Downsizing an Engineered panel

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ggunn said:
If you are adding phase currents to come up with line current, you do not understand the system. I don't have an issue with you trying to learn more; I am encouraging you to do some research on how this stuff works instead of relying on answers from an internet forum.
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not sure why you are part of an internet forum then...

The correct response would be "hey buddy it looks like you mis-calculated your phase amps, this is how you should do it next time"

you could do all the research in the world and still mis a small detail that throws everything off, like i did, i used 3ph math when calculating the per phase load. simple mistake anyone could make if not in the grove of the math.
 
Spencer125 said:
haha I just don't get why electricians never what to help each other. our trade is so cut throat its sad really...
I understand not wanting to teach non electricians something they shouldn't be doing but not helping your own brothers seems a shame.
Nobody can understand or know perfectly how to do everything, our trade is so broad.
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You know he was joking, right? If you think that I am not trying to help you, I am not communicating effectively. I am encouraging you to learn how this works on a fundamental level, and for that you'll need to do some homework. Once you understand something and work from that knowledge rather than doing things by rote, it is like riding a bike.
 
Spencer125 said:
Hello all, my customer is looking to VE the current design. the panel is shown as 600a but the calculated per phase KVA is:
A=58.41
B=54.41
C=49.03
for a total KVA of 161.85.

if i break this down to amps i believe i get the following:
A=162.13a
B=151.02a
C=136.09a
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This was calculated using 3ph math and cause my error. thank you for the people with the useful input that helped me discover my mistake
Spencer125 said:
for a total of 449.25a

So theoretically couldn't we get away with a 200a panel? I'm going to suggest a 400a but I'm trying to understand the logic behind the math...

I apologize in advance as i do not do many design build jobs so this stuff gets rusty
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The corrected numbers are as follows:
A=486.75
B=453.41
C=408.58
 
Spencer125 said:
not sure why you are part of an internet forum then...

The correct response would be "hey buddy it looks like you mis-calculated your phase amps, this is how you should do it next time"

you could do all the research in the world and still mis a small detail that throws everything off, like i did, i used 3ph math when calculating the per phase load. simple mistake anyone could make if not in the grove of the math.
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Adding phase currents to determine line current is not a small detail; it shows a lack of understanding of the fundamentals. I am not slamming you for that, I am encouraging you to educate yourself so that the answers you get make sense.
 
If you had a single phase 240V panel with a 200A main breaker you wouldn't say it could handle 400A. For a 240V load on a 2 pole breaker the two poles are actually in series with the load between them; when the voltage on L1 is at its positive maximum, the voltage on L2 is at its maximum negative, so at that instant the current is flowing from the panel through the L1 pole, through the load, and back through the L2 pole. The current through each pole of the breaker is the total current.

Three phase is essentially the same, except that when the voltage on the A phase is at its positive maximum the voltages on the B and C phases are at half their negative maximums, so the current is going out through the A pole, through the load, and back through the B and C poles, split between them.

In both cases, the current in any phase is the same as the total line current. Once you understand this you won't make the computational error again.
 
I have to agree with the op. You guys are engineers and this is second nature to you. I often have trouble with calculation from 3phase especially when the loads are single phase. It may be lack of understanding the system but in most situation electricians don't need to know most of that. Would it be better to know more? Of course, but that often is not the case as engineers usually do the load calculations.
 
Total power in a three-phase system is 3 * phase voltage * phase current. In a wye system, phase current and line current are the same. Phase voltage is line voltage divided by square root of 3 (we're ignoring phase angle but would matter for complex loads). Current (phase or line) then equals total power (161.85 kVA) / phase voltage (120 V) / 3 = 450.0 A.
 
Dennis Alwon said:
I have to agree with the op. You guys are engineers and this is second nature to you. I often have trouble with calculation from 3phase especially when the loads are single phase. It may be lack of understanding the system but in most situation electricians don't need to know most of that. Would it be better to know more? Of course, but that often is not the case as engineers usually do the load calculations.
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If the OP is in a position where he needs to make design decisions, then IMO he should understand the basics behind the calculations he is running to make those decisions. Blindly applying formulas by rote is prone to error.
 
