3 phase Demand load

[Roger]

It occurred to me today that I overlooked a critical part of a project at work. We are going to add 30 fans as it turns out to a 208Y/120 3 phase panel. This panel is fed by a 30 KVA dry type transformer. I'm working on the demand load of the panel and to make sure the transformer is big enough to handle the load we are going to add. The panel is rated 250 amps. It presently has a 100 amp 3 phase main breaker. There are presently 35 fans connected to this panel and no additional loads. We are going to add 30 more fans. These fans (including the ones already installed) are 120 volt 2.8 amp fla 1/4 h.p. so each fan is 336 VA. I'm showing 11,760 VA (98 amps at 120) volts))for the existing 35 fans as connected load and 10,080 VA (85 amps at 120 volts) for the additional 30 fans. 21,840 VA total load. Thats 183 amps total for all three phases or 61 amps per 120 volt leg. Then I'm dividing 21,840 VA by 208 volts getting 105 amps total demand load. I'm not 100% confident that I am handling this three phase panel, load calculation correctly. Would any of you guys know if this is correct? Now the transformer is 30 KVA so do you just divide this by 208 volts and get 144 amps? Would this be per phase leg? Or is there other factors to consider. Does anybody know a link to a site that would be helpful?

Anyway, have one of our skilled electricians coming tomorrow to check all this out. Would like to see if I can try to calculate this and compare to his calculations.

[mdshunk]

Since you have all 120 volt loads, it's pretty easy to figure the load on the panel. 65 fans total divided by 3 phases equals approximately 22 fans on each phase. 22 fans times 2.8 amps each equals about 61 amps on each phase.

To find the find the amp capability of the transformer, you need to devide the 30kva by the voltage (like single phase) but you have to multiply the voltage times the square root of 3 (1.732). The math is:

30,000 va/208*1.732= 83 amps

Many transformer manufacturers permit their transformers to be permanently overloaded at about 140%. This makes your 30kva transformer basically a "100 amp 3 phase transformer".

[Put another way, 3 phase VA=volts x amps x 1.732]

Don't forget that square root of 3 for 3 phase calculations. It'll mess you up every time.

[Roger]

Thanks Marc, that was why I wasnt very confident in the transformer calc. I wasnt sure if the square root of three (1.732) came into play.

[mdshunk]

Sounds like you're gonna end up with a "fan transformer" and a "fan panel"!!

Go figure.

Tell the bean counter that if you could have gotten 277 or 480 fans from the very beginning, you could have avoided all that expense!