Feeder from UPS to loadcenter

[jcook980]

Another engineer in our office sized and spec'ed a 9kVA UPS unit with appropriately sized conductors and CB on the output. The client purchased a modular 9kVA unit that the manufacturer can easily increase the output to 18kVA. Even though only 9kVA is needed, only 9kVA will be purchased and no future growth is anticipated, the manufacturer is insisting that the conductors and CB be sized for 18kVA.

Is this a Code requirement?

 

[bennie]

Re: Feeder from UPS to loadcenter

Why is the manufacturer insisting on wiring for twice the capacity?

 

[plechtenberg]

Re: Feeder from UPS to loadcenter

Is this downstream of the UPS (i.e. wire from UPS to main circuit breaker in a panel)? If so, and the overcurrent protection on the output of the UPS is rated for 18KVA, what is protecting this wire? You could be able to utilize the tap rules if you are within appropriate distance.

 

[jcook980]

Re: Feeder from UPS to loadcenter

The vendor claims that they won't warranty the unit unless the output conductors and the OCPD is rated for the highest possible capacity of a fully expanded unit i.e. 18kVA.

Let's say we leave it with the 60A CB designed instead of the 125A CB they are insisting on. Down the road ten years, some maintenance type gets the idea of doubling the size of the UPS without benefit of a licensed electrician, permit or inspection. What's the worst that could happen if the UPS is doubled in size and the load tries to draw more than the rating of the 60A CB?

 

[jtb]

Re: Feeder from UPS to loadcenter

This makes me wonder about the UPS design itself.
Perhaps it is the internal components that are limiting power output by changing power factor characteristics?

UPS systems are known for thier very low power factor in some situations.

I would follow the manufacturers instructions and consider the 18KVA as the 'nameplate' rating.

 

[gregory]

Re: Feeder from UPS to loadcenter

I agree with JTB. Follow the manufacturers instructions. I think that the Nec2002 460.24 may be the reason for the manufacturer wanting this. I think that they are dealing with inrush current. I know that it is not over 600 volts but I think that it is the same idea. it talks about faults and inrush current & interupting capacities.

II. Over 600 Volts, Nominal
460.24 Switching.
(A) Load Current. Group-operated switches shall be used for capacitor switching and shall be capable of the following:
(1) Carrying continuously not less than 135 percent of the rated current of the capacitor installation
(2) Interrupting the maximum continuous load current of each capacitor, capacitor bank, or capacitor installation that will be switched as a unit
(3) Withstanding the maximum inrush current, including contributions from adjacent capacitor installations
(4) Carrying currents due to faults on capacitor side of switch

 

[bennie]

Re: Feeder from UPS to loadcenter

There is no logical reason to size a feeder at twice the load. This sounds like a sales gimmick.

Have the manufacturer's rep give a written explanation for the reason.

Oversizing feeders also increases available fault current.

 

[dereckbc]

Re: Feeder from UPS to loadcenter

Is this a standby or static UPS? Also what size is the input OCPD?

[ June 04, 2003, 11:06 PM: Message edited by: dereckbc ]