Calculating Three Phase UPS Capacitiy Serving Three Phase Distribution Panels

A 15kva UPS feeds a 3 Phase 208V distribution Panel. 

Almost every branch circuit in the panel is occupied by a single pole breaker in the ON position.  According to the human interface on the UPS, the output is currently 20 amps, 30amps and Zero amps on phases A, B, and C.   The interface also states a 0.86 Power Factor.

The customer would like to place an existing nearby 3 Phase 208V distribution panel on UPS power.  You remove the panel cover and load check the three incoming panel feeders.  The meter reads 20 amps, 10, amps, and 12 amps.

The customer has verified that all loads in both panels are on prior to checking the output on the UPS, and the incoming feeder loads on the second panel.

Assuming a power factor of 0.80 on the second panel, can the customer feed the second panel with a correctly sized three pole breaker off of a circuit in the UPS panel without overloading the UPS, or, will the customer have to purchase a new UPS?

Note: assume you do not know if the panels are phased the same but that you would be able to correctly identify phasing and move loads as necessary to balance the total load.

All Loads are Wye-Connected.

 

Click for the solve:

 

Yes.

The answer is found by calculating what the total load connected to the UPS would be if it fed both panels, and compare that to the rating of the UPS.   Calculate loads in KVA since that is the units the rating of the UPS is given in.

 

 

I. Solve for the existing load for the UPS connected panel, which is equal to the existing UPS output.

UPS:  15KVA Rated,  0.86 PF and 20A, 30A, 0A output.

Load: 208 3phase Panel

Total Load in KVA:

P in Vars = IV

(20A+30A+0A)*120 V l-n =3.6 KVA

 

Even though this is a three phase panel, the load is given in amperage per phase. So to find the total output of the UPS and load of the existing UPS panel, you must add all three phase currents and multiply by the line to neutral voltage.  The line to neutral  phase voltage of 208 Wye is 208/1.73 = 120v

 

II.  Solve for the load of the second panel in KVA

P in Vars = IV

(20A+10A+12A)*120 V l-n =5.04 KVA

III. Solve for what the total load would be if the second panel is connected, and compare to what the UPS is rated for

Total Load:

3.6 KVA + 5.04 KVA = 8.64 KVA

 

There are two ways to interpret the results:

First:

15 KVA – 8.64 KVA =6.36 KVA

The UPS can handle both panels with still having an additional 6.36 KVA capacity if allowed to be 100% loaded by the manufacturer (which most likely is not allowed or recommended).

Second:

Percentage of rated UPS Output:

8.64/15 = 58%

The UPS would be at 58% capacity.   Compare this to manufacturer documentation which most suggest running UPS’s to 80% capacity as fairly safe.

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