You are here: Home > Support > Knowledge Base > Voltage Drop Calculations in Revit

Voltage Drop Calculations in Revit

Thursday, October 12th, 2017

How to do voltage drop calculations in Revit using Design Master Electrical RT.



So welcome. This is David Robison with Design Master Software. We are going to be doing training on voltage drop calculations in electrical for Revit. If you have any questions, you can type them into the chat box in your [inaudible] interface there, and I will answer them as we’re going along. So we’re going to walk through how you do voltage drop calculations, the relevant parameters you can set in our Revit software, so that you can have understanding of how we’re doing the calculations and make sure that you’re getting your model set up right to have that calculated properly.

So voltage drop is basically a function of three things. You’ve got your load, you’ve got the resistance of the wire, and then you have the length of the wire. So those are the three main inputs to voltage drop that you kind of grind through our calculations, and then out comes the voltage drop number on the end. So you have to make sure that those three values are all correct.

We do voltage drop on the feeders and the branch circuits, and so how those are set varies, depending on whether you’re doing it in the feeder or the branch circuit. So we’re going to look at feeders first. So the first thing that we have to set, you have to get right is the load on the feeder. Currently, the load on the feeder is the load that is calculated by Revit as the calculated load.

So if you run our panel as it command, we display the connected load and then the calculated demand load for all of your panels, and that value is the one that we are using for the load for the voltage drop calculation. So this assumes that you have your devices connected, that you have your demand factors set right, and then we use that load for your feeder voltage drop.

A lot of people want to do a voltage drop based upon anticipated load or full loading, even if they don’t have the load in the device yet. We don’t have that. For the next release, we are going to add a new option where you can do the voltage drop based upon 80% of the panel capacity. So if you don’t have everything connected up or you want to make sure that you can handle any additional loading in the future, that will be an option.

But currently, it’s the actual devices connected that gives you the load for your feeder calculations, for your voltage drop. So we have load, resistance, and wire length. So for the resistance, that’s based upon the X and the R values of the wire that you’ve chosen. So in your feeder settings, you have your conductor amps, and that’s basically going to choose what the size wire and the ampacities that are being used.

This transformer doesn’t seem to connect to anything, it doesn’t have a feeder, but if it was a switchboard, this is the feeder size that we’re doing that calculation based upon. For the Conductor Amps, if it’s set to size automatically, it is going to size it based upon the main/bus amps of the value, so we’re getting a 400-amp feeder. If you need to override that, you can change the value here to select a different size wire.

So if we wanted a (2) 4/0 AL or (2) 250kcmil, you would select it in the Conductor Amps. The X and the R values are set in the Wire Ampacities and Customization, so if you go to Customization, Wire Ampacities, we have a list of all of the ampacities in your project and then the corresponding X and the R values.

So if we go down here and look at the 100-amp ampacity, we have the X and the R value specified here. You also, obviously, specify your wire sizes, your ground sizes. Those are not directly correlated with these X and the Rs. The X and R are input values by you, so you need to go…if you want to change these, you have to go to NEC Table 9, look up what the values are, and type them in if you’re using a different size wire or if you want to use a different conduit type.

These are based upon just our default. There’s EMT for 2-inch conduit and less and PVC for 2-1/2-inch and above, so that’s what our X and R values are based upon. So if you want to use different conduit types, you would want to adjust these X and R values appropriately. It’s also…if you have multiple parallel runs, we do that division for you, so you just enter the X and the R for a single run of wire.

And then if there are multiple runs, we will do that division for you, so you don’t have to account for that. So if you ever need to adjust your X and the R, this is where you do it. So you can see, for the copper and the aluminum feeders, we have different values there. So that is where the resistance comes from.

There’s one more factor in the resistance, and that is the power factor of these circuits. And the power factor we pull from the Revit model, so when you have devices in Revit, all of your loads have a power factor associated with them. We assume that has been set correctly, and then we use that value.

So you just need to make sure that when you’re creating your families, when you’re looking at the families in addition to the load value that’s being set, make sure that the power factor is also set. As an example of that, I’ll go ahead and take a look at one of the receptacles here in this project. So if I take this receptacle and I open up the corresponding family and select the Connector, it has a load on it.

It also has a power factor. And so that power factor is what we are going to be using for the power factor in the voltage drop calculation, so make sure those values are set properly. So we’ve got the load that we’ve looked at, resistance, and then the final input to the calculation is the wire length.

In the panel as it command in the Feeder settings, we display the wire length that we are using. We can calculate that two different ways. We can calculate it as a straight between the two panels, so if you’re running it underground or something like that, you can just do a straight line

[inaudible] directly between them. So that’s the straight-line option. We also have the right angles, and that’s where we’re following the X and the Y axis of the building to kind of model, you know, following the walls around a little bit. For both of those, if there’s any change in elevation between the two panels, we will include that.

