3 phase panel calculations Help needed

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embryo3

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So to try to simplfy this

When I have (3) loads at (30amp 3phase 208v) I would calculate the panel as such:
(30a x 208v X 1.37 / 3 ) or do I enter the total VA to each phase 10,807.68va?

A phase B phase C Phase
3607 3607va 3607va
3607va 3607va 3607va
3607va 3607va 3607va
10821va 10821va 10821va

Add all va together = 32463va
Divide by 208v = 156
Divide by 1.73 = 90.17a

And if I added a 120 load it would be added to the phase total

Total Amps for panel size 90.17amp​
 
embryo3 said:
So to try to simplfy this

When I have (3) loads at (30amp 3phase 208v) I would calculate the panel as such:
(30a x 208v X 1.37 / 3 ) or do I enter the total VA to each phase 10,807.68va?

A phase B phase C Phase
3607 3607va 3607va
3607va 3607va 3607va
3607va 3607va 3607va
10821va 10821va 10821va

Add all va together = 32463va
Divide by 208v = 156
Divide by 1.73 = 90.17a

And if I added a 120 load it would be added to the phase total

Total Amps for panel size 90.17amp​
Click to expand...

Yes, three 30A (3 ph) loads would be a 90A (3 ph) total load.
 
embryo3 said:
SO you do divide the va total by three and add it to the phase?
Click to expand...
Yes... or simply multiply the 3? current by L-N voltage 30A ? 120V = 3600VA each phase. Don't forget 208 is actually 120√3 (or 207.846...) rounded. It's actually 30 ? 120√3 ? √3 ? 3 = 3600VA
 
Last edited:
Per Phase VA
Circuit
Load
Voltage
Phase
Divided by 3
a
b
c
1
30
208
1.732
3
3602.56
3
30
208
1.732
3
3602.56
5
30
208
1.732
3
3602.56
7
30
208
1.732
3
3602.56
9
30
208
1.732
3
3602.56
11
30
208
1.732
3
3602.56
13
30
208
1.732
3
3602.56
15
30
208
1.732
3
3602.56
17
30
208
1.732
3
3602.56
Total Phase VA
10807.7
10807.7
10807.7
Total VA
32423
Voltage
208
1.732
Panel Amps Required
90
 
Thank you for your time, haven?t done this in a while andwhen trying to lookup refresher references you find a lot of things on the net?
Ok.. So you can just take the three phases X 120 each L-N (or) L-L *1.732/3 and get the va perphase. Same should apply in a 480/277v system/

Now when you select a panel size do you select it by thelargest Phase load or do you add all three pases up then divide by voltage thenby 1.732?
Please see panel layout above this is how I understand it, is this correct?
 
embryo3 said:
Thank you for your time, haven’t done this in a while andwhen trying to lookup refresher references you find a lot of things on the net…
Ok.. So you can just take the three phases X 120 each L-N (or) L-L *1.732/3 and get the va perphase. Same should apply in a 480/277v system/

Now when you select a panel size do you select it by thelargest Phase load or do you add all three pases up then divide by voltage thenby 1.732?
Please see panel layout above this is how I understand it, is this correct?
Click to expand...
Largest phase (actually LINE) load. In the example presented, it makes no difference though.
 
You would need to size according to highest loaded line, but in general you should try to arrange loads so they are as balanced as possible which could make a big difference in maximum amount of load on any one line. Put all load on one 120 volt line and you could have as much as 3 times the current on that one line as you may have if current were balanced across all three phases.
 
Please help me understand why he would need a 90amp rated panel board? He has 3 loads at 30amps, each bus on the panel would only see 30amps. Where is this 90 amps coming from?

Edit: must have miss read the part of 3 - 3 phase loads at 30amps. Thought it was 3 loads at 30amps.
 
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