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Thread: Amps per phase for panel different than feeder circuit

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    Default Amps per phase for panel different than feeder circuit

    I have uploaded screenshots from a test model.

    My ultimate question is this: shouldn't the amps per phase for the circuit feeding the panel be the same as the amps per phase on the panel (and reported on the panel schedule)? Refer to the screenshots. Shouldn't the amps per phase on MS 208 - 1 be the same as the amps per phase on Panel 208 panel schedule?

    BTW, this discrepancy occurs using all OOTB settings. families, etc. as well. So I feel reasonably confident it isn't something in our template or families.

    I have a panel with a 3 phase load and a single phase 2 pole load. Refer to Panel_208. The load for the entire panel is correct, as is the current for the entire panel. However, how it is calculating total amps per phase is what is wrong. (And I am not talking about whether it divides the load shorthand by L-G voltage or more mathematically accurately by RMS. ) This problem becomes more apparent as you add more load to the panel and especially if you had single phase two pole loads. I am not an engineer by training, but my engineers and designers tell me that the amps per phase is particularly important to consider in correctly sizing a panel. Granted, in my research thus far the amps per phase is always high rather than low, but it is still wrong.

    From the screenshots, for example:
    8333VA/120V=69A (Load per Phase/L-G Voltage)=Amps per Phase OR (Load per Phase*3/(208*sqrt(3))=Amps per Phase

    If you select the panel, the properties display the current for phase A as 76A (screenshot Equipment). If you select the circuit (screenshot Circuit) from the panel to the switchboard, it displays the current for phase A as 69A. The load for phase A is the same for both. This is the root of the problem. The panelschedule only reports panel information, not circuit information, in this location on the panel schedule. If you look at Panel_208 you notice that I created rows for the correct calcuated values...for whatever reason that same formula in the phase columns of the schedule template produces incorrect values as well.

    In case I am not communicating clearly...amps per phase as calculated for the panel and shown in the panel schedule is different(and wrong) than what it calculates for the amps per phase for the circuit (correct) from the panel to whatever feeds it.

    I can also upload the test model if that would help. Thanks in advance for any help.
    Attached Images Attached Images

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    Default Re: Amps per phase for panel different than feeder circuit

    ok so right or wrong I'm not sure but I think I was able to determine what the calc is doing to give you the 76A value. So here is what is happening.

    Let's take phase A... you have
    (1) load @ 3333 VA @ 120V power... that equals approx 27.8A
    (1) load @ 5000 VA @ 208 power... The amperage on this takes into account the complete load so it's 10000 VA combining both poles because each pole sees that amperage. 10000 VA @ 208 single phase = 48.1A

    It appears the panel schedule phase amperage is simply just adding the Amps off each circuit so... 27.8 + 48.1 = 75.9 or 76A

    When the load is transferred up it is transferred up as a VA value. So the 8333 VA on phase A is what is being calculated upstream. So 8333 VA @ 120v = 69 A

    Now again I am not sure if this is done correctly or not... I have never really cared so much as to the amperage on each phase as much as the VA on it. Hope that helps clears up "What" Revit is doing even though I know it doesn't give you the "Why" or if it's even correct.

    Please post anything else you find out, I am curious now.

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    Default Re: Amps per phase for panel different than feeder circuit

    Thanks for the response. Like you said, right or wrong, that does help me understand the math going in the background. We have been using Revit panelschedules exclusively for 4 projects over the last year and this issue never came to light. Now, however, a particular project is requiring two 42 pole panels of 240 single phase power on 120/208 building service. So, on the panel upstream from the transformer the amps per phase are incredibly high on two phases, even with diversity/demand factor, and even though the amps for the panel are correct/acceptable...the panel itself needs to be sized off the amps per phase. In our case the amps per phase Revit is showing for A and B are 130A higher than if calculated using L-G voltage or RMS for each phase.

    I am not still not totally satisfied that it does it this way: why doesn't it calculate the same in both places? Seems like this would be explained somewhere or pointed out previously but in all my research it seems like I am in uncharted territory. You aren't the first person familiar with Revit electrical to tell me they never noticed this before. I will let get back to you after discussing this with my designers/engineers.

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    Default Re: Amps per phase for panel different than feeder circuit

    Just wanted to give an update on this issue. I also updated my post over at Autodesk Discussion forums http://forums.autodesk.com/t5/Autode...4308866#M19570

    Basically it was on oversight on the part of our team AND an issue with Revit that is now "being worked on by our developers for a future release."

    Hopefully this will be helpful to anyone else out there that runs into this problem.

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