So if they’re on different floors, that will add just the difference in elevation to the length. If they’re on the same floor, we do not take into account the fact that you probably are going to go up the wall and then back down. So if you have particularly high ceilings, you might need to account for that manually. And the way you do that is with the fixed value. This is where you can specify the feeder length directly.

If our automatically-calculated values are not giving you a number that’s close enough to what you think it’s going to be, you can enter fixed values. That’s if you are, you know, going up and down the wall or you’re going down the corridor and back because you want to up the resistance for your fault calcs. Whatever reason you have, you can enter the fixed values, you always have the option to just tell us what the length of the feeder is.

There also is listed here the default, and that’s always going to be either right angles or straight-line. But that’s the default value for the project, and so that’s set in a number of different places. We kind of have a cascading level of defaults, to give you, you know, good default values, but also the option to override it at whatever level you want to.

If we go to Customization options, this is where the first level of defaults is set. So here, for feeder length, we have the option of whether the default is right angles or straight lines, so this is the default for the whole project. If you have a feeder and it’s not overridden anywhere, this is how it’s going to calculate that feeder length.

You can then override that for each panel. So if I select the Switchboard and I go to the Circuit settings, I have the feeder length calculation method, and so this is for all downstream feeders, basically everything connected to it. How should it calculate the lengths? Should it use that default value from the options, or should it be overridden with a specific straight-line or a right-angle calculation.

And then, if you go…so our default’s right angle. If I change this to straight line and we go to a downstream panel to MVP, you can see here this feeder length is based upon that straight line by default. And so this is the last place where you can set it for this specific panel. The feeder here is the feeder length based upon that default from the upstream panel, which might be based upon the options or overriding it for this specific feeder.

So that is how the feeder length is calculated. There are two other options that come into play for calculating a feeder length. One, we have the feeder makeup length, and this is an additional distance that is added to the end of every single feeder in the project.

This is to take into account, you know, making the actual connection to the panel, whatever extra wire you need. So that’s just a makeup value you can add in that’s global for the whole project. We don’t have any option for adjusting that for different panels or anything. And then, for right angles, we have a building angle, so if you’re doing right angles, we need to know what to treat as the two axes if…

Typically, it’s going to be kind of X and Y, kind of left and right and up and down on the drawing, but if you have something that’s tilted, the building that’s rotated, you can actually rotate what we consider those X and the Y values. Particularly if you have multiple wings of a building, you can set the building angles differently for different panels which are serving different areas.

So we do have that building angle. You can set the default for the whole project in the Options, and then, also at every place you can override it, you can set a specific value. So you can set it at the panel level for all the downstream panels, and then, when you’re overriding the value at a specific panel, you can set it for that specific panel.

So those are the options there. So those are the three basic inputs for voltage drop, the load, the resistance, and the wire length. We do the calculation using those values, and then we output everything to shared parameters.

So when you go to Schedule and Voltage Drop, we’re pushing all this information out as a shared parameter. I’m going to go ahead and run the calculation to fill all those values in. So these are standard Revit shared parameters. There’s nothing trick or special about them. They’re just a typical shared parameter.

That means that you can use your standard scheduling to display them either in the schedule or anywhere else you want to show them. Looking at this one here, we have two different columns, shared parameters, for the voltage drop.

We have one that has a warning and one that doesn’t, so I’m going to go ahead and put that other one in there because we’re, by default, including the one with a warning. But there’s also a voltage drop with no warnings on it, and so, if we include that, you’ll see that this feeder here that’s over the 3% that the NEC recommends and the column with the warnings, we have the exclamation point saying, “You should change this.”

And this one here, we don’t have those, so if you want to turn the warnings off, you just use the different field. And then you’ll get a value without those exclamations. Where we warn is set in Options. There is a…for branch circuits, it’s always 3%.

For feeders, we have two choices. You can either warn at 2% or you can warn at 3%. The NEC recommends checking and making sure they don’t exceed 3%. Certain energy codes and jurisdictions say, “No, you should really limit your feeders to 2%.” So if you’re in one of those jurisdictions using California Title 24 or anything like that, you can override it and say, “Hey, let me know if my feeders are over 2%.”

So you have the option for whichever one you need for your specific project. We do not resize your wires based upon the voltage drop. So we’ll tell you, “Hey, this is above 3%.” But then we leave it to you, as the engineer, to figure out what should be done about that.This is a good example in this project here because, at EP2, we’re above 3%.

But we probably don’t actually want to make the change at EP2 because the panel up the stream of it is at 2.9%. So we don’t really have a whole lot of play there. We’ve only got 0.1% before that one’s going to be over the 3%. So even if we make that wire huge, you know, we’re still probably going to exceed the 3%.

We’re better off going up one level PP2B, where, you know, we’ve got a 1% or 2% voltage drop in just that feeder because it happens to be particularly long, and to upsize that one a little bit. So rather than making the change here, you’d want to come and make the change on this feeder. And so if we use our Panel Edit, that’s where you can come change your conductor amps.

