Calculating 3 Phase Load

[dsmith411]

Good evening guys, I'm rather new to this but I have a wall with several 3 Phase boxes on it. There is a 600 amp main panel and then 8 smaller ones feeding out of it. I took amp readings on all 3 phases in each of the smaller panels and came up with a total amp reading for each one. Example: A 30amps, B 25 amps and C 30 amps. So after I averaged them out I came up with 28 amps on that panel. So if the other 7 has the exact same amount of amps per panel do I add them up for a total of 224 amps or do I need to divide that by 3 to get the total 3 Phase Amps ?
Thanks

 

[Ingenieur]

Good evening guys, I'm rather new to this but I have a wall with several 3 Phase boxes on it. There is a 600 amp main panel and then 8 smaller ones feeding out of it. I took amp readings on all 3 phases in each of the smaller panels and came up with a total amp reading for each one. Example: A 30amps, B 25 amps and C 30 amps. So after I averaged them out I came up with 28 amps on that panel. So if the other 7 has the exact same amount of amps per panel do I add them up for a total of 224 amps or do I need to divide that by 3 to get the total 3 Phase Amps ?
Thanks

yes, just add them up (no divide by 3) assuming all relatively balanced 3 phase (avg +/- 10%)
I would use the highest from each branch, in you example 30 A

 

[Besoeker]

yes, just add them up (no divide by 3) assuming all relatively balanced 3 phase (avg +/- 10%)
I would use the highest from each branch, in you example 30 A

You can't just add the currents arithmetically.

 

[Dennis Alwon]

Why not go back to the main and measure your load there. Also these loads don't mean anything if everything isn't on. Is the heat or a/c running, refrigerations, motors etc

 

[dsmith411]

Why not go back to the main and measure your load there. Also these loads don't mean anything if everything isn't on. Is the heat or a/c running, refrigerations, motors etc

So after I get a amp reading on all the panels per phase for Example: Disconnect #1 A=20 B=25 C=22
Disconnect #2 A=10 B=12 C=10
Disconnect #3 A=15 B=14 C=12
Total on all three panels A=45 B=51 C=44
So this would mean I have a max load of 51 amps and not a total of 45+51+44 = 140 amps ?

 

[Ingenieur]

You can't just add the currents arithmetically.

sure you can KCL, main must sum to branches
perfectly fine for total load
they are essentially 3 phase current magnitudes

consider:
use avg current per branch
i1, i2....in

S1 = sqrt(3) v i1
S2 = sqrt(3) v i2
Sn = sqrt(3) v in

S total = sum(Sx) [for x = 1 thru n] = sqrt(3) v i1 + sqrt(3) v i2 + ... sqrt(3) v in = sqrt(3) v (i1 + i2 + ... in)

i total = S total / (sqrt(3) v) = [sqrt(3) v in = sqrt(3) v (i1 + i2 + ... in)] / (sqrt(3) v) = i1 + i2 + ... in

 

[Ingenieur]

So after I get a amp reading on all the panels per phase for Example: Disconnect #1 A=20 B=25 C=22
Disconnect #2 A=10 B=12 C=10
Disconnect #3 A=15 B=14 C=12
Total on all three panels A=45 B=51 C=44
So this would mean I have a max load of 51 amps and not a total of 45+51+44 = 140 amps ?

yes, 51 not the sum of the a/b/c line currents

 

[Besoeker]

sure you can
perfectly fine for total load

What if one is a 10A motor load and the other a 5A PFC?
Would that really amount to 15A from the supply?

 

[Ingenieur]

What if one is a 10A motor load and the other a 5A PFC?
Would that really amount to 15A from the supply?

still close enough
the reactive power will generally be on the order of10%, if some correction added a small % of a small %
either way, he'll be on the high conservative side

 

[Smart $]

still close enough
the reactive power will generally be on the order of10%, if some correction added a small % of a small %
either way, he'll be on the high conservative side

As such you should state it is a conservative estimate... and only for energized loads at the time of measurement. There could easily be loads de-energized or drawing less than full load when the readings are taken.

 

[Smart $]

So after I get a amp reading on all the panels per phase for Example: Disconnect #1 A=20 B=25 C=22
Disconnect #2 A=10 B=12 C=10
Disconnect #3 A=15 B=14 C=12
Total on all three panels A=45 B=51 C=44
So this would mean I have a max load of 51 amps and not a total of 45+51+44 = 140 amps ?

What is the purpose of taking these readings and attempting to sum them.


To be compliant loads are calculated per Article 220. No other method is truly compliant.

 

[GoldDigger]

So after I get a amp reading on all the panels per phase for Example: Disconnect #1 A=20 B=25 C=22
Disconnect #2 A=10 B=12 C=10
Disconnect #3 A=15 B=14 C=12
Total on all three panels A=45 B=51 C=44
So this would mean I have a max load of 51 amps and not a total of 45+51+44 = 140 amps ?

Yes. Your 600A main panel can handle up to 600A on each line (not on each phase).
Whether all of the lines are equally loaded or only one line and the neutral are carrying all of the current, the limitation of the panel and its OCPD are the same: the current in the most heavily loaded line.

If you were trying to size a three phase transformer to supply the 600A panel, you would need to look at the total power consumption, not just the power consumption corresponding to each phase or to each line.
If your 600A service was 208Y/120, then your total VA corresponding to disconnects 1, 2 and 3 only would be approximately 45*120 + 51*120 +44*120. Which is mathematically equal to 140*120.

 

[Besoeker]

still close enough
the reactive power will generally be on the order of10%, if some correction added a small % of a small %
either way, he'll be on the high conservative side

The answer would be about seven or eight Amps depending on motor PFC.

Mod(s), please try to be reasonable. Play nice.
But, no doubt, this post will also get the treatment. So I have saved this and others.