Unbalanced Loads in three phase panel

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W@ttson

Senior Member
Location
USA
Hello all,

I follow the guidance found in this thread whenever doing anything with panels:
http://forums.mikeholt.com/showthread.php?t=140398

So I only talk to people in KVA on my day to day because of it.

My question is, when you have a three phase panel with a mix of single phase and three phase loads and you end up with and unbalanced load, and you add up all three phase total KVAs to get total 3phase KVA connected, how come there is never any mention of utilization of symmetrical components?

In other words, I have an unbalanced three phase system why is there no mention to utilize symmetrical components, is that part of the magic of using KVA instead of currents?

Other than fault calculations when are symmetrical components used?
 

JoeStillman

Senior Member
Location
West Chester, PA
Symmetrical components are used in short circuit calculation software, undergraduate problems lab and the PE exam. Our ability to predict panelboard loading doesn't warrant the level of accuracy.

It would be like using a CNC robot to operate a Ouija board.
 

W@ttson

Senior Member
Location
USA
So technically, you could use symmetrical components to add the load currents, but using the KVA to add up gives a very useful approximation?

Symmetrical components are used in short circuit calculation software, undergraduate problems lab and the PE exam. Our ability to predict panelboard loading doesn't warrant the level of accuracy.

It would be like using a CNC robot to operate a Ouija board.
 

Smart $

Esteemed Member
Location
Ohio
So technically, you could use symmetrical components to add the load currents, but using the KVA to add up gives a very useful approximation?
With panel load balancing we are mainly concerned with the "net" line current. We use the kVA method because current values do not add arithmetically. To do a symmetrical component analysis, you will have to already have the "net" line current information through vector addition... and at that point, there's no need for the symmetrical component analysis. :D
 

philly

Senior Member
Can you get the neutral current by simply looking at the kVA on each phase? I know that technically the neutral current would be derived by looking at each of the current phasors as stated above and there is also a square root function that uses the currents that gives a close approximation, but I've never seen neutral current derived from the kVA values on each phase?

I always thought neutral current in a panel was based on accounting for neutral current on L-N loads per the NEC factors.

If the system only consist of 3-phase loads there will be no neutral current regardless if system is 4-wire.
 

Ingenieur

Senior Member
Location
Earth
Symmetrical components are a transformation from phase to a sequence domain
I have no idea how (or why) you would use them to balance a 3 ph load
for 2 main reasons
they are used for fault analysis
and they are done on a single ph basis
(you can do 3 ph but requires linear algebra and usually software)
 

Smart $

Esteemed Member
Location
Ohio
Can you get the neutral current by simply looking at the kVA on each phase? I know that technically the neutral current would be derived by looking at each of the current phasors as stated above and there is also a square root function that uses the currents that gives a close approximation, but I've never seen neutral current derived from the kVA values on each phase?

I always thought neutral current in a panel was based on accounting for neutral current on L-N loads per the NEC factors.

If the system only consist of 3-phase loads there will be no neutral current regardless if system is 4-wire.
In the design process, we are only concerned with the maximum possible unbalanced neutral current... and that's only if you want to reduce the size of the neutral compared to the ungrounded conductors. You can determine that by adding up the kVA of the L-N loads connected to each line separately and dividing by the voltage. Demand factors may apply, so weigh that into the summation accordingly.

We do not need to know the actual neutral current magnitude for the design process.
 

W@ttson

Senior Member
Location
USA
How does the approximation differ with change in unbalance? Lets say, the more you deviate from a balanced total 3PH load, how does that effect just using the KVA method by adding up the individual phase KVAs?

I would imagine that the more unbalance there is the worse the approximation. I bet someone will say, well if you take the largest KVA value and multiply by 3 to get the total it will never matter. But what if you just want to say what the three phase load is and not size and upstream breaker or transformer?


 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Balancing the loads by looking at the totals on a panel schedule is not enough. I had one inspector almost fail my design because I showed one phase (of a 3-phase panel) about 5000 watts higher than the other two. He wanted me to shift the loads to create a better balance. But then I pointed out that the one load that cause the imbalance was a point-of-demand water heater that only runs for a matter of seconds at a time. He withdrew the comment.
 

W@ttson

Senior Member
Location
USA
Panel balancing can sometimes be an art, as you try to determine the best layout based on what you think will be the most likely situatuion.

Balancing the loads by looking at the totals on a panel schedule is not enough. I had one inspector almost fail my design because I showed one phase (of a 3-phase panel) about 5000 watts higher than the other two. He wanted me to shift the loads to create a better balance. But then I pointed out that the one load that cause the imbalance was a point-of-demand water heater that only runs for a matter of seconds at a time. He withdrew the comment.
 

Carultch

Senior Member
Location
Massachusetts
Balancing the loads by looking at the totals on a panel schedule is not enough. I had one inspector almost fail my design because I showed one phase (of a 3-phase panel) about 5000 watts higher than the other two. He wanted me to shift the loads to create a better balance. But then I pointed out that the one load that cause the imbalance was a point-of-demand water heater that only runs for a matter of seconds at a time. He withdrew the comment.

I never realized it was a requirement to balance panelboard loads in the first place. I know it is good strategy, but I didn't realize that you'd fail inspection because of any imbalance that may exist
 

topgone

Senior Member
I never realized it was a requirement to balance panelboard loads in the first place. I know it is good strategy, but I didn't realize that you'd fail inspection because of any imbalance that may exist

Inspectors are there to act as inspectors!:) They are there to comment if he sees something out of t he ordinary. They know whats there to inspect but will be willing to add on top of the legit things and say their little things.:)
 

W@ttson

Senior Member
Location
USA
Hello all,

I follow the guidance found in this thread whenever doing anything with panels:
http://forums.mikeholt.com/showthread.php?t=140398

So I only talk to people in KVA on my day to day because of it.

My question is, when you have a three phase panel with a mix of single phase and three phase loads and you end up with and unbalanced load, and you add up all three phase total KVAs to get total 3phase KVA connected, how come there is never any mention of utilization of symmetrical components?

In other words, I have an unbalanced three phase system why is there no mention to utilize symmetrical components, is that part of the magic of using KVA instead of currents?

Other than fault calculations when are symmetrical components used?


This forum updated the way the link structure works so often old links point you to the wrong place. The above link now points to a Rotary Phase Converter which is obviously not applicable.

Here are some new links that are applicable to what I meant above:



If anyone knows if there is a conversion from the old forum link format to the new ones that would be helpful.

Thanks
 

W@ttson

Senior Member
Location
USA
This forum updated the way the link structure works so often old links point you to the wrong place. The above link now points to a Rotary Phase Converter which is obviously not applicable.

Here are some new links that are applicable to what I meant above:



If anyone knows if there is a conversion from the old forum link format to the new ones that would be helpful.

Thanks

Here is another one:

One of the above links may have actually been the one I posted. But you get the point. Use KVA not Amps.
 
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