Maybe some clarification is needed...
Maybe some clarification is needed...
I see where my mistake was made - I figured an Open Delta connected Transformer, simply because the Loads are L-L. As so, the total KVA was added up in an "Open Delta" fashion - hence the 150 KVA Rating.
Per the Delta Secondary, the qualifying "Phase" description would be at the connection points for Two individual Windings' leads... i.e.: Terminal X1 being "Phase A" is made at the Left-Hand side Lead of Secondary Winding "A", along with the Right-Hand side Lead of Winding "C".
This Open Delta would be setup as a Right Triangle, with Phase A derived from the lower Right end, Phase B at the top of the Triangle, and Phase C from the lower left end of the Triangle.
With each separate Winding being capable of supplying 75 KVA, the combination of both will allow for Two 75 KVA Single Phase L-L Loads to be connected as:
Load #1 connected between Phase "A" and Phase "B",
Load #2 connected between Phase "C" and Phase "A".
Phase "A" will be common to both Loads as far as Termination and Feeder (individual Feeders to each Load, or a single Feeder connected to both Loads), but the Windings will have the same Apparent Power overall drawn by the Loads - in this case, each Winding will have 66.5 KVA drawn by the Load, with each Winding having a Maximum Apparent Power Capacity of 75 KVA.
If we apply this to a Wye Connected Secondary, the common Load on Phase "A" will be 66.5 KVA (66,560 VA), whereas the Load on Phase "B" or Phase "C" will only be 33.28 KVA (33,280 VA).
In this case, the "Per Phase" Load is also the "Per Winding" Load, so the Apparent Power drawn per Phase Winding "A" is 66.5 KVA.
A 150 KVA Wye connected Secondary will have a Maximum "Per Phase" (per Winding) Apparent Power Capacity of 50 KVA, which is 75% of the required Apparent Power needed on the "Common Phase A", but is 227% of the required Apparent Power for Phase "B" and Phase "C".
This Design _MIGHT_ perform without issues, since we might have Non-Continuous Loads at hand (some factor of Load Diversity where the Phase A Load would be at Maximum KVA draw for less than 180 Minutes), but for me I would rather compile the Design per the highest Phase Load of 66.5 KVA, so a 225 KVA Wye Connected Transformer would be applied to the Project.
The 225 KVA Transformer will have a Per-Winding Apparent Power Rating of 75 KVA, resulting in 112% of the highest Load KVA draw per Phase (Ph. A).
The 225 KVA Wye Transformer is more "Overkill" for these Loads, as compared to the 150 KVA Open Delta Transformer.
Availability would be more of a key decision than anything else.
Question to david luchini
If you were to compile a Panel Schedule for this scenario, how would you apply the Load VA per Circuit, then the Panelboard's Rating?
Examples:
Load #1: Ckt 1/3,
Load #2: Ckt. 5/7
System is derived from a 208V 3 Phase Wye Connected SDS.
Method #1:
Ckt #1 (Phase A): 33,280 VA
Ckt #3 (Phase B): 33,280 VA
----------------------------
----------------------------
Ckt #5 (Phase C): 33,280 VA
Ckt #7 (Phase A): 33,280 VA
----------------------------
----------------------------
Total Loads: Phase A: 66,560 VA; Phase B: 33,280 VA; Phase C: 33,280 VA.
Highest Phase: 66,560 VA (554.67 Amps)
...............................................................................................................
...............................................................................................................
Method #2:
Ckt #1 (Phase A): 66,560 VA
Ckt #3 (Phase B): 66,560 VA
----------------------------
----------------------------
Ckt #5 (Phase C): 66,560 VA
Ckt #7 (Phase A): 66,560 VA
----------------------------
----------------------------
Total Loads: Phase A: 133,120 VA; Phase B: 66,560 VA; Phase C: 66,560 VA.
Highest Phase: 133,120 VA (1109.34 Amps)
...................................................................................................
...................................................................................................
Method #3:
Ckt #1 (Phase A): 33,280 VA
Ckt #3 (Phase B): 33,280 VA
----------------------------
----------------------------
Ckt #5 (Phase C): 33,280 VA
Ckt #7 (Phase A): 33,280 VA
----------------------------
----------------------------
Total Loads: Phase A: 66,560 VA; Phase B: 33,280 VA; Phase C: 33,280 VA.
Highest Phase: 66,560 VA (320.00 Amps)
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Just curious...
Scott