Downsizing an Engineered panel

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Mustang125

Member
Location
New Hampshire
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
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
 
J

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
You don't total the amps. Just size for the largest phase value and include any anticipated growth. In your case you would need a minimum of a 200A panel.
 
david luchini

david luchini

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Staff member
Location
Connecticut
Occupation
Engineer
Spencer125 said:
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
Click to expand...
449.25A is more than 200A and more than 400A.

the 600A panel sounds correct.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
david luchini said:
449.25A is more than 200A and more than 400A.

the 600A panel sounds correct.
Click to expand...
I think he came up with the 449 amps by adding the three phases together. He doesn't seem to understand how these things work very well.

I'm not sure how he came up with the amps per phase either they don't seem to make much sense. If his total KVA is correct this is about 192 amps per phase if this is a 480 volt panel and it is evenly distributed. I don't think a 200 amp panel would work but a 400 amp panel probably would.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Spencer125 said:
yes but that's the total amps. technically on a 200a panel you have 600a total?? 200a per phase...
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No. If you have three phases with 200A on each phase the total current is 200A. We could write voluminous texts explaining this, but I encourage you to do your own research; there is plenty of information on line.
 
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david luchini

david luchini

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Location
Connecticut
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Engineer
petersonra said:
I think he came up with the 449 amps by adding the three phases together. He doesn't seem to understand how these things work very well.

I'm not sure how he came up with the amps per phase either they don't seem to make much sense. If his total KVA is correct this is about 192 amps per phase if this is a 480 volt panel and it is evenly distributed. I don't think a 200 amp panel would work but a 400 amp panel probably would.
Click to expand...
The load is 161.85kVA, which is 450A at 208V, 3ph.

He calculated the amps/phase incorrectly.
 
M

Mustang125

Member
Location
New Hampshire
petersonra said:
I think he came up with the 449 amps by adding the three phases together. He doesn't seem to understand how these things work very well.

I'm not sure how he came up with the amps per phase either they don't seem to make much sense. If his total KVA is correct this is about 192 amps per phase if this is a 480 volt panel and it is evenly distributed. I don't think a 200 amp panel would work but a 400 amp panel probably would.
Click to expand...
its a 120/208 system
i do not understand these things very well which i why im reaching out for input
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If it is a 208 system then 450 amps per phase is about right.

Maybe he could put three 200 amp panels in. Might be cheaper than a single 600 amp panel.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Spencer125 said:
its a 120/208 system
i do not understand these things very well which i why im reaching out for input
Click to expand...
I don't want to be insensitive, but why are you trying to do work on a system that you do not understand?
 
M

Mustang125

Member
Location
New Hampshire
ggunn said:
I don't want to be insensitive, but why are you trying to do work on a system that you do not understand?
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I do understand the system. I don't understand how the engineer came up with his answer because sizing panels is not something I typically do, some humans forget how to do things when they don't do them often, sizing a panel is not like riding a bike. I'm trying to learn and become better at my trade, is that an issue for you?
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Spencer125 said:
I do understand the system. I don't understand how the engineer came up with his answer because sizing panels is not something I typically do, some humans forget how to do things when they don't do them often, sizing a panel is not like riding a bike. I'm trying to learn and become better at my trade, is that an issue for you?
Click to expand...
I think you made an arithmetic error in your calculations. If you divide the VA in each phase by 120 volts you will come up with a lot more than what you suggested for amps.

The easiest way to do these things is just to add up the total VA and assume it's distributed evenly.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Spencer125 said:
I do understand the system. I don't understand how the engineer came up with his answer because sizing panels is not something I typically do, some humans forget how to do things when they don't do them often, sizing a panel is not like riding a bike. I'm trying to learn and become better at my trade, is that an issue for you?
Click to expand...
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.
 
M

Mustang125

Member
Location
New Hampshire
petersonra said:
Because you are mean, mean as a snake.😀
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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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