Questions on 120v loads on a 120/208 3 phase panel with NO 3 phase loads? Panel size?

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R

re-evaluating

Member
Location
Illinois
I have a few questions about single phase loads on a 3 phase 120/208 panel. Prior to my joining the site I have searched dozens of threads on here trying to find the answer and did not find conclusive answer.
To start I do estimating for a company that builds large prewired chef’s counters for commercial kitchens in restaurants and stadiums etc.
I have a customer who’s engineer sent me a change to a panel layout that in one way makes sense in another seems to not. Due to the nature of the business most of these will be constant loads as they will all be on during business hours. Several are food warmers that are inductive loads and the others are refrigerators or freezers. No lighting or non-dedicated outlets. All are 120v there are No 3 phase items on this panel. This is UL listed and NO additions or modifications are allowed after the fact without voiding the UL listing.
My main question is in sizing the panel (biding the right panel), when converting to amps as per normal the total is 97.1 amps @ 120v this was originally a 125amp single phase panel that the engineer now wants to change to a 3 phase 60amp panel.

Can some of you guys help me understand how this panel should be sized. I get that the load per leg is what it is, but the overall total load is 97.1amps right????? I am hoping to understand more not just an answer of XXX amp panel if you know what I mean. Thank you all in advance for your help. Dave
3 Phase Panel layout as follows

Bus A Bus B Bus C
.95kva .95kva .95kva
.95kva .876kva .876kva
1.1kva 1.1kva 1.1kva
.360kva .924kva 1.1kva
.360kva__________________
3.7kva 3.8kva 4kva = 11.5kva Total
 
kwired

kwired

Electron manager
Location
NE Nebraska
re-evaluating said:
I have a few questions about single phase loads on a 3 phase 120/208 panel. Prior to my joining the site I have searched dozens of threads on here trying to find the answer and did not find conclusive answer.
To start I do estimating for a company that builds large prewired chef’s counters for commercial kitchens in restaurants and stadiums etc.
I have a customer who’s engineer sent me a change to a panel layout that in one way makes sense in another seems to not. Due to the nature of the business most of these will be constant loads as they will all be on during business hours. Several are food warmers that are inductive loads and the others are refrigerators or freezers. No lighting or non-dedicated outlets. All are 120v there are No 3 phase items on this panel. This is UL listed and NO additions or modifications are allowed after the fact without voiding the UL listing.
My main question is in sizing the panel (biding the right panel), when converting to amps as per normal the total is 97.1 amps @ 120v this was originally a 125amp single phase panel that the engineer now wants to change to a 3 phase 60amp panel.

Can some of you guys help me understand how this panel should be sized. I get that the load per leg is what it is, but the overall total load is 97.1amps right????? I am hoping to understand more not just an answer of XXX amp panel if you know what I mean. Thank you all in advance for your help. Dave
3 Phase Panel layout as follows

Bus A Bus B Bus C
.95kva .95kva .95kva
.95kva .876kva .876kva
1.1kva 1.1kva 1.1kva
.360kva .924kva 1.1kva
.360kva__________________
3.7kva 3.8kva 4kva = 11.5kva Total
Click to expand...
If I understand this correctly you have total load on A of 3.7 kva, B is 3.8 and c is 4.

4 is your highest so that leg will draw 4000/120=33.33 amps. You need at least 33.33 amp supply circuit to this panel to handle that load.

An actual balanced 11.5 kva is only 31.92 amps but your max imbalance is 33.33 amps on line C, you need to be able to carry that.
 
R

re-evaluating

Member
Location
Illinois
kwired said:
If I understand this correctly you have total load on A of 3.7 kva, B is 3.8 and c is 4.

4 is your highest so that leg will draw 4000/120=33.33 amps. You need at least 33.33 amp supply circuit to this panel to handle that load.

An actual balanced 11.5 kva is only 31.92 amps but your max imbalance is 33.33 amps on line C, you need to be able to carry that.
Click to expand...

Yes you read correctly (even though it changed my chart in the way it looked from when I did it) that is the value I came up with as well.

So when I am calculating this I basically view a 3 phase panel as if it were 3 separate single phase panels when using solely 120v loads (highest leg considered) to size the panel? This makes sense as I said in the original post but my panel supplier keeps telling me that I have to account for total amperage when sizing this panel (97.1/.8=121.4amp = 125amp panel). I get that the current draw will be divided between the 3 legs (and feed conductors) which is part of the reason for the 60amp panel but I just want to make completely sure we are bidding and building it correctly. Additionally I want to learn for the future opportunities. Please feel free to expand a little more if you would like.
Thank you again. Dave
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
re-evaluating said:
Yes you read correctly (even though it changed my chart in the way it looked from when I did it) that is the value I came up with as well.

So when I am calculating this I basically view a 3 phase panel as if it were 3 separate single phase panels when using solely 120v loads (highest leg considered) to size the panel? This makes sense as I said in the original post but my panel supplier keeps telling me that I have to account for total amperage when sizing this panel (97.1/.8=121.4amp = 125amp panel). I get that the current draw will be divided between the 3 legs (and feed conductors) which is part of the reason for the 60amp panel but I just want to make completely sure we are bidding and building it correctly. Additionally I want to learn for the future opportunities. Please feel free to expand a little more if you would like.
Thank you again. Dave
Click to expand...

You cannot add the amps that way. Period.

Your 97A total is incorrect.

Are all your loads continuous? Why the .8?
 
kwired

kwired

Electron manager
Location
NE Nebraska
re-evaluating said:
Yes you read correctly (even though it changed my chart in the way it looked from when I did it) that is the value I came up with as well.

