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3-Phase Load Demand Calculation


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#1 wilheldp_PE

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Posted 13 October 2008 - 02:13 PM

This is probably going to sound like a really stupid question to power people, but I'm a computer engineer, so I don't know how to do it. I recently took some load readings off of some switchgear at a site I'm working on. I got the current draw on each phase, and the line-to-line voltages. I need to know the demand reading for the panel in both Amps and kVA. There is an example below:

Phase 1: 420A
Phase 2: 370A
Phase 3: 400A

L1-L2: 390V
L2-L3: 180V
L3-L1: 205V

#2 benbo

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Posted 13 October 2008 - 02:41 PM

QUOTE (wilheldp_PE @ Oct 11 2008, 12:13 PM) <{POST_SNAPBACK}>
This is probably going to sound like a really stupid question to power people, but I'm a computer engineer, so I don't know how to do it. I recently took some load readings off of some switchgear at a site I'm working on. I got the current draw on each phase, and the line-to-line voltages. I need to know the demand reading for the panel in both Amps and kVA. There is an example below:

Phase 1: 420A
Phase 2: 370A
Phase 3: 400A

L1-L2: 390V
L2-L3: 180V
L3-L1: 205V

You're not alone. I'm a complete idiot on three phase power. One thing I am not even sure of is if it matters whether it is delta or wye. Common sense would tell me both current and voltage should be phase or line . But common sense is not always right, and like I said, I admit to being a complete idiot in this. Anything I knew at the time of the test I have since forgotten. Most EEs around here are power people so you should get an answer soon.

#3 jregieng

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Posted 13 October 2008 - 02:57 PM

I am a complete idiot when it comes to this stuff as well - I am not even an EE! However, I do work on some environmental remediation systems where the technology utilizes three-phase current to resistively heat large electrodes in the ground to the point of creating a 'boiling' condition for the volatile contaminants in the groundwater (and adsorbed onto soil matrix).

I would like to hear the explanation as well .... popcorn.gif

JR

#4 Kuku

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Posted 13 October 2008 - 04:50 PM

Your voltages are wacked out! So I will just assume your system is 480V

(420 + 370 + 400) / 3 = 397A demand

Voltage * Amperage * sqrt(3) = kVA

397A * 480V * sqrt(3) = 330kVA (at time of your reading)


#5 Kuku

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Posted 13 October 2008 - 04:53 PM

And just for the heck of it, if you found the kVA and not the amps (#'s are made up for example):

A: 105
B: 120
C: 111

kVA = 105+120+111 = 336kVA

336kVA / (480*sqrt(3)) = 404A

Notice how you average the amps and add the VA across the phases.

Edited by Kuku, 13 October 2008 - 04:54 PM.


#6 wilheldp_PE

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Posted 13 October 2008 - 05:00 PM

QUOTE (Kuku @ Oct 13 2008, 12:50 PM) <{POST_SNAPBACK}>
Your voltages are wacked out! So I will just assume your system is 480V

(420 + 370 + 400) / 3 = 397A demand

Voltage * Amperage * sqrt(3) = kVA

397A * 480V * sqrt(3) = 330kVA (at time of your reading)

I noticed that the voltages were whacked out...what the hell is up with that? That was a voltmeter built directly into the panel, and I read those line-to-line voltages right off the meter. Ever seen anything like it?

I appreciate the help. I knew the kVA calc, but didn't know if it was as simple as just averaging the current demands. Also, the crazy voltages were throwing me off.

#7 Kuku

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Posted 13 October 2008 - 05:04 PM

QUOTE (wilheldp_PE @ Oct 13 2008, 01:00 PM) <{POST_SNAPBACK}>
I noticed that the voltages were whacked out...what the hell is up with that? That was a voltmeter built directly into the panel, and I read those line-to-line voltages right off the meter. Ever seen anything like it?

I appreciate the help. I knew the kVA calc, but didn't know if it was as simple as just averaging the current demands. Also, the crazy voltages were throwing me off.


