Determining KVA For 480/277 Panel

[lumbee]

I have two existing lighting panels that I am trying to size a generator for. Both are 480V 4 wire. The lights are 277 volts. Panel 1 has (6) 20amp light circuits at 277 volts. Assuming 100% circuit usage and evenly dividing 40 amps across all three phases I calculated 33.25 kva (40 amps x 480 volts x 1.732). Panel 2 has (4) 20 amp light circuits at 277 volts. Since you can't balance the load among the three phases, do you assume 1 phase will have to have 40 amps which means this panel will also have 33.25 kva.

 

[alehman]

For most 3-phase generators, each phase can accept a maximum of 1/3 of the 3-phase kVA rating. You need to confirm that with the manufacturer, because some will tolerate only a limited amount of unbalance.

For continuous duty (which lighting is usually assumed to be), the NEC permits loading to only 80% of the circuit rating (16A). Also, most generators shouldn't be loaded to more than 80% of their standby rating.

 

[mpparent]

lumbee,

As far as sizing the genset goes, you don't need to worry about balancing phases. Just calculate the kVA(kW) required. Power is Power at any voltage.

Mike

PS If you assume 80%, as alehman rightly indicates, you would get 44.3kVA for your total lighting load.

 

[ccjersey]

mpparent says
"As far as sizing the genset goes, you don't need to worry about balancing phases. Just calculate the kVA(kW) required. Power is Power at any voltage."

True. Power is power at any voltage, but you must not overload any one of the generator windings unless approved by the manufacturer. Some 3 phase sets for instance are "extended stack" and can supply the full kVA rating as single phase power on only 2 lines, but most cannot and only deliver 2/3 safely. While this is an extreme example of unbalance, it illustrates the limitation placed on power output by the size of each winding.

If you know the amperage of each line, you can size the generator set to handle the max amperage. You can see the advantage of balancing the loads across the 3 phases when you spec. the generator.

If you didn't balance at all, and worst case all the loads were on one line that is 200amps. When you balance as much as possible, the amperage is only 80 maximum, BIG difference. I think calculating the kVA and assuming a balanced load will not matter much if you fudge and oversize the generator quite a bit, but I would check the continuous amperage available from the generator. This is lower than standby rating....maybe the 80% derating mentioned above.

If the breakers are properly sized, the load is only 80% of rating already, is it not? May depend on the breaker design. Could be doubling up on the oversizing for continuous loads here.


Jim