UPS secondary OCPD

[Gategator37]

Question: The engineer shows a 500kva ups module and the secondary side is being protected by a 1000AT. I am coming up with 800AT. Am I suppose to be using something other than 125% for my calc? or is the engineer wrong?

Thanks

 

[ron]

600A full load.
Typically an 800A trip on the output is sufficient.
Is it possible that the designer mixed up the input with the output for sizing? Was the input the same size.

 

[markstg]

Question: The engineer shows a 500kva ups module and the secondary side is being protected by a 1000AT. I am coming up with 800AT. Am I suppose to be using something other than 125% for my calc? or is the engineer wrong?

Thanks

Ya, I get the same thing, 800AT for a 500KVA, 3 phase, 480VAC output. Maybe the UPS is modular and can be upgraded in KVA, say to 550KVA, so the engineer is allowing for this. But it would seem CB for the 500KVA operation would be set for 800AT, and if increased, the trip unit changed to 1000AT when the larger capacity is installed.
Seems like a good idea to find out what UPS manufacturer and model is being purchased to check on this.

What wire capacity is shown for the output, I would figure 1000A?

 

[Gategator37]

Yes, they were showing 1000A for the output. The input was rated at 1200 AT

 

[jdsmith]

Think about selective coordination on a UPS - the bypass source breaker, the output breaker, all the downstream output breakers, and possibly the input breaker if it can be used as a bypass source all have to be coordinated. The 1000 AT may be required for coordination purposes.

 

[dbuckley]

Is there some specific electrical or regulatory reason there needs to be a OCPD on the UPS output at all?

Theres usually no electrical reason for it to exist, and its presence is a risk to the continuity of supply output, which is the whole reason the UPS is there in the first place.

 

[ron]

Is there some specific electrical or regulatory reason there needs to be a OCPD on the UPS output at all?

Since the rectifier/inverter will not represent a output side overload/fault on the input with the same characteristics, the UPS is designed to go to bypass where the bypass breaker should protect.
Unfortunately, since the UPS protective characteristics that make it go to bypass are not listed as a protective device like a breaker or fuse, the output breaker is needed to protect the output conductors.
In addition, some low end UPS's don't have a bypass to go to anyway, so there would be not protection of the output conductors.

 

[dbuckley]

All true, except that in either scenario the inverter cannot produce sufficient current to damage the output side wiring, their overload capability is extremely limited.

So is this just a listing issue? A manufacturers instructions issue?