Stand By Generator Load Question?

[steveng]

i have a generator stand by duty, i want to power up an area approx 1600 sq ft with power for lighting,rec, and 1 -5 ton hvac with gas heat, rec circuits only computers and miscellaneous 120v loads. load calc <40a per ph

35 kva, 480 3ph,
100 amp main brkr SDS, neutral is switched, ats asco.

i want to feed an 30 kva 3 ph xfmr with this genset.

any comments or suggestions on this amount of load for this size generator?

i am working with an ec on this project, and he has the bid at present, I want to learn as much as i possibly can, so i ask questions?

i would like to add that i have learned a great deal from you guys from doing searches on subjects and reading the threads.

you guys are ok in my book,

 

[bob]

Check here http://www.kohlerpower.com/resident...nNumber=13561&nodeNumber=1&contentNumber=103#
for a Kohler gen set. Enter your load an the program will give you a size.

 

[ron]

Depending on the power factor of the loads, you might not be able to do it with a 35kVA generator. The generator probably has a 28kW rating. If the loads are close to a unity power factor (easier for calculations), then when you've reached 28kW (28kVA @ 1PF), you have maxed out the gen.
The load would be approximately 33A per phase which is less than your expected 40A per phase. If the HVAC unit starts, then all bets are off, the load will be > than steady state.

 

[mdshunk]

I'm concerned about that generator breaker tripping immediately just trying to excite that particular transformer.

 

[don_resqcapt19]

Marc,
Have you seen that happen? My experience with generators tells me that most cannot produce enough current to trip a breaker in the instantaneous trip range.
Don

 

[mdshunk]

Marc,
Have you seen that happen? My experience with generators tells me that most cannot produce enough current to trip a breaker in the instantaneous trip range.
Don

I've seen it happen with breakers and transormers sized about like he's talking about, but not with this same equipment associated with generators.

 

[ron]

Good point Marc. Although I haven't seen breakers tripping because of the low HP of the prime mover compared to the generator and the inrush, but I have seen them stall because of the inadequacy of the engine to drive past the inrush.

 

[steveng]

i can't see the responses

i can't see the responses

thanks for the responses, i will take the advise and consult with the ec on this project.

ps; i hit a wrong button on this laptop that caused the reponses to disappear.

 

[steveng]

one more question?

one more question?

ron, others, can you answer this question?

when you power up a xfrmr with no load , is the primary current constant or is it determined by the amount of load on the secondary?

for example my primary fla is 33 amp per phase for a 30kva xfmr
the secondary current would be approx 80 amps per phase

if my load was ,let's say for example: 40 amps per phase secondary
would the primary current be 15 amps per phase?

is this assumption accurate?

 

[chaterpilar]

Hello Steve,

The primary inrush current (even with no load) when the transformer is energised can be upto 12 times the primary full load current for (few milliseconds)

Many times this is an issue with the tripping of primary side breaker.

Once on load your assumption is right.

 

[don_resqcapt19]

I agree that when connected to a utility system that the inrush of a transformer can trip the OCPD, but most generators of this size cannot provide enough current to trip a breaker in the instantaneous range even if you directly short the conductors together. It will trip in the short time trip range, but the inrush is a much shorter event than is the short time trip.

 

[mdshunk]

I agree that when connected to a utility system that the inrush of a transformer can trip the OCPD, but most generators of this size cannot provide enough current to trip a breaker in the instantaneous range even if you directly short the conductors together. It will trip in the short time trip range, but the inrush is a much shorter event than is the short time trip.

Sounds like even if it didn't trip the breaker, it might stall the engine.