VA and Panel Schedules

[Karl H]

I'm having a difficult time wrapping my brain around this. I hope you guys can help.
Let's say, I have a 16 amp load at 208v.The power Factor or eff. is not known. 16 x 208v is 3328VA, So Phase "A" would be 16X120= 1920 Phase"B" would be 16 x 120 = 1920. If it were 16 amp @ 480 3 ph it would be 16 x 830 = 13280. So phase "A" would be, 277 x 16 = 4432, "B" would be 277 x 16 = 4432 and phase C would be 277 x 16 =4432. Is this correct? For some reason , when I check my math, these numbers don't add up to equal the value of a 208v single phase circuit and a 480v 3 phase circuit when determining VA.E x I. What am I missing?

 

[charlie b]

You are missing the relationship between three phase and single phase, and you are mixing 120/208 systems with 480/277 systems.

 

[hurk27]

To cross calculate between single phase and 3-phase you need to use the square root of 3 (1.732)

if you have a load that is 3328va about at 208, then at 480 3-phase it would be:
3328/480/1.732=4 amps per leg so that will be 1920 va per leg

 

[BJ Conner]

Draw three arrows in a star pattern, with 120 degrees between each ( like the star on the front of your 500SLK).
Draw the arrow heads out. Draw each exactly 1.2 inches long. Label them A,B, & C
From the center of the star ( the system neutral ) to the tip represents 120 volts. A single phase load is connecte from line to neutral and "sees" 120 volts.
Now draw an arrow from the tip of one arrow to one next to it. PReciscely measure that arrow and it will be 2.08 inches long. That represents the voltage a two phase load in a 3 phase sytem "sees"

10 amps in a single phase load is 10 amps x 120 volts =1,200 VA
10 amps in two plase (line to line ) load is 208 volts x 10 amps = 2,080 VA
I am not tryiong toinsulting or smart but drawing the vectors out helps a lot people ( as it did me)
If you get in a book store that has the following

http://www.mhprofessional.com/product.php?isbn=0071425829

Look it ove IMO it's one of the best for explaining three phase theory, math etc. Lots of problems with answers.

 

[hurk27]

I should add that it depends upon what is your constant, in your calculation you kept the current as a constant, in most electrical calculations the VA will be the constant, except in a resistance heating load where the resistance will be the constant if the resistance is not changeable for different voltage, like running a 208v heating element on 480 but in most cases you will be figuring the current that a va load will pull at a different voltage with the va load configurable to the newer voltage and or 3-phase from single phase.

each will depend upon the design of the load as to how to figure the result you need to know.

so what kind of a load are you trying to figure?

 

[Smart $]

... What am I missing?

First, as charlie b said.

Next, 1? L-N load VA goes in appropriate "line" column (A, B, or C)

3? load VA is divided by 3 and that value goes in each of the three columns.

1? L-L load VA is divided by 2 and that value goes in each of the connected "line" columns. The main thing with these loads is that you want to balance them out as best possible. Say for example you have three 200VA L-L loads. You should have 100+100VA in each of the three columns... and thus 200VA total for these loads in each column. As you can see 200VA times 3 is 600VA total. The only time this does not work out is when the L-L loads are unbalanced, but that's just the way it goes

 

[jghrist]

Do it the way Smart$ says. The VA method is meant to make things simple, so keep it simple. If there are unbalanced L-L loads or different power factors on different phases, the results are not completely accurate, but are conservative and good enough.