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Thread: Voltage Drop - How does your firm do it?

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    Default Voltage Drop - How does your firm do it?

    I am interested in hearing how other electrical engineers and firms do their voltage dropping, mainly for circuits with multiple fixtures (ie, lighting runs). The reason I ask is because the way we do it seems to be completely different than the way Revit does it...

    The way we have done it for years is to use the "electrical center" of a circuit, and then run parallel and/or perpendicular to walls all the way back to the panelboard. So, for instance, if you have a run of lights that are the same, and equally spaced, then the "electrical center" of the circuit will be in the middle of the run. If you have a circuit with different loads spaced sporatically around the area, then it is obviously harder to find the exact center. On a project I am currently working on, I had determined that the electrical center of the circuit was ~75' from the panelboard.

    Now, from what I can gather (since I don't think its actually spelled out anywhere), Revit uses the entire length of the circuit for voltage drop. I built a schedule to return values similar to our in-house voltage drop spreadsheet, and found that Revit returned a value of 175'.

    This now caused the circuit to go from #12 wire to #8 wire. When I plugged the same values into our in-house spreadsheet, I found that with #10 wire, the resulting voltage drop was 3.02% (I had it specified in Revit for 3%). So for us, we feel the built in wire sizing in Revit is invalid and not usable, which sucks. So my question is, is anyone else under the same opinion? Or have you even checked it against the way you were doing it before Revit?

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    All AUGI, all the time JoelLondenberg's Avatar
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    Default Re: Voltage Drop - How does your firm do it?

    I don't know of any firm using Revit's voltage drop/wire sizing.
    Joel Londenberg
    BIM Box Biz - Serving all MEP Contractors

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    Default Re: Voltage Drop - How does your firm do it?

    Maybe that should be a note for Autodesk then...

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    Default Re: Voltage Drop - How does your firm do it?

    Scott with RLBA shared a nice pdf of his effort to determine how various length of various circuits are defined. It boils down to an X/Y relationship to the furthest parts of the circuit, adding the X distance and Y distance to arrive at an "arbitrary" estimate for length. I'm not sure how they can ever be "accurate". You'd have to run actual wiring to even get close, up and over doors/openings, into plenum and down under floors...over the wood and through the snow.

    I suppose it would be better to be able to define some "length calculation" criteria like take the total distance and "divide by 2". Take your scenario and codify it??

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    Default Re: Voltage Drop - How does your firm do it?

    That is an interesting evaluation. I attempted that method with my circuits, and I still can't seem to get anywhere close to what Revit is returning. If it factors in switching, it might be slightly closer, but I still don't see it even within 20%... I have included the lighting and circuitry I am currently working on (small tenant space). The panelboard is not shown, but it is right below the 3-way switch on the far right of the page. For circuit 1, Revit returned 138'. For circuit 3, Revit returned 174'. I have no measurements shown, but you can get a general idea of size based on the 2x4 troffers shown on circuit 1.
    Attached Files Attached Files

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    Revit MEP Moderator mjdanowski's Avatar
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    Default Re: Voltage Drop - How does your firm do it?

    In Excel/SKM

    The fact that Revit just spits out a number without any explanation or display of factors which it used to get there is simply not something we can use.

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    All AUGI, all the time JoelLondenberg's Avatar
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    Default Re: Voltage Drop - How does your firm do it?

    The last time I experimented, Revit gave the correct length to within 10%. It reads the cumulative distance in x/y/z orthogonally from the panel board to the farthest fixture.

    The reason it can't be used for voltage drop is that it assumes that the entire load goes the entire length of the circuit instead of dropping off at each device. Depending on your firm's design criteria, this may be desired in some instances (one uses this method on receptacles), but certainly not in all circuits.
    Joel Londenberg
    BIM Box Biz - Serving all MEP Contractors

    "The best you can hope for in this life is that your delusions are benign and your compulsions have utility." - Dilbert author Scott Adams

    This is the correct date format

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    Default Re: Voltage Drop - How does your firm do it?

    Quote Originally Posted by jlondenberg View Post
    The last time I experimented, Revit gave the correct length to within 10%. It reads the cumulative distance in x/y/z orthogonally from the panel board to the farthest fixture.
    For the example I attached earlier, the X/Y orthoganal distance for circuit 3 is ~80' to the furthest fixture. And I know I don't have 94' vertical feet. There has to be other factors involved.

    The reason it can't be used for voltage drop is that it assumes that the entire load goes the entire length of the circuit instead of dropping off at each device. Depending on your firm's design criteria, this may be desired in some instances (one uses this method on receptacles), but certainly not in all circuits.
    Agreed. As stated above, we measure to the electrical center of a circuit, which can vary due to device loading, spacing, etc. The electrical center may be closer to or further from the actual circuit center, depending on where the larger of the loading is placed on the circuit. If you have 90% of the load in the first 50% of the circuit length, your electrical center is going to be much closer to the panel than if everything was spaced evenly across the circuit. I'm sure, if it was thought out, it could be programmed. I mean, look at all the other analysis they are doing. Surely, this wouldn't be that hard to put into an algorithm.

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