I have a 50kVA, 208-wye, UPS servicing my electrical cabinet. I'm pulling 25A, 3 phase, 208V to service motors and their drives. I also want to pull 25A, 120V off one leg to service all the other peripherals. Will I have any balancing problems when all these units are turned on at FLA?
Pulling 120V from 208-wye, 4wire
- Thread starter rcarlis
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tryinghard
Senior Member
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- California
Your need to calculate your load as per Article 220. Your motors need to be calculated from Table 430-250 (plus an additional 25% of your largest motor). Then your ?peripherals? calculated as per Article 220 and simply spay them evenly across the three phases best you can.
Also notice Article 645-11 for UPS?s and the manufactures specifications for constant load.
Also notice Article 645-11 for UPS?s and the manufactures specifications for constant load.
kingpb
Senior Member
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- Engineer, Registered
rcarlis said:
The rating of the 50KVA UPS is 138+ Amps. Assuming this is the input rating and given some losses, you probably have somewhere around 125+ amps on each phase available.
With the motor load of 25A per phase and adding the 25A of 120V on say phase A, you still have quite a bit of capacity remaining. Just to be sure, double check with the manufacturer that there won't be an issue with the unbalance. There shouldn't be, but you never know for sure til you ask.
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Why would you want to pull all the 120 off a single leg? Is there a good reason not to spread it around? Also, with the numbers 50,25, and 25 showing up so neatly, I must ask whether you meant to say that the "25's" were KVAs, and not amps? If you really did mean amps, then I agree with kingpb that an imbalance will not cause problems, when you are that far under the rating of the transformer.rcarlis said:
tryinghard
Senior Member
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- California
rcarlis said:
Yes because you are placing ?all other peripherals? on one phase. This will cause unbalanced between A, B, & C phases. It doesn?t sound like an overload but it does affect harmonics and should not be done if avoidable.
The calculated load is best known at first because it is needed to determine sizes for: branch wiring, panel, feeder & minimum UPS. I?m guessing this UPS has a 100A breaker so you shouldn?t be overloading it. You need to calculate your loads from VA, start with your motors and use 430-22 and or 430-24 coupled with Table 430-250. Follow the examples in the Annex D8. I?d guess you are using 7 ? HP motors so your VA = 24.2x(1.732x208) meaning 8,718 VA. Divide this across 3 phases meaning 2,906VA per phase for 1 motor.
Are you getting your amperes from Table 430-250?
weressl
Esteemed Member
rcarlis said:
Motors? Drives - presuming you are talking about ASD's?
The HORROR of it!
UPS's are not built nor designed to handle motors as a general rule and would "look" with disdain feeding another L/C chopper load. Better check with the UPS manufacturer with the specific motos and drives. When they see and inrush they tend to switch over to bypass and the inductive kickback on the load side could severly influence your sensitive electronics, to which UPS's are designed to serve. The UPS's would most likely need to be oversized. I did design a system some time ago with mixed loads but it was the 350kVA range with 10HP motors on it. Lucked out that time, but since I was young and foolish and the ratio was so big, I did not have a problem.
I would be very careful if I would need to put another system together and would use a generator to feed standby loads that can take a 3-10 second outage that motoric loads usually tolerate but sensitive electronic components can not.
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