Line-to-Line kVA of 3 phase Motor

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Kaizoku_Juffy

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Thanks for accepting me here!

This could be an "easy" question for you so I beg your pardon. My 3-phase knowledge is still shaky so I look forward to learn from you here.

Supposed that I have a 6 kVA 3-phase Motor to be installed in a 120/208V, 4-wire system. If I'm going to prepare for a Panel Board Schedule, I know that the Line-to-Neutral kVA would be 2 kVA per phase. But what if I show a Line-to-Line kVA on my Schedule? What would be the Line-to-Line kVA of this load? Is it 6 kVA per Line? Or is it 3.47 kVA? (Line Current is equal to 16.67A, multiplying by 208V Line Voltage)

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line-to-line-kva

Image line-to-line-kva hosted in ImgBB
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I hope your explanation can provide clarification and remove my confusion regarding this matter.
Thank you!
 
Your 6 kVA 3-phase motor is inherently balanced by itself per phase. Since you are connecting it to 2 "phases" (which is actually 2 lines), your line to line kVA should show 6 kVA per line and NOT 3.47 kVA
 
The line to line kVA and the line to neutral kVA are both approximate values which you put into a panel schedule so that you can come up with an approximate total for the entire panel and have a reasonable understanding of the panel load and load balance.

In a l-n panel schedule you need numbers that you can total up for each of the A, B, and C busses.

In a l-l panel schedule you need numbers that you can total up for your A-B loading, your B-C loading, and your C-A loading.

In both cases you have rough and ready kVA assignment rules that you use.

For a line to neutral panel schedule you assign each connected (to that phase) l-n load directly, 1/2 of each connected l-l load, and 1/3 of each connected 3 phase load.

A 6kVA l-n load on phase A counts as 6 kVA towards the A total.
A 6 kVA l-l load between phase A and B counts as 3vkVA towards the A total.
A 6 kVA 3-phase load counts as 2 kVA towards the A total.

For a l-l panel schedule you assign 1/2 of each l-n load, full of each connected l-l load, and 1/3 of each connected 3 phase load.

A 6 kVA l-n load on phase A counts as 3 kVA towards the A-B total. (The other 3 kVA count towards the C-A total.)
A 6 kVA l-l load between phases A and B counts as 6 kVA towards the A-B total.
A 6 kVA 3-phase load counts as 2 kVA towards the A-B total.

The _exact_ way to do this is to tabulate each load and the phase angle of the current drawn by the load, and then do the vector addition of all the loads on each bus. I intentionally left this description in the dense 'gobbledygook' form because it is rarely necessary to actually do this, but it is important to understand what you are approximating.

Jon
 
winnie said:
For a l-l panel schedule you assign 1/2 of each l-n load, full of each connected l-l load, and 1/3 of each connected 3 phase load.

A 6 kVA l-n load on phase A counts as 3 kVA towards the A-B total. (The other 3 kVA count towards the C-A total.)
A 6 kVA l-l load between phases A and B counts as 6 kVA towards the A-B total.
A 6 kVA 3-phase load counts as 2 kVA towards the A-B total.
Click to expand...

Hi Sir Jon! Thank you for giving a very informative explanation. However, I'm getting hard time accepting the fact that the 6kVA 3-phase load counts as 2 kVA per line A-B. May you further explain the reason why? I think the source of my confusion is I see the 3-phase load connected this way:

ibb.co

balanced-wye-wye-connection

Image balanced-wye-wye-connection hosted in ImgBB
ibb.co ibb.co

As you can see, each of the winding of the load is connected per phase (Phase A-n, etc.) and the way I see the 2 kVA is between 1 line and the neutral, not between two lines. May I ask you a sample of panel board schedule from your old project? Maybe using one as reference may help me to fully grasp this.

Thank you!



--
I may sound dumb but if that's what it takes to learn, I will take it.
 
I don't have a l-l panel board schedule to share. I don't use l-l panel schedules. I simply grasp the concept.

This is about accounting, not physics.

The motor could internally be connected wye (motor folk call it star) or delta. Externally you don't know the difference and you don't care. For the panel schedule you simply have the motor kVA rating.

If you are doing a normal L-N panel schedule, when you connect the motor you assign 1/3 of its kVA to each bus.

If you are doing a L-L panel schedule (why are you doing an L-L calculation in the first place?), when you connect the motor you assign 1/3 of its kVA to each bus-bus column. It is simple symmetry. You have 6kVA going to a symmetric load and you have 3 symmetric supplies.

-Jon
 
Are you sure the motor is really 4 wire? An overwhelming majority of 3 phase 208V motors are 3 wire, even if they are powered from a 4 wire panelboard, you wouldn't bring a neutral to the motor, since it is un-needed and there would be no place to terminate it, as it is generally a delta connection.
If you truly have a wye connected motor, please feel free to ignore my comment.
 
ron said:
Are you sure the motor is really 4 wire? An overwhelming majority of 3 phase 208V motors are 3 wire, even if they are powered from a 4 wire panelboard, you wouldn't bring a neutral to the motor, since it is un-needed and there would be no place to terminate it, as it is generally a delta connection.
If you truly have a wye connected motor, please feel free to ignore my comment.
Click to expand...
In my career I only saw one motor with a T0 connection. It was a 3 phase, 480 volt, 500 hp motor. The drawings did not show any connection to that terminal point. I have no idea of what its function would be.
 
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