How old is the system you were looking at? In my very limited experience, I've only seen voltages screwy like that if the main feeders coming into the panel were not secure. But I'm only talking about a drop of 10-15volts. You'll have to ask someone with more knowledge than me on this one!

Edited by Kuku, 13 October 2008 - 05:05 PM.


#8 Dark Knight

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Posted 13 October 2008 - 05:07 PM

QUOTE (Kuku @ Oct 13 2008, 01:04 PM) <{POST_SNAPBACK}>
How old is the system you were looking at? In my very limited experience, I've only seen voltages screwy like that if the main feeders coming into the panel were not secure. But I'm only talking about a drop of 10-15volts. You'll have to ask someone with more knowledge than me on this one!


I would suspect an unbalanced load as the reason for the crazy voltages. Who knows....

#9 wilheldp_PE

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Posted 13 October 2008 - 05:09 PM

QUOTE (Kuku @ Oct 13 2008, 01:04 PM) <{POST_SNAPBACK}>
How old is the system you were looking at? In my very limited experience, I've only seen voltages screwy like that if the main feeders coming into the panel were not secure. But I'm only talking about a drop of 10-15volts. You'll have to ask someone with more knowledge than me on this one!

Those readings came off of a panel that was installed in the mid-1980's. It was the only one that had funky voltages which is why I used it as my example. The other panels were averaging around 460 to 470V or 205V.

And while I'm asking n00b questions... I'm looking at the single-line diagram, and whenever there is a high-current circuit breaker (like 1000A plus), there are two numbers listed for the breaker followed by AF and AT. For instance, the breaker feeding the main distribution panel has "3000AF, 2000AT" written next to it. My simplistic mind associates those with 3000 Amp Fault, 2000 Amp Trip (i.e. the breaker trips before it's rated fault current), but I'm not sure.

#10 Kuku

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Posted 13 October 2008 - 05:19 PM

QUOTE (wilheldp_PE @ Oct 13 2008, 01:09 PM) <{POST_SNAPBACK}>
Those readings came off of a panel that was installed in the mid-1980's. It was the only one that had funky voltages which is why I used it as my example. The other panels were averaging around 460 to 470V or 205V.

And while I'm asking n00b questions... I'm looking at the single-line diagram, and whenever there is a high-current circuit breaker (like 1000A plus), there are two numbers listed for the breaker followed by AF and AT. For instance, the breaker feeding the main distribution panel has "3000AF, 2000AT" written next to it. My simplistic mind associates those with 3000 Amp Fault, 2000 Amp Trip (i.e. the breaker trips before it's rated fault current), but I'm not sure.


I believe those refer to frame and trip ratings. There are certain frame ratings that can hold a range of breakers.

#11 wilheldp_PE

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Posted 13 October 2008 - 05:20 PM

QUOTE (Kuku @ Oct 13 2008, 01:19 PM) <{POST_SNAPBACK}>
I believe those refer to frame and trip ratings. There are certain frame ratings that can hold a range of breakers.

Ah shit...I bet you're right. I should have known that one from my machine controls days.

#12 Chimpoco

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Posted 03 May 2012 - 03:40 PM

Posted 13 October 2008 - 09:13 AM
This is probably going to sound like a really stupid question to power people, but I'm a computer engineer, so I don't know how to do it. I recently took some load readings off of some switchgear at a site I'm working on. I got the current draw on each phase, and the line-to-line voltages. I need to know the demand reading for the panel in both Amps and kVA. There is an example below:

