208/120V Wye Source Delta Load Power Calculations

[Fmt662]

Hi all,

Long time lurker, first time poster. I am hoping that the minds here can help me overcome a mental block.

I am trying to set up formulas to calculate power for various circuits in a data center. We have 208/120V power distribution panels from which we provide typically 208V circuits, usually L6 single-phase but sometimes 3 phase. The distribution panels have metering CTs on each pole measuring line current. All the equipment connected are single phase, typically 208V, servers.

For a simple L6 circuit, I know that power = V * A * Pf, so for a 24A load at .9pf, i get 208 * 24 * .9 = 4,492w.
For a balanced 3 phase circuit, power = V * A * 1.73 * Pf, so for the same 24A per line load I calculate 208 * 24 * 1.73 * .9 = 7,772w

But when I try to calculate the unbalanced load, I can't seem to make sense of it. Since the loads are effectively delta loads, a L6 circuit connected L1 - L2 should be the same power as a 3 phase circuit with all the loads on the L1 - L2 receptacle bank. My metering should still show 40A on L1 and L2 and 0A line 3. But my power calculation shows a consumption of 5,181w.

I'm sure there is some conceptual issue I am missing here. How can I fix the formulas or am I just missing a concept?



Thanks.

 

[david luchini]

But when I try to calculate the unbalanced load, I can't seem to make sense of it. Since the loads are effectively delta loads, a L6 circuit connected L1 - L2 should be the same power as a 3 phase circuit with all the loads on the L1 - L2 receptacle bank. My metering should still show 40A on L1 and L2 and 0A line 3. But my power calculation shows a consumption of 5,181w.

How do you arrive at 5,181w?

 

[Fmt662]

My current formula is (L1 Amps + L2 Amps + L3 Amps)/3 * 208(V) * 1.73 * .9(pf). I know that is for balanced 3ph circuits which this isnt.

I think I need to set up a formula to calculate each phase of the 3 phases separately since all my loads are single phase. Any advice how to do that?

 

[jumper]

How do you arrive at 5,181w?

What is a "w"?

I know what "W" stands for?

 

[Besoeker]

What is a "w"?

I know what "W" stands for?

And P is for pedant...........

 

[Besoeker]

My current formula is (L1 Amps + L2 Amps + L3 Amps)/3 * 208(V) * 1.73 * .9(pf). I know that is for balanced 3ph circuits which this isnt.

I think I need to set up a formula to calculate each phase of the 3 phases separately since all my loads are single phase. Any advice how to do that?

You can't simply add the currents like that. Calculate them as sinngle phase loads. VIcos(phi).

 

[charlie b]

I will start by talking about your sketch. If, in fact, there are no loads on the L2-L3 and L1-L3 legs, then the three-phase circuit will draw exactly the same 4992 watts as the single phase circuit. Therefore, you are correct when you say,

Since the loads are effectively delta loads, a L6 circuit connected L1 - L2 should be the same power as a 3 phase circuit with all the loads on the L1 - L2 receptacle bank.

However, I don’t know where you get the 40 amp from, in this statement: `

My metering should still show 40A on L1 and L2 and 0A line 3.

No, the meter should show 24 on L1 and L2, and 0 on L3.

But my power calculation shows a consumption of 5,181w.

That calculation methodology is not valid. Calculating unbalanced loads on a three phase system is far, far more complex than taking the average current as though all three phases shared that same value of current. When one phase is zero amps, it is easy enough, as shown above. When all three phases are non-zero, and are not the same value, then the best thing to do is to hope that you really don’t need to predict, by calculation, what the meters will show, by measurement.

For a simple L6 circuit, I know that power = V * A * Pf, so for a 24A load at .9pf, i get 208 * 24 * .9 = 4,492w.
For a balanced 3 phase circuit, power = V * A * 1.73 * Pf, so for the same 24A per line load I calculate 208 * 24 * 1.73 * .9 = 7,772w

These two formulas are correct.

 

[jumper]

And P is for pedant...........

Does that mean a G can be for Goofball, like me, since the mho has been replaced by the siemens(S).

 

[loveng]

Delta Load has different phase and load currents

Delta Load has different phase and load currents


The difference between the power calculated and the one you measured are voltage drops, a fp greater than 0.9 or simply the load to be consuming more than 24 A.

Greetings from El Salvador

 

[loveng]

Delta 3phase connections have different line and phase currents

Delta 3phase connections have different line and phase currents

Hi all,

Long time lurker, first time poster. I am hoping that the minds here can help me overcome a mental block.

I am trying to set up formulas to calculate power for various circuits in a data center. We have 208/120V power distribution panels from which we provide typically 208V circuits, usually L6 single-phase but sometimes 3 phase. The distribution panels have metering CTs on each pole measuring line current. All the equipment connected are single phase, typically 208V, servers.

For a simple L6 circuit, I know that power = V * A * Pf, so for a 24A load at .9pf, i get 208 * 24 * .9 = 4,492w.
For a balanced 3 phase circuit, power = V * A * 1.73 * Pf, so for the same 24A per line load I calculate 208 * 24 * 1.73 * .9 = 7,772w

But when I try to calculate the unbalanced load, I can't seem to make sense of it. Since the loads are effectively delta loads, a L6 circuit connected L1 - L2 should be the same power as a 3 phase circuit with all the loads on the L1 - L2 receptacle bank. My metering should still show 40A on L1 and L2 and 0A line 3. But my power calculation shows a consumption of 5,181w.