So when I am calculating this I basically view a 3 phase panel as if it were 3 separate single phase panels when using solely 120v loads (highest leg considered) to size the panel? This makes sense as I said in the original post but my panel supplier keeps telling me that I have to account for total amperage when sizing this panel (97.1/.8=121.4amp = 125amp panel). I get that the current draw will be divided between the 3 legs (and feed conductors) which is part of the reason for the 60amp panel but I just want to make completely sure we are bidding and building it correctly. Additionally I want to learn for the future opportunities. Please feel free to expand a little more if you would like.
Thank you again. Dave
Click to expand...
You have a fixed level of VA, but current is distributed differently if you try to balance it as best as possible across three phases, compared to balancing it across 120/240 single phase, and the VA is across just two leads if you only had a 120 volt two wire feed, so if you for some reason wanted to only supply with 120 volt two wire you would have a amp draw of 95.8 amps - total VA is still the same.
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
Welcome to the forum. You need a 3ph panel that can carry the maximum load per leg, which as kwired calculated was 33.33A. A 60A panel is more than capable of that. You need a minimum 5 wire #8 feed, which sounds terribly small for a commercial kitchen; what are the exact loads you have?

Three phase power is kw = VL-L x I x pf x √3 ÷ 1000 or kW = (VL-L× I × PF × 1.732) ÷ 1,000.

As mentioned above, you cannot add amps, tho if each line were perfectly balanced, you could use total kW or kva with the above formula, reworked as such:

I = KW ÷ (VL-L x √3 x PF )

In other words, if you had a single 11.5kW 3ph 208V load, it would draw 31.9A per leg. If you were sizing a breaker for this, and it were a constant (continuous) load, you'd need a 40A breaker, and #8 wire.

A 60A 3ph panel is plenty for what you list. Is this a package kitchen, like what would be on a food truck? I ask because a single 208V 3ph commercial deep fryer can pull 40A (11.5+kW) on its own. It looks to me you have 6 208V 1ph loads and 1 120V load to 13 120V loads that are all relatively small.
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
To add to what jflecther and kwired have said:

By assigning kVA values to all the loads and then distributing them, you will be able to work with single and three phase loads and panels.

Most panels are goings to be a combination of loads and if you do it this method we are trying to show you, it will always work.
 
R

re-evaluating

Member
Location
Illinois
jumper said:
You cannot add the amps that way. Period.

Your 97A total is incorrect.

Are all your loads continuous? Why the .8?
Click to expand...

Okay from spec sheets it comes to 97.1 if added together (I'm listening to try and learn why this isn't correct)


Yes the loads are continuous or all on when open for business.

/.8 = panel size 125% of load
 
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R

re-evaluating

Member
Location
Illinois
JFletcher said:
Welcome to the forum. You need a 3ph panel that can carry the maximum load per leg, which as kwired calculated was 33.33A. A 60A panel is more than capable of that. You need a minimum 5 wire #8 feed, which sounds terribly small for a commercial kitchen; what are the exact loads you have?

Three phase power is kw = VL-L x I x pf x √3 ÷ 1000 or kW = (VL-L× I × PF × 1.732) ÷ 1,000.

As mentioned above, you cannot add amps, tho if each line were perfectly balanced, you could use total kW or kva with the above formula, reworked as such:

I = KW ÷ (VL-L x √3 x PF )

In other words, if you had a single 11.5kW 3ph 208V load, it would draw 31.9A per leg. If you were sizing a breaker for this, and it were a constant (continuous) load, you'd need a 40A breaker, and #8 wire.

A 60A 3ph panel is plenty for what you list. Is this a package kitchen, like what would be on a food truck? I ask because a single 208V 3ph commercial deep fryer can pull 40A (11.5+kW) on its own. It looks to me you have 6 208V 1ph loads and 1 120V load to 13 120V loads that are all relatively small.
Click to expand...

No this is a Chef's counter with 13 120v circuits 8 inductive warmers 4 @(1100w) and 4 @(950w) 2 refrigerators @(876w) 1 freezer @(924w) and 2 printers @(360w).

Thank you for taking the time to help me understand the concepts here. Here is the layout.
Capture.JPG
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
Re-evaluating, the only time you add amps is when dealing with equipment on a single line. Example: you have 12 1.5A and 6 1.0A loads, all 120V. In that case you have 24A total @ 120V. Now, if you had a 1 ph panel, and the loads were balanced, you would have 6 1.5A and 3 1.0A per leg, or 12A total, at 240V. If you had a 3ph panel that was balanced, you'd have 4 1.5A and 2 1.0A to give (you'd think) 8A per leg, but across 208V, not 360V. The ONLY way this works tho is factoring the varying voltages; you are no longer working with 120V in all 3 cases:

24A @ 120V = 2880W - 24A on a single 120V circuit
12A @ 240V = 2880W - 12A on a 240V circuit
8A @ 208V = .... 1664W... oops! when working with three phase, we have to multiply by the square root of 3: 1664 x 1.73205... = 2880W, or 13.8A

eta: just saw your above post. Yes, everything there is correct. To answer again your initial post, a 60A 3ph panel is plenty for your application.
 
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F

FountainGuy

Member
Location
Metro Atlanta, GA
Your panel supplier is incorrect in adding amperage for three phases together. kwired was correct on his first post. With 33.33A being the highest leg and adding 25% to all on this leg for continuous duty, highest case scenario is 41.66A. The 60A 120/208V 3-phase panel will work fine!
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
And my POCO representative tells me that I should always run the heat and the A/C at the same time so I have a zero energy balance. :angel:
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
infinity said:
Never mind
Click to expand...

Rob, please do not go there.

I and many members here will understand your point, but that is not the proper way to learn load distribution.

That idea could work here if one knows all what applies, but the full method always works.

Throw in one 20A 208V single or three phase load and it goes kaput.
 
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