Phase 1: 420A
Phase 2: 370A
Phase 3: 400A

L1-L2: 390V
L2-L3: 180V
L3-L1: 205V

Response to wilheldp_PE


Unfortunately the electrical person(s) posting to this thread are making a wrong assumption: that the system is a 3ph 3w power supply, hence all the comments posted are incorrect. The system is a 200V/400V 1ph 3w system (some people refer to it as a 2ph 3w) with L3 being your neutral or "return". The instantaneous voltage would be only obtained if you measure all three phases at the same time, this will account for the variance when you add them, so when you add L2-L3 to L1-L3 (180+205=395V) it will equal approximately 390V. I forgot the power calculations but I am sure it can be easily found once since you now know the correct power supply configuration that you looking at. (I know the question may no longer be relevant but this is posted for the benefit of other people who may read this post. )

#13 Wildsoldier PE

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Posted 04 May 2012 - 01:05 AM

Looks like that system have a Hi leg.....extra careful you need to have on those systems like for example 120/240 3phase 4wire delta...there is going to be a configuration that from line to neutral is going to measure 208v instead of 120.

A-n=120
B-n=120
C-n=208

A-b=240
B-c=240
C-a=240

If some dont know what he is doing...well you know..

#14 Nittany_Lion

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Posted 04 May 2012 - 04:59 PM

Looks like that system have a Hi leg.....extra careful you need to have on those systems like for example 120/240 3phase 4wire delta...there is going to be a configuration that from line to neutral is going to measure 208v instead of 120.

A-n=120
B-n=120
C-n=208

A-b=240
B-c=240
C-a=240

If some dont know what he is doing...well you know..




Looks good except Hi-leg delta should be B phase per NEC.....

#15 knd107

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Posted 05 May 2012 - 04:22 AM

QUOTE (wilheldp_PE @ Oct 13 2008, 01:09 PM)
Those readings came off of a panel that was installed in the mid-1980's. It was the only one that had funky voltages which is why I used it as my example. The other panels were averaging around 460 to 470V or 205V.

And while I'm asking n00b questions... I'm looking at the single-line diagram, and whenever there is a high-current circuit breaker (like 1000A plus), there are two numbers listed for the breaker followed by AF and AT. For instance, the breaker feeding the main distribution panel has "3000AF, 2000AT" written next to it. My simplistic mind associates those with 3000 Amp Fault, 2000 Amp Trip (i.e. the breaker trips before it's rated fault current), but I'm not sure.


I believe those refer to frame and trip ratings. There are certain frame ratings that can hold a range of breakers.


Yes, those stand for 3000 Amp Frame and 2000 Amp Trip. There are standard frame sizes and then there are breakers that fit into the frame. Check the NEC for standard ones available or go directly to a manufacturer website and check out what each one offers. The 2000A circuit breaker is most likely a electronic breaker so you can adjust the trip unit on it but it is not a 3000A breaker that trips at 2000A.

#16 ndekens

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Posted 10 May 2012 - 02:32 PM

Posted 13 October 2008 - 09:13 AM
This is probably going to sound like a really stupid question to power people, but I'm a computer engineer, so I don't know how to do it. I recently took some load readings off of some switchgear at a site I'm working on. I got the current draw on each phase, and the line-to-line voltages. I need to know the demand reading for the panel in both Amps and kVA. There is an example below:

Phase 1: 420A
Phase 2: 370A
Phase 3: 400A

L1-L2: 390V
L2-L3: 180V
L3-L1: 205V

Response to wilheldp_PE


Unfortunately the electrical person(s) posting to this thread are making a wrong assumption: that the system is a 3ph 3w power supply, hence all the comments posted are incorrect. The system is a 200V/400V 1ph 3w system (some people refer to it as a 2ph 3w) with L3 being your neutral or "return". The instantaneous voltage would be only obtained if you measure all three phases at the same time, this will account for the variance when you add them, so when you add L2-L3 to L1-L3 (180+205=395V) it will equal approximately 390V. I forgot the power calculations but I am sure it can be easily found once since you now know the correct power supply configuration that you looking at. (I know the question may no longer be relevant but this is posted for the benefit of other people who may read this post. )


Thats why the first thing you do is read the nameplate on the panel!!! I have to admit that a 200V/400V 1ph 3w system is rather odd. Why would one use this system and where?




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