UPS Wire Sizing

[steve66]

Planning for a 160KVA, 144KW, 208V, 3 phase UPS.

From the UPS cut sheets, the maximum input current is 480 amps.

The UPS manual lists the maximum input breaker size as 700 amps, and it calls for (2) sets of 400KCM wire.


Question: Can I feed this UPS with a 600 amp breaker and (2) sets of 300 KCM? Two sets of 300's would have an ampacity of more than 500 amps even if I include derating for 4 current carrying conductors. That seems to satisfy the NEC.

Or do I have to follow the manufacturer's reccomendations even though I'm not using a 700 amp breaker?

Steve

 

[augie47]

IMO, If the mfg states "maximum" breaker then there is no Code reason that you can't go smaller.
The use of the 300 kcmil with the 600 amp breaker would be dependent on your calculated load based on this thread:
http://forums.mikeholt.com/showthread.php?t=122364&highlight=240.4

 

[ron]

Steve,
I'm sure the manufacturer know best, but that unit would have 444 FLA. The battery recharge current and losses for the double conversion itself which would be in addition, it sounds like 480A for max input current is low.
I think that a 600A 100% rated breaker would be okay as it is the next standard size CB above the 480A. If you were to use a regular molded case breaker, then go with 700A.

Generally they list maximum breaker size, you can provide less but then you need to be sure you program the recharge current low, so the breaker doesn't trip.

 

[dbuckley]

Just a note - although you can use any size breaker smaller than the maximuim stated to feed the UPS, the charging current consumed by the UPS is very roughly proportional to thie size of it. So (being silly) if you only have a 20A load on that UPS, and that might indicate that perhaps a 30A breaker would be plenty, the charging current will likely open your breaker.

Of course, as batteries arrived fully charged, you'll only see that after the batteries have been dischaged by running them for a while... Just the time you want the CB to open...

 

[ron]

dbuckley,
On the units I work with, they typically have a current limit circuit so that it would limit the maximum amount of charging current permitted based on it's setting. That way the input breaker doesn't have to be humongous as compared to the bypass breaker, although the input is always somewhat larger because of losses and the limited charging current.
I like to set the current limit for charging current to 15% of the module rating if I have the room on the input breaker capacity, but have lowered it to 10% on many occasions due to breaker size.

 

[dbuckley]

Thats nice flexibility to have.

 

[ron]

Many of the larger units can do the current limit. I'm usually doing parralleled 750's, but I've had it with 400's too.

 

[mayanees]

I think I'd supply this as if the 160 kVA were a continuous load at 125%. That's 444 * 1.25 = 555 amps. That gives room for the intermittent charging current up to 25% as well as a fully loaded UPS - which it should be designed to accommodate. So that's in support of the OP's original design using parallel 300s for an ampacity of 570 amps and a 600 amp, 100% rated breaker.
Ron - I think you erred with your statement that the next standard rating above 480 is 600. 240.6(A) lists a 500. Am I missing something?
John M

 

[ron]

John,
You are correct. 500A is in there.
I'll correct my statement by saying that some UPS inputs (front ends) have transformers, so I'd stay with 600A 100% rated or 700A for a regular molded case breaker to help overcome the inrush.
I still think 480A is low for a max input current to the rectifier.

 

[steve66]

Thanks for all the responses.

I also thought 480 amps seemed low. That only leaves 35 amps for losses, and charging current (which would be less than 10% charging current.)

I tried to find exactly how much charging current the UPS needs, but all the cut sheet says is "adjustable for generators".

I'm pretty sure this one doesn't have an input xformer, but the cut sheet does say to size the input breaker for inrush as if you were feeding a transformer.

Mayanees: I think you are right about using 125% of the 160 KVA as a continuous load. If you combine that with derating to 80% for 4 CCC's in a conduit, that seems to justify the two sets of 400's they list in the manual.

I'll probably just go with the 700 A breaker and the two sets of 400's. But I have had complaints from the electricians before that its really hard to fit the wire specified by the UPS manual in the bypass cabinet. The bypass cabinet usually doesn't even seem to have enough room for the conduit entries.

Steve