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

  1. #1
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    Questions on 120v loads on a 120/208 3 phase panel with NO 3 phase loads? Panel size?

    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

  2. #2
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    Quote Originally Posted by re-evaluating View Post
    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
    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.

  3. #3
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    I agree with kwired. The 60 amp panel should be enough even for a few extra circuits
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
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  4. #4
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    While you can total kVA, you cannot total amps. Amps do not add together like that.
    "Electricity is really just organized lightning." George Carlin


    Derek

  5. #5
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    Quote Originally Posted by kwired View Post
    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.
    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

  6. #6
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    Quote Originally Posted by re-evaluating View Post
    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
    You cannot add the amps that way. Period.

    Your 97A total is incorrect.

    Are all your loads continuous? Why the .8?
    "Electricity is really just organized lightning." George Carlin


    Derek

  7. #7
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    Quote Originally Posted by re-evaluating View Post
    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
    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.

  8. #8
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    Quote Originally Posted by jumper View Post
    While you can total kVA, you cannot total amps. Amps do not add together like that.
    In just this case or in every case? Thank you!

  9. #9
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    Quote Originally Posted by re-evaluating View Post
    In just this case or in every case? Thank you!
    You are incorrect here and trying to do it that way will prolly be wrong 90% of the time.

    Do you want to learn methods that are always correct or play around and try and find a trick that works once in a blue moon?
    "Electricity is really just organized lightning." George Carlin


    Derek

  10. #10
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    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.
    Electricians do it until it Hertz!

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