OH NO! Another unblanced WYE calc question

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grasfulls

grasfulls

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Before consuming the large portion of humble pie I have just consumed in putting this together, I think I looked all over for an answer I could understand. Please, when you look at the drawing, I already know I went from a single line to a three line, did not complete things, etc. I only made to hopefully make the question as simple as I am hoping the answer may be. We are being required to furnish calculations, etc before being issued a permit. I took all of the single phase loaded, 3-phase panel, and merely divided the wattage on each phase by 120. So you see original watts and original amps. The chargers are 7200 watts at 208 (2pole). Is the dividing each phase watt sum by 120 off-base? If yes, how should it be performed?

Thanks.
(off to get one more slice)
 

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The basic notion of adding the phase A watts and dividing it by 120 to get the phase A amps is not valid. I think that was the essential question you are asking. The mathematics of unbalanced phase addition is much more complex than that. I would not suggest trying to calculate any kind of "amps per phase."

There are four quick things I wish to point out. First, you have 10 things called "equipment" in the panel schedule, and 11 things by that same name in the list of loads. Secondly, you do not have the correct correlation between the phases to which the chargers are connected and the phases to which the loads are added in the load list. For example, EVC 1 is connected from phase B to phase C, and the load list shows the load on phases A and C. Third, if the EVCs have a load of 7200 watts and are fed via two-pole breakers, then the load list should assign 3600 watts to each of the two phases. Finally, I strongly suggest taking the word "watts" off the load list, everywhere it appears. Substitute "VA" instead. Do all the math in VA. Please note that there is a total watts and a total KVA (by the way, you forgot to divide by 1000 here - OK that makes five things), but no correlation between them. I don't know what the "equipment" is, but the chargers should have a power factor of 1.0. So watts and VA are the same for that load.

My suggestion is that you correct these issues, and then add the total VA for the entire panel. Then divide that total by 208 and divide again by the square root of 3. This will give you one value for amps, not three. It will be essentially an average, but as long as you make an effort to balance the phases as best you can, that average will be close to the current you would actually measure in any of the phases.
 
Two more comments. You are showing a total load, after the addition of the chargers, that is very close to the rating of the panel. But you are double-counting the load on the chargers. So the actual final load will be much lower, and you won't really be getting close to the limit. Also, I just noticed the "0.95 PF" at the top of your load list. That number does not correlate with the total watts and kva entries. For example, a total watts of 43200 and a kva of 48.900 corresponds to a PF of 0.88.
 
https://drive.google.com/file/d/0By0rzU3UuNh7UWNkZ0hGeEg4Q2M/view?usp=sharing

Don't say I never gave you anything... :D

Made assumption of same mistake with Gen Battery Charger load.
grasfulls.gif
 
Way off everything

Way off everything

charlie b said:

The basic notion of adding the phase A watts and dividing it by 120 to get the phase A amps is not valid. I think that was the essential question you are asking. The mathematics of unbalanced phase addition is much more complex than that. I would not suggest trying to calculate any kind of "amps per phase."
Click to expand...


If the city (San Jose, CA), will let the calcs be done that way, that will be nice. All I have seen is that unbalanced three phase calcs are complex.

charlie b said:

OK that makes five things), but no correlation between them. I don't know what the "equipment" is, but the chargers should have a power factor of 1.0. So watts and VA are the same for that load.

Two more comments. You are showing a total load, after the addition of the chargers, that is very close to the rating of the panel. But you are double-counting the load on the chargers. So the actual final load will be much lower, and you won't really be getting close to the limit. Also, I just noticed the "0.95 PF" at the top of your load list. That number does not correlate with the total watts and kva entries. For example, a total watts of 43200 and a kva of 48.900 corresponds to a PF of 0.88.

My suggestion is that you correct these issues, and then add the total VA for the entire panel. Then divide that total by 208 and divide again by the square root of 3. This will give you one value for amps, not three. It will be essentially an average, but as long as you make an effort to balance the phases as best you can, that average will be close to the current you would actually measure in any of the phases.
Click to expand...

I think I corrected most things, except cutting down the VA per phase to 3600 if the total VA of a unit is 7200. And yes, the question was how to do the calcs, but I really appreciate your scrutinizing the drawing. I am still working it, I have attached it with most of what you told me to correct. This is an existing panel, I have no idea what the EQUIPMENT is either, or how many there really are. The PF is on the original drawing, it may need to be changed.
I have a meeting to go to and will try and finish it in about 7 hours.
Thanks!
 

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grasfulls said:
Thanks!! Nice :)
I just saw this after I replied to the first two posts.
I should probably build a spreadsheet to do this as well.
gare
Click to expand...
Link at top of post is the spreadsheet (download to computer, open in Excel, not compatible with other ss programs)
 
Relying on balanced

Relying on balanced

charlie b said:
The basic notion of adding the phase A watts and dividing it by 120 to get the phase A amps is not valid. I think that was the essential question you are asking. The mathematics of unbalanced phase addition is much more complex than that. I would not suggest trying to calculate any kind of "amps per phase."

My suggestion is that you correct these issues, and then add the total VA for the entire panel. Then divide that total by 208 and divide again by the square root of 3. This will give you one value for amps, not three. It will be essentially an average, but as long as you make an effort to balance the phases as best you can, that average will be close to the current you would actually measure in any of the phases.
Click to expand...

Is this not relying somewhat on all loads being on at the same time? I do need to find out what all of the "EQUIPMENT" labels stand for. As an example, I had to move most of the equipment off of phase C to balance phases, although changing the va to 3600 per phase will help. but what happens when all of the equipment is running and only chargers across A and B?
 
Fixed all - sweet! thank you

Fixed all - sweet! thank you

grasfulls said:
Is this not relying somewhat on all loads being on at the same time? I do need to find out what all of the "EQUIPMENT" labels stand for. As an example, I had to move most of the equipment off of phase C to balance phases, although changing the va to 3600 per phase will help. but what happens when all of the equipment is running and only chargers across A and B?
Click to expand...

Putting the proper VA in worked out great. Also, I see that the balanced 3 phase calc works out to be very close to what the individual phases see... I need to clean the drawing up, but I wanted you to sdee the results of your inputs, thank you. Oh, and I need to find out if I need to apply a power factor at all.
 

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