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Thread: Volt Drop & Wire Size

  1. #11
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    Default Re: Volt Drop & Wire Size

    Yes please do, that is why it is there. I hope to improve and expand on it later. If you like it, please tell your friends.

    FYI I tryed to using the side car calc in my program and saw maximum length to be 621 feet, otherwise it seems to be accurate.

  2. #12
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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by bts View Post
    Single load of 1800VA, 100ft away from the source, with a 90% PF at 3% drop yields a wire size of #10 (2.77% total voltage drop).

    10 general receptacles, spaced 10ft apart, with the furthest being 100ft away from the source, maintain PF and 3% drop yields a wire size of #12 (2.29% total voltage drop).

    I have a feeling (might try it out later) that revit will still size the 2nd example, even with the setting of 6% with a #10 wire because it uses the entire distance, not the electrical center of the circuit. I could be completely wrong, though. Got a busy day ahead of me, but if I get some time, I will try out both examples and report back my findings.
    Ok, so I set my percentages higher (6% branch, 4% feeder), and ran 2 different circuits, as mentioned above (PF was 100%, though). Created a voltage drop schedule, and added a calculated parameter for VD %. Both came back with identical loads (which they should have) as well as identical VD % (which they shouldn't have, at least for the way we do voltage dropping). Voltage drop was 5% for both, and with the doubled percentage allowances, both resulted in #12 wire. The problem is, for a single 15A load, 100ft away from a panel, you are going to want #10 wire. So it works for some circuits, but not for others...

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    Default Re: Volt Drop & Wire Size

    As a proof in point as to why it is wrong, lets break down the voltage drop at each device. For the single load (1800VA), 100ft away, the voltage drop will be 6 volts (5%). For 10 loads of 180VA, spaced equally apart, each device is going to drop as follows:

    100ft - 0.6v
    90ft - 0.5v
    80ft - 0.5v
    70ft - 0.4v
    60ft - 0.4v
    50ft - 0.3v
    40ft - 0.2v
    30ft - 0.2v
    20ft - 0.1v
    10ft - 0.1v

    This results in an overall voltage drop of 3.3 volts, or 2.75%. The electrical center of the 10 device circuit (since they are equal loads) is 55ft. 1800VA at 55ft results in a voltage drop of 3.3 volts. That is the way the calculation needs to be determined, and if so, will result in the correct wire size for any given situation.

  4. #14
    I could stop if I wanted to Misteracad's Avatar
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    Default Re: Volt Drop & Wire Size

    So in other words, the workaround I mentioned doesn't actually work? I'm not an electrical engineer by trade, but it would be good to know what to tell new Revit MEP users who have certain expectations about the program in its current form
    Timothy S. Kramer
    AEC BIM Specialist

    "Just because you can doesn't mean you should!!!"
    "You don't know what you don't know!"

    What am I doing HERE??? I should be working on my Nash!

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    Default Re: Volt Drop & Wire Size

    I wouldn't use it.

    With the explanation I gave, I think it is a very viable option for Autodesk to create a voltage drop system that actually works properly. Each component would be individually voltage dropped to the panel, and the individual voltage dropped summed together for the circuit to create a true voltage drop for the entire circuit. I don't see it being something hard to do, since its such a simple solution and all the information should already be there (component location relative to the panel, load, voltage, etc).

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    Revit MEP Moderator mjdanowski's Avatar
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    Default Re: Volt Drop & Wire Size

    We use Revit in conjunction with SKM to do our calculations. Revit sums up a fairly accurate load, and then SKM does the power flow, voltage drop and stupidity check with our sizing. Worst case voltage drop (full load) is done in excel as SKM gets annoying with it.

    Revit is less transparent than Apple with their calculations, so I wouldn't trust it at all.

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    Default Re: Volt Drop & Wire Size

    I think the base problem is that length of conductor needs to be EDITED and not something they think might be right. If the formula they use is correct, and I'm sure it is, then the only variabel that could be throwing it off is length of conductor. Reguardless of the excuss, it is serious and wrong.

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    Default Re: Volt Drop & Wire Size

    Mathew, how do you export your data for the electrical SKM?

    Is there a way to export electrical circuits to comma delimited or other that have load data? Is it best to do this from a schedule?

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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by Jimbob View Post
    I think the base problem is that length of conductor needs to be EDITED and not something they think might be right. If the formula they use is correct, and I'm sure it is, then the only variabel that could be throwing it off is length of conductor. Reguardless of the excuss, it is serious and wrong.
    Although the length of the conductor has the ability to be slightly off (it doesn't take into account the fact that the wire will go up into the ceiling and then back down to the panel), I don't feel that is the root of the problem. As stated above, the main issue is that they are using the entire length of the run as the distance by which the calculation is done. This is fine for single loads on a run, but once you have multiple loads on a single run, it is overkill (as I showed in my example above, where the total voltage drop is almost twice what it should be).

    There are other items that, if they continue to use the generic formula, as stated in the 2010 Revit MEP User's Guide, are going to cause issues:

    Revit MEP uses the following formula to calculate voltage drop in wiring runs:

    VD=(L*R*I)/1000

    Where:

    VD = Voltage Drop
    L = One-way length of circuit in feet
    R = Conductor resistance in Ohms per thousand feet, from the wire size impedance tables for the specified wire type.
    I = Load current in amperes
    I'm not even sure where they came up with the values listed in the wire size impedance table (http://docs.autodesk.com/RVTMPJ/2010...umber=d0e35180), but it doesn'e seem to match anything I've seen before. Besides that fact, they are using a specific value based on an ambient temperature, when they also need to be taking into account the power factor on the circuit, as that is going to affect the final wire size as well.

    As Jimbob stated above, they really need to use the following formula to determine the effective impedance:

    Ze = R x PF + Xl x SIN[ARCCOS(PF)]

    and then determine the voltage drop for each component individually using the formula listed above, including the ability to add (or define in settings) the up and down distance for the run. When the circuit is defined, do the calculation for each component individually, then add up the voltage drop of each component to come up with the overall voltage drop for the entire circuit. Doing it this way will result in the most accurate wire sizing.

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    Default Re: Volt Drop & Wire Size

    Just to show how impedance can affect wire size, when I developed our in-house voltage drop spreadsheet, I also included an impedance graph which plots out where the NEC minimum wire size is, as well as the calculated wire size. As an example, I have a 208v/3ph circuit, at 45 amps. The circuit length is 165ft, voltage drop is 3%, and my power factor is 90%. This results in a 1-step upsise, from #8 to #6 (see attachment). As shown on the graph, it is very close to the curve. If the power factor is changed to 95%, a #4 wire size is required. #6 is still very close (it ends up being like 3.01%), but as the chart shows, there are some larger wire sizes that will be affected much easier with a change in PF.
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