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Thread: Volt Drop & Wire Size

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    Default Volt Drop & Wire Size

    Has anyone figured out a way to manually set the wire size in Revit 2011? I have found that Revit seems to be measuring from the fixture to the panel, and then back when calculating volt drop, so the wire sizes are usually incorrect when it comes time to upsize.

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    Default Re: Volt Drop & Wire Size

    The method of sizing is hard coded into the program. You can upsize by changing the circuit's rating parameter.

    You can always add a custom project parameter to create a field for a manually added wire size.

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    Default Re: Volt Drop & Wire Size

    The distance is actually measured with parallel and perpendicular runs, from the furthest device, to each subsequent device, and finally to the panel. Fairly inaccurate, and not the way we (as I'm sure several others) do voltage dropping. When I posted a similar question last year, it was pretty unanimous across the board that no one used the wire sizes Revit was spitting out.

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    Default Re: Volt Drop & Wire Size

    I saw a tutorial that noted the inaccuracy of wire lengths and sizing in Revit, and mentioned that there was an inherent flaw in the program. The suggested workaround was to DOUBLE the Max Voltage Drop values. The end result in the model should be lengths and sizes that more closely match what you would expect. However, calculating the true Voltage Drop would then have to be done manually.

    Interestingly, I've yet to see anyone else identify the root of this problem, or offer a similar workaround. I can't help but wonder if Autodesk is going to get this fixed in an upcoming release. The info is not much good to you otherwise lol

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    Default Re: Volt Drop & Wire Size

    Quote Originally Posted by Misteracad View Post
    I saw a tutorial that noted the inaccuracy of wire lengths and sizing in Revit, and mentioned that there was an inherent flaw in the program. The suggested workaround was to DOUBLE the Max Voltage Drop values. The end result in the model should be lengths and sizes that more closely match what you would expect. However, calculating the true Voltage Drop would then have to be done manually.

    Interestingly, I've yet to see anyone else identify the root of this problem, or offer a similar workaround. I can't help but wonder if Autodesk is going to get this fixed in an upcoming release. The info is not much good to you otherwise lol
    When you say double the max voltage drop values, are you talking about the percentages? I could possibly see that making the sizes closer, but not the lengths. The way it determines the length is always going to be constant, and there really is no way (that I've found) to make them any closer to what we typically use (electrical center of the circuit). We have an in-house voltage drop calc which is very powerful, and I see us continuing to use it, even if Autodesk figures out a way to improve the built in functionality.

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    Default Re: Volt Drop & Wire Size

    Hope I am reading this correctly but when we were going through training in 2010 I found (errors) problems in voltage drops and wire sizes, but actually they are right. I know in a lot of cases that the wire sizes shown in revit are something we would never design for or use but can be worked around by increasing the voltage drop settings to 10% instead of 3%. This will keep constant values throughout the project.

    Go to manage > mep settings > wiring and see what your percentages are set for and increase by 10%

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    Default Re: Volt Drop & Wire Size

    Yes, Brad and JimBob that is what I was talking about...the percentages. JimBob suggests using 10%, my source suggested doubling the OOTB setting of 3% to 6%. I guess you'll have to test it out and see what works best. Let us know how you make out! Cheers

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    Default Re: Volt Drop & Wire Size

    Interesting thought. I might look into it when I have some time, although I still don't think it'll give wire sizes that we are used to. As an example:

    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.

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    Default Re: Volt Drop & Wire Size

    Revity uses a simple math problem to solve voltage drop while my progam is more complex in it result, but delivers almost the same result.
    The NEC reccomends the following, but of course it is not this simple when dealing with single phase circuits. The formula is written out in my program if anyone would like to try it.
    Voltage drop was calculated using the formula found in 2005 NEC®, Handbook, Chapter 9, Example 2. Result is within ±5%. Zc = (R x cos Ѳ) + (Xl x sin Ѳ)
    There will alwasy be variations in wire seperation and moisture differances.

    http://www.powersofteng.com/eps2010/download.html
    Its free.

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    Default Re: Volt Drop & Wire Size

    That is the same equation we use for our calculator. I've included a screen shot. I plan on downloading your just to check it out, as it looks pretty cool!
    Attached Images Attached Images

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