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Thread: Volt Drop & Wire Size

  1. #21
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    Default Re: Volt Drop & Wire Size

    I used to be under the impression that you don't have to double the length either BUT you do for single phase circuits. Only a three phase doesn't have to be doubled when using that formula. I you see the examples shown in the NEC handbook as I mention in my program, the example shows a differant formula when solving for single phase. I choose to use the "engineering" formula because I have all the information available to me, along with being easier to deal with and done correctly, can be very accurate, which is what I want. Please try my program and see if you get similar results when using single phase wiring. The formula is written out for you.
    As another test, you may try the excel sheet on Mikle Holts web site that is also very accurate. I checked all my calcs using this and other methods.
    http://www.mikeholt.com/documents/fr...Calculator.xls

    The simple math formula can be accurate if all the info is entered correctly.

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    Default Re: Volt Drop & Wire Size

    I'm not sure I would say that the distance is necessarily doubled for single phase loads, its just that the multiplier is set to 2, whereas for 3-phase loads its set to square root of 3.

    I still have yet to look at yours (can't DL and install it at work, and keep forgetting at home), but I looked at the one on Mike Holt's site and I found some things I don't like about it. First off, the fact that it will recommend a wire size with an ampacity LESS THAN the load entered isn't good (it returns an error, so that's at least helpful). Also, the fact that it is using the resistance from Table 8 (DC resistance) instead of Table 9 (AC resistance) is kind of wierd.

    With that said, it really doesn't matter how other VD calcs work, as that's not the point here. The point is to try to pass on to Autodesk what improvements they need to make to the program to make voltage drop and wire size more accurate out of the box.

  3. #23
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    Default Re: Volt Drop & Wire Size

    You are right, though, with such a crude formula, you will get a crude result, but hey, there are actually a lot more things that needs huge improvment than just vd, things which I will not elaborate on, till I see how 2012 will turn out.

  4. #24
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    Default Re: Volt Drop & Wire Size

    Not sure when you started using Revit, but 2011 is leaps and bounds better (on the Electrical side) than it was in 2010, and its hard to even compare to Revit Systems 2 (ie, Revit MEP 2006/7...?). Although I haven't had a chance to look at the beta for 2012, reading up on the Beta release notes, not much changes for Electrical. There are a few things I'm looking forward to trying out, though.

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    Default Re: Volt Drop & Wire Size

    Yea, we'll see. I have no ideas what they are doing either.

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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by bts View Post
    I'm not sure I would say that the distance is necessarily doubled for single phase loads, its just that the multiplier is set to 2, whereas for 3-phase loads its set to square root of 3.
    If memory serves me correctly, the single phase drop must be multiplied by 2 for unbalanced loads where there is current in the hot and neutral and the wire length involved is therefore double the run length.

    Three phase voltage drop is sqrt(3) x the single phase drop calculated.

  7. #27
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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by sparkie001 View Post
    If memory serves me correctly, the single phase drop must be multiplied by 2 for unbalanced loads where there is current in the hot and neutral and the wire length involved is therefore double the run length.

    Three phase voltage drop is sqrt(3) x the single phase drop calculated.
    Yes and no. The calculation is as follows:

    Voltage Drop = Distance * Phase Constant * (Ze/1000) * Current

    Where Ze is the effective impedance (as calculated above somewhere for various power factors, or from Table 9 of the NEC for a general number based on 85% PF) and where Phase Constant equals 2 for single phase and 1.732 (sqrt3) for 3 phase. So yes, you are multiplying by 2 for single phase, but you are not multiplying the single phase drop by sqrt3 (the way I read what you are saying is you effectively multiple by 3.464).

    I guess I just never pictured it as doubling the distance, I always just thought of it as multiplying by a phase constant...

  8. #28
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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by bts View Post
    Where Ze is the effective impedance (as calculated above somewhere for various power factors, or from Table 9 of the NEC for a general number based on 85% PF)
    As I have done more research, there are actually few circuit which will use a 85% power factor, in this case a bad low efficent motor, so the power factor side of table 9 is not applicable to most circuits. The formula I use in my program is not based on power factors. I use the left side of table 9 in first two columns only.

    Again that program can be found at: There is a conduit fill and voltage drop formula as a side bar.
    http://www.powersofteng.com/eps2010/download.html

    There are also two very useful pdf's there which have some very useful for design information on them.

  9. #29
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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by Jimbob View Post
    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?
    We just do it manually. I don't think there is a way to do it automatically, and if there was I would be wary to trust it.
    Matthew Danowski, PE, LEED AP BD+C
    Project Electrical Engineer
    Baltimore, MD

  10. #30
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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by mjdanowski View Post
    We just do it manually. I don't think there is a way to do it automatically, and if there was I would be wary to trust it.
    I am pondering Revit API and EPS with 2 way communications. Revit seroiusly lacks in overall electrical continuity and work flow I think, although everthing else is great. Integrating a great program like EPS into revit would be the ultimate, provided I could force values back into revit and lock calculations.
    If you try eps please let me know what you think.

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