Volt Drop & Wire Size

[Jimbob]

I have created a voltage curve chart. Thought I would post it so everyone could see the surpising results of power factors I saw.

.1 power factor has less voltage drops than a more efficient 1. I was very surprised. The slopes are linier.

[brad-TEC]

To maybe get a better view of what you are seeing, have a look at my attachments. This is impedance curves for various wire sizes, and plotted impedance points for entered information. You can see how the impedance of the wire varies with power factor, and the curves are different for each wire size. The curves shown are based on copper wire in steel conduit.

The first curve is a 208v/3ph, 180A circuit, 95%PF, with a 3% voltage drop allowance and a length of 275ft. The NEC required is the minimum size wire for the ampacity of the circuit, and the calculated is the required upsize of the wire in order to have a 3% drop. In this case, the NEC required is 3/0, but the calculated is 250kcmil.

In the next example, all things stay the same, except the length of the circuit is decreased to 150ft. You can now see that the calculated wire size is only 1/0 for the 3% drop, but the minimum size is still 3/0 per the NEC. This results in a voltage drop of only 2.05% for the 3/0 wire.

[Jimbob]

Your chart is difficult to read. Not sure where those height bubbles or for that matter what the height is. Voltage drops are linier.

What did you use to generate this chart and what formula did you use?

[brad-TEC]

They aren't really "heights". If you scroll all the way to the bottom, you will see power factor percentages. The "bubbles" are the calculated impedance (Zeff) of the wire for the given conditions. The Zeff is calculated by the equation listed in Table 9 of the NEC...

Zeff = R x PF +Xl sin(arccos(PF))

where R is the resistance of the wire and Xl is the reactance of the wire, all based on values in Table 9 of the NEC.

[amanda353909]

I know this is an old thread. Just wondering if there has been any changes. Does anyone know if Autodesk is working on the wire size and voltage drop issue? It seems like most companies either do it manually or with a different program. It is often difficult to encourage the use of Revit for Electrical when there are other programs that include the calculations.

[mhartmann]

no changes, the wire length of a circuit that revit is coming up with is completely inaccurate, so it cannot be used for tagging on documentation or in voltage drop calcs. i've tried to walk them thru it, or convince them to let us override the circuit length and wire size. so, until it works right or they let us override, we continue to do all wire sizing and voltage drop calcs manually, blah...

[Misteracad]

Not sure who is still following this thread, but something related came up on the LinkedIn Revit MEP group 7 months ago. I was just on there again just now and thought I would share...

http://www.linkedin.com/groups/Is-an...88.gmp_1550697

Specifically I would look at Martin Schmid's responses. Cheers

[MEP1]

from mine experience and from the previous post, I understood, that VD (voltage drop) seems to be quietly "ununderstandable" in terms of calculation origin.
With a simple test based on 2 identical panels (230V), I found the reason that the cable setting (Ampere, dimension, diameter, used in calculation) represents the most important scenery to be taken into consideration.
Unfortunately, by the preset cable (revit default file, USA standard, es. THWN), the drop voltage has a sense, but by simple personalized cable, there is no way out to figure with drop voltage calculation different from zero.
What's gonna be the wrong/mistake/error?
waiting your response, I thank you.

[JoelLondenberg]

The method Revit uses for calculating wire size vs voltage drop is consistent and matches the method used by all of the Electrical engineers that I've personally analyzed it with.

The short description of calculations:
wire size is driven by max voltage drop
voltage drop is calculated by using the distance (cumulative x,y,z from panel to farthest load) and the load (the largest of either the breaker size or connected load)
it also assumes that the entire load for the entire circuit all goes to the farthest load

More detailed descriptions are available here: http://revitoped.blogspot.com/2012/0...culations.html

[David Robison]

no changes, the wire length of a circuit that revit is coming up with is completely inaccurate, so it cannot be used for tagging on documentation or in voltage drop calcs. i've tried to walk them thru it, or convince them to let us override the circuit length and wire size. so, until it works right or they let us override, we continue to do all wire sizing and voltage drop calcs manually, blah...

Revit 2018 added the "Circuit Path" tool to allow you to set the circuit length. It's a step in the right direction, but you still don't have control over the wire size.

While you wait for them to implement that missing piece, take a look at our electrical add-in for Revit. We size your feeder and branch circuit wires for you, but also give you the ability to override the sizes when necessary.

You can learn more and download a free trial on our website:
http://www.designmaster.biz/revit/voltage-drop.html