Contractors calculation of 3 phase KVA compared to 3 phase Amps

[scott3229]

Hi all,
I've received a ton of great information here, but there's one thing I've looked for endlessly that I just can't quite wrap my head around at this point, so perhaps someone could help me out.

We are installing a service in a commercial building, so to get an accurate readings of the machines continuous amp draw, I put my clamp meter on the machines, and read a running current of 25 amps per phase at 480v 3 phase, balanced across A B and C

I understand a 25 amp 3 phase loads draw 25 amps per phase, and to convert to KVa Im using S_(kVA) = √3 × I_(A) × V_L-L(V) / 1000 where V =480 and I =25 which puts me at about 20.785 kva. So the contractor provided me a panel schedule the reflected 20.785/3 which equals about 6928 VA and put that in each of the phases. So A = 6928 va B= 6928 va c=6928va.

So here is where Im questioning the calculation, we have (3) of the same machines. So from a logic standpoint I would say 25 amps per phase for one machine should give me around 75 amps per phase for all 3 connected. 75 amps on A, 75 amps on B, and 75 on C.

What he is doing is adding the 6928 (3) times to add up the VA per phase for all (3) machines to get 20785 va per phase on A B and C, then calculated each individual phases current draw using the calculation _{I(A) = 1000 × S(kVA) / (√3 × VL-L(V) ) where V=480 and S = 20.785.
Notice the 3 phase calculation .}

That gave him a calculation of 26 amps per phase. Wouldn't he have to use a single phase calculation if he is just going to add up the VA values from each of the phases and calculate them individually?

So shouldn't he be calculating these at single phase 277 volts to get the current draw per Phase? Or am I missing something here?


Thanks In advance!

Scott

 

[charlie b]

You are not missing anything. He used the total load on a single phase as the input to a three phase calculation.

Welcome to the forum.

 

[GoldDigger]

... He used the total load on a single phase as the input to a three phase calculation.

Which, in case is was not clear, is a mistake.

PS: When discussing the load current in a three phase circuit, it is more accurate and leads to less confusion to refer to the currents on A, B and C as line currents and the currents, which you cannot measure directly except inside the loads, from A to B, B to C and C to A as the phase currents.
It is not a critical issue as long as you recognize which current value you need to plug into which 3 phase power formula.

If you have line to line lighting loads, for example, the total power is just the sum of the line to line currents times the line to line voltages.

 

[scott3229]

You are not missing anything. He used the total load on a single phase as the input to a three phase calculation.

Welcome to the forum.

Okay great and thanks for the confirmation!

Which, in case is was not clear, is a mistake.

PS: When discussing the load current in a three phase circuit, it is more accurate and leads to less confusion to refer to the currents on A, B and C as line currents and the currents, which you cannot measure directly except inside the loads, from A to B, B to C and C to A as the phase currents.
It is not a critical issue as long as you recognize which current value you need to plug into which 3 phase power formula.

If you have line to line lighting loads, for example, the total power is just the sum of the line to line currents times the line to line voltages.

Thanks for that response, and note taken on the line/phase (I'll get that down one of these days after 30th time I've been told) so to clarify, in this instance, I am referring to Line currents of 25 amps whic should equal phase currents because of the wye secondary, and here I believe his end calculation of line current was incorrect based on attempting to convert the sum of a single phases' Kva back to current in Amps using a 3 phase calculation, when in fact he should have been using a single phase calculation.

As far as the line to line lighting loads, you say that because they are single phase (480v for example) loads correct?

Thanks for the clarification!

 

[scott3229]

Okay great and thanks for the confirmation!



Thanks for that response, and note taken on the line/phase (I'll get that down one of these days after 30th time I've been told) so to clarify, in this instance, I am referring to Line currents of 25 amps whic should equal phase currents because of the wye secondary, and here I believe his end calculation of line current was incorrect based on attempting to convert the sum of a single phases' Kva back to current in Amps using a 3 phase calculation, when in fact he should have been using a single phase calculation.

As far as the line to line lighting loads, you say that because they are single phase (480v for example) loads correct?

Thanks for the clarification!

And to clarify that single phase 277v calculation in my example.

 

[charlie b]

The choices are:

• Take the 6928 times 3 (for three machines) and times 3 again (for three phases), then divide by 480 volts and divide again by the square root of 3. Result: 75 amps.
• Take the 6928 times 3 (for three machines), then divide by 277 volts. Result: 75 amps.

 

[Besoeker]

Hi all,
I've received a ton of great information here, but there's one thing I've looked for endlessly that I just can't quite wrap my head around at this point, so perhaps someone could help me out.

We are installing a service in a commercial building, so to get an accurate readings of the machines continuous amp draw, I put my clamp meter on the machines, and read a running current of 25 amps per phase at 480v 3 phase, balanced across A B and C

I understand a 25 amp 3 phase loads draw 25 amps per phase, and to convert to KVa Im using S_(kVA) = √3 × I_(A) × V_L-L(V) / 1000 where V =480 and I =25 which puts me at about 20.785 kva. So the contractor provided me a panel schedule the reflected 20.785/3 which equals about 6928 VA and put that in each of the phases. So A = 6928 va B= 6928 va c=6928va.

So here is where Im questioning the calculation, we have (3) of the same machines. So from a logic standpoint I would say 25 amps per phase for one machine should give me around 75 amps per phase for all 3 connected. 75 amps on A, 75 amps on B, and 75 on C.

What he is doing is adding the 6928 (3) times to add up the VA per phase for all (3) machines to get 20785 va per phase on A B and C, then calculated each individual phases current draw using the calculation _{I(A) = 1000 × S(kVA) / (√3 × VL-L(V) ) where V=480 and S = 20.785.
Notice the 3 phase calculation .}

That gave him a calculation of 26 amps per phase. Wouldn't he have to use a single phase calculation if he is just going to add up the VA values from each of the phases and calculate them individually?

So shouldn't he be calculating these at single phase 277 volts to get the current draw per Phase? Or am I missing something here?


Thanks In advance!

Scott

Are the machines single phase or three phase?

 

[dkidd]

Are the machines single phase or three phase?

He said

I put my clamp meter on the machines, and read a running current of 25 amps per phase at 480v 3 phase, balanced across A B and C

 

[Besoeker]

He said

But are they three phase machines?

 

[GoldDigger]

...
As far as the line to line lighting loads, you say that because they are single phase (480v for example) loads correct?

Yes, that is why I chose that as an example.
And because they will be specified as a particular power input, which then corresponds simply to the line to line current (if set up for near unity power factor).