I'm sure there is some conceptual issue I am missing here. How can I fix the formulas or am I just missing a concept?

View attachment 15916

Thanks.

Here are my calculations
The difference between the measurement of power and results are maybe due to voltage drop, fp greater than 0.9 or just the load not to consume exactly 24 A.

 

[Smart $]

...
For a simple L6 circuit, I know that power = V * A * Pf, so for a 24A load at .9pf, i get 208 * 24 * .9 = 4,492w.
For a balanced 3 phase circuit, power = V * A * 1.73 * Pf, so for the same 24A per line load I calculate 208 * 24 * 1.73 * .9 = 7,772w

But when I try to calculate the unbalanced load, I can't seem to make sense of it. Since the loads are effectively delta loads, a L6 circuit connected L1 - L2 should be the same power as a 3 phase circuit with all the loads on the L1 - L2 receptacle bank. My metering should still show 40A on L1 and L2 and 0A line 3. But my power calculation shows a consumption of 5,181w.
...

24A L1-L2 is 24A on L1 and [the same] 24A on L2.

FWIW, when your calculation changes from phase current to line current, you shift the voltage reference (208 versus 120) and thus the voltage and power factor angles (±30° relative).

 

[engr.tahir26]

How to Select the transformer size??

How to Select the transformer size??

Dear All,
I am new here, Hopefully i will got a good response from all of u guys, i am new in electrical also, i want to select the transformer size of voltage 480/208-120, My load currents are as AB=10A, BC=11A, CA=13A, AN=12A, BN=12A, CN=14A, 3Phase= 15A. where A,B,C are phase 1,2,3 and N is for neutral. Can any body help me to size the transformer?? actually i want to know that in formula (KVA=1.732*VOLTAGE*CURRENT/1000 ) which current i should i have to mention, either it should be AB or BC or CA or AN or BN or CN or 3 phase??? please help

 

[Ingenieur]

Dear All,
I am new here, Hopefully i will got a good response from all of u guys, i am new in electrical also, i want to select the transformer size of voltage 480/208-120, My load currents are as AB=10A, BC=11A, CA=13A, AN=12A, BN=12A, CN=14A, 3Phase= 15A. where A,B,C are phase 1,2,3 and N is for neutral. Can any body help me to size the transformer?? actually i want to know that in formula (KVA=1.732*VOLTAGE*CURRENT/1000 ) which current i should i have to mention, either it should be AB or BC or CA or AN or BN or CN or 3 phase??? please help

How did you measure the currents?
the phase to neut currents are line currents (eg AN)
AB is a phase current...how was it measured (summed AN + (-BN)?)
what is a 3 phase current? The neut current?

typically use the largest line current

 

[Smart $]

Here's the panel schedule method....

	A	B	C
AB	1040	1040
BC		1144	1144
CA	1352		1352
AN	1440
BN		1440
CN			1680
3Ø	1800	1800	1800
Total	5,632VA	5,424VA	5,976VA
Amps	46.9A	45.2A	49.8A

 

[topgone]

Dear All,
I am new here, Hopefully i will got a good response from all of u guys, i am new in electrical also, i want to select the transformer size of voltage 480/208-120, My load currents are as AB=10A, BC=11A, CA=13A, AN=12A, BN=12A, CN=14A, 3Phase= 15A. where A,B,C are phase 1,2,3 and N is for neutral. Can any body help me to size the transformer?? actually i want to know that in formula (KVA=1.732*VOLTAGE*CURRENT/1000 ) which current i should i have to mention, either it should be AB or BC or CA or AN or BN or CN or 3 phase??? please help

For the loads on the phases, use the formula: 208V x phase current = kVA load
For the line to neutral loads, use the formula: 120 x line current.
Your total connected load are: Phase loads =12.27 kVA and Line-neutral loads = 4.56 kVA = 16.83 kVA total! ~ 25 kVA!

Alternately, from my quick calcs (assuming PF = 1 for all loads): Ia = 46.94A, Ib = 44.32A, Ic = 48.94A, In = 2 A.
I'd use the highest line amps to get the needed transformer size = 1.732 x 48.94x 208/1000 = 17.63kVA ~ 25kVA!

I'd say you need a 25kVA or bigger, if you want to consider future loads!

 

[Smart $]

For the loads on the phases, use the formula: 208V x phase current = kVA load
For the line to neutral loads, use the formula: 120 x line current.
Your total connected load are: Phase loads =12.27 kVA and Line-neutral loads = 4.56 kVA = 16.83 kVA total! ~ 25 kVA!

Alternately, from my quick calcs (assuming PF = 1 for all loads): Ia = 46.94A, Ib = 44.32A, Ic = 48.94A, In = 2 A.
I'd use the highest line amps to get the needed transformer size = 1.732 x 48.94x 208/1000 = 17.63kVA ~ 25kVA!

I'd say you need a 25kVA or bigger, if you want to consider future loads!

25kVA not a common 3Ø xfmr size. Probably have to go with a 30kVA... but who knows whats available in Pakistan...?

http://www.schneider-electric.us/en/faqs/FA91532/