadn't thought of. I assumed everything was straight KVA of output power.

Steve

 

[ron]

Re: UPS calculations / loading?

The input breaker has to carry the charging current and the losses within. Output breaker just serves the load.
Regarding kVA and kW of the loads: Many newer power supplies attempt to power factor correct within the supply, so the load will have a power factor close to 1.

 

[bwyllie]

Re: UPS calculations / loading?

Most manufacturers of the UPS will supply this information to you.

 

[brian john]

Re: UPS calculations / loading?

BYW:

The firm I work for does independent testing, the number of UPS's systems we see with improperly sized line and load conductors and OCP surprises me. Also in proper rotation of bypass and proper rotation but phases rotated. Just did a job where the contractor and UPS tech blew up a CB due to this.

Read the UPS literature for the most (ALMOST) reliable information.