3? kVA Calculation

[tkb]

If I wanted the total kVA on a 3? 208/120v panel would I average the curents on the three phases or use the highest one in the formula below?

Phase A = 20 amps
Phase B = 40 amps
Phase C = 23 amps

KVA = E x I x 1.73 \ 1000

KVA = 208 x ? x 1.73 \ 1000

 

[charlie b]

There is no single "right" answer. If this is a test question, then it is an unfair test question. Everything depends on why you are doing the calculation, and what you intend to do with the results. Averaging the three currents will give you the more reasonable representation of the total load on the system. I would use this value if I were giving the mechanical engineers information for them to use in estimating the performance of their ventilation system. Taking the highest current (the conservative or "worst case" value) will give you a better answer, if you are trying to decide whether there is capacity for adding load to the system.

 

[busman]

Charlie,

Am I missing something. 120/208V would be a Wye connected load. The total KVA is just the sum of the three phase currents times the L-N voltage of 120V (assuming a PF of 1.0).

Mark

 

[tkb]

There is no single "right" answer. If this is a test question, then it is an unfair test question. Everything depends on why you are doing the calculation, and what you intend to do with the results. Averaging the three currents will give you the more reasonable representation of the total load on the system. I would use this value if I were giving the mechanical engineers information for them to use in estimating the performance of their ventilation system. Taking the highest current (the conservative or "worst case" value) will give you a better answer, if you are trying to decide whether there is capacity for adding load to the system.

Not a test question.
I am sizing a UPS for this system and was trying to determine the existing kVA on the panel.
I know the load needs to be balanced, but that will come later.
I used the highest current for my calculation.
If I average the current I get ~10kVA. If I use the highest current I get ~14kVA.
Along with the existing load and the future capacity requested it is the difference between a 20 and 30 kVA UPS.

 

[jim dungar]

If I was sizing something, I would use the highest load current, not the average.

For example: if you need to supply loads of A= 30A, B=20A, and C=10A, running a 20A 3-phase circuit would mean that one leg is overloaded.

 

[charlie b]

Am I missing something. 120/208V would be a Wye connected load. The total KVA is just the sum of the three phase currents times the L-N voltage of 120V (assuming a PF of 1.0).

Which is right:
? 2 plus 5 minus 3 equals 4, or
? 5 minus 3 plus 2 equals 4?

If you can get to the same answer using two different methods, is one right and the other wrong? Perhaps not. But perhaps one is better, because it will always be a valid method, and the other might sometimes lead you astray.

Which is right:
? (A + B + C) divided by 3, then multiplied by 208, and multiplied again by the square root of three, or
? (A + B + C) multiplied by 120?

If you note that 208 is the same as 120 times the square root of 3, you will see that the two expressions will give the same answer. But if you always take the line current times the phase-to-phase voltage times the square root of three, as your standard method, you are more likely to get the same answer, regardless of whether you are dealing with WYE or delta.

 

[charlie b]

I know the load needs to be balanced, but that will come later.

Pretend you will do it first. The answer you get will be the 20KVA unit. I see no reason that is not sufficient. But the final decision might be based on the likelihood that future growth might make the 20 KVA unit too small.

 

[Besoeker]

Charlie,

Am I missing something. 120/208V would be a Wye connected load. The total KVA is just the sum of the three phase currents times the L-N voltage of 120V (assuming a PF of 1.0).

Mark

kVA does not depend on power factor.

Anyway, to answer the original point, I would go with the highest current.
The UPS will have an equal output current rating on all three phases.
There is no trade-off for lowering the current on one phase to get more on another.

 

[Besoeker]

Further to post #8, you also need to consider the kW rating of the UPS.
Typically, a UPS is rated at say 15kVA at 0.8 pf.
If your phase A 40A is all unity power factor, you may hit the kW limit.
I had that happen to me once.
I had expected a UPS with a 0.8 pf rating.
What I got was one at 0.7 pf.
The load to be supported was within the kVA rating, but not the kW rating.
A $4,000 cock-up.
Sometimes learning can be an expensive pain.

 

[petersonra]

Not a test question.
I am sizing a UPS for this system and was trying to determine the existing kVA on the panel.
I know the load needs to be balanced, but that will come later.
I used the highest current for my calculation.
If I average the current I get ~10kVA. If I use the highest current I get ~14kVA.
Along with the existing load and the future capacity requested it is the difference between a 20 and 30 kVA UPS.

go with the bigger ups.

 

[tkb]

After talking with our engineer about UPS'. I think the kVA rating should be ignored and just use the kW rating since this is the actual power available.
With the different PF ratings on the UPS', some have .8 or .9 even seen 9.7 for 3? units.
You could run out of kW before you run out of kVA.

I used the highest value for my calculation and am going to use a 30 kVA UPS to provide some future expansion.