Dennis Alwon said:
I have to agree with the op. You guys are engineers and this is second nature to you. I often have trouble with calculation from 3phase especially when the loads are single phase. It may be lack of understanding the system but in most situation electricians don't need to know most of that. Would it be better to know more? Of course, but that often is not the case as engineers usually do the load calculations.
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thank you. I apricate this. I think engineers forget that sometime we HAVE to do their jobs and its a shame, but there is never an instance when they HAVE to do ours.

I do wish that our code book was broken down into 3parts.
Commercial/industrial
Residential
Engineering

Then we would be required to be licensed in each. In most states our license is far to broad. id wish for nothing more than to say, this is outside my code book which in turn is outside my license.

I don't do residential but I'm required to waste so much time in code updates listening to residential changes when id much rather spend more time being better versed in the codes that pertain to me.

Anyone who sees this that has any say in the world should push for our book to be split, pleaseeee hahah
 
ggunn said:
If the OP is in a position where he needs to make design decisions, then IMO he should understand the basics behind the calculations he is running to make those decisions. Blindly applying formulas by rote is prone to error.
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it wasn't blindly, I could tell there was an error right out of the gate, which is why I reached out here, for help. if it was blind then i would of not had the sense to ask the question in the first place.
 
david luchini said:
Except there wasn't an error. The engineer's design for a 600A panel was correct for the calculated load.
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when I did the math on my own and came up with only needing a 200a panel is when I thought there was an error, what I wasn't sure about at the time was if the engineer over designed the system, which is extremely common these days or if the error was in my math. come to find out it all stemmed from my unfamiliarity with the math, I now understand it much better thanks to some here. there is a big cost diffrence between needing a pad mount xfomer vs a pole mount, the utility typically requires pad after 400a, i was asked by the customer to look for cost savings and i thought i might of found some.
 
infinity said:
Keep asking your questions that's the purpose of this forum. Ignore those who are critical.
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If I gave the impression of being critical, I apologize. What I was trying to get across is that if one is in the position of making design decisions or evaluating those which were made by someone else, it would be useful to understand the underlying principles upon which the decisions are made. I hope that what I posted in #25 was informative.
 
ggunn said:
If I gave the impression of being critical, I apologize. What I was trying to get across is that if one is in the position of making design decisions or evaluating those which were made by someone else, it would be useful to understand the underlying principles upon which the decisions are made. I hope that what I posted in #25 was informative.
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Yes, it was thank you. Much more useful then "why are you working on something you don't understand". which just for the record, I'm trying to understand it better, hence the questions.
 
Spencer125 said:
Yes, it was thank you. Much more useful then "why are you working on something you don't understand". which just for the record, I'm trying to understand it better, hence the questions.
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My concern was only that decisions might be made with less than optimal results if one is making them without understanding the principles underlying them. I did not mean to slam you personally, and I am sorry if what I said looked that way.
 
ggunn said:
My concern was only that decisions might be made with less than optimal results if one is making them without understanding the principles underlying them. I did not mean to slam you personally, and I am sorry if what I said looked that way.
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No worries. trust me I don't take anything on the internet personal. I'm just trying to point out that a comment like that doesn't do anybody any good, if your not interested in helping an OP I don't see any reason you wouldn't just scroll past the post. Thanks again.
 
If you want post up a pic of the schedule and we can walk you though some of it.
Looking at the loads the engineer did a good job balancing the loads per phase.

It tells me there is some single phase loads. So be careful if you decide to move circuits around or use two to three separate panles.
 
Spencer125 said:
No worries. trust me I don't take anything on the internet personal. I'm just trying to point out that a comment like that doesn't do anybody any good, if your not interested in helping an OP I don't see any reason you wouldn't just scroll past the post. Thanks again.
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At the risk of flogging a deceased equine mammal, I thought that I was helping you. Personally, I remember much better how to do something when I understand why I am doing it. YMMV.
 
This will make you crazy.

A=58.41
B=54.41
C=49.03
for a total KVA of 161.85.
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(58410/208) / (sqr(3)) = 162
wrong answer that the OP got

58410 / (208/(sqr(3)) = 486
right answer

Just put the implied parenthesis in the wrong place.

😀
 
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