This is a 300-amp panel. I’m going to upsize it to a 400-amp feeder. And then we’ll run our calculation, and it’ll update those values. So we do not do any resizing automatically based upon voltage drop. That is something for you, as the engineer, to make the judgment and figure out how to best handle that situation.

So now we’re down to the appropriate values there. For feeders, we also have the choice of what we are going to do with voltage drop when we get to a transformer. Our software gives you three different options, depending on what you feel is the right thing to do, because there’s a bit of engineering judgment here as well.

You can choose to just ignore the transformer, where you’re not going to pre…you’re going to pretend there’s no voltage drop through it and just carry on as if nothing happened. You can choose to include the transformer. If you do that, typically, you’re going to exceed your 3% voltage drop because transformers actually tend to have pretty good voltage drop in them. So we defaulted to ignore.

I change it to include. And when I rerun our calculation value to these transformer points, we’re going to get significant voltage drop. The third option, which I’ll show you once this is done calculating, is you can reset your voltage drop to 0 at the transformer. That’s where you’re assuming that, at the taps, they’re going to account for the voltage drop and kind of increase that voltage in the field.

So you’re basically back at 0% voltage drop once you go through the transformer because of the adjustment at the taps. And so if you want to, you know, assume that that’s going to be what’s happening, you can reset the voltage drop. So here, you’ll see that, you know, we’ve got, at this TP1A, we’ve got 1% voltage drop, so it’s got voltage drop through there.

If we recalculate, we’re going to get significantly lower values. The PP2B and EP2 also have, you know, nearly 4% voltage drop, so that obviously, that calculation, if that’s how you’re thinking it’s going to happen. Is it going to work? If you reset to 0 at the transformer, you get much lower numbers.

[silence] So those are the three options that we have for transformers for voltage drop. Again, that’s set in Options, and you need to decide what is the appropriate calculation for your building. That’s voltage drop for feeders. We have a corresponding calculation for branch circuits.

It’s handled a little bit differently just because there’s a little bit different stuff going on. And so, in our software, we consider feeder as the wires between anything that has other things then connected to it, and branch circuits are all the stuff that don’t have anything connected to them just any other kind of terminal connections. So again, we have the voltage drop is based upon load, resistance, and wire length.

So the load is based upon what is connected to the circuit. If you run our circuit as a command and find a circuit with a load on it, the load that is listed here is the load that we are using, so this 2.46 amps, that is the load that we are using in our calculation. So that’s where taking that from the Revit model, assuming you have your devices modeled correctly with the appropriate loads, we pulled those values.

So it’s basically the connected load. There’s really no demand factors happening at the branch circuit level. The resistance is, again, based upon the wire size. So that same ampacity dialog box controls the X and the R values, and that’s being used for the resistance calculation.

And again, you have for your circuits the ability to override the conductor size to give you a different voltage drop, so this is currently 0.26%with the #12 wires. If I downsize it to #10, I get 0.16% voltage drop. So increase the wire size, the voltage drop goes down, so that is where the resistance is being set.

Same for the power factor. Again, that’s being pulled from the Revit model based upon the power factor in your connectors. I should note that, if you upsize your wires for a voltage drop, we do not currently upsize the ground.

The NEC has a rule that, if you upsize your conductors for voltage drop, you actually need to upsize the ground and it’s a corresponding percentage. We just size it as if it hasn’t been upsized, so you end up with some ground wires that could be too small if you don’t account for that. So you need to manually upsize your ground wires as well if you’re upsizing for voltage drop.

That is something that has been pointed out to us over the last year. And we are addressing that in the next release of the software, so it will upsize the grounds automatically in the next major release we put out. Currently, it doesn’t, so just make sure, if you are upsizing for voltage drop, upsize your ground to match. And then the last input for branch circuits, we have load, resistance, and the last one is the wire length.

And so the wire length for branch circuits is a bit more of an estimation than it is for feeders because feeders have a start point and an end point. You have a single wire running between them, a single load on that wire. That’s pretty straightforward. For branch circuits, you have the home run from the panel to the first device, and then you have the individual wires for all the other devices.

So the length is increasing, but your load is decreasing as you go. So to do a complete, accurate calculation, you have to know a lot of information about the specific wires, what’s connected to each wire, how much load there is, what the worst case is. So you know, we have to make some simplifications. Because the Revit models don’t always have complete information for us to do that, we don’t necessarily know what’s going to be connected, certainly not…

You know, we don’t have a whole lot of confidence that, you know, necessarily, what you’re even going to draw is what the contractor’s going to put in, depending on where you sit, and you know, in the design/build versus design/bid world world. So we do a calculation for the circuit length that’s just the average distance of the devices to the panels.

So we take all the devices, we figure out how far they are from the panel, we take the average. We say, “That’s pretty good for a circuit length for branch circuits.” It’s not going to be exactly correct, but for a voltage drop calculation, where you have all of your other, you know, assumptions that are being made, particularly for a large project with tons of branch circuits, it’ll at least get you in the ballpark if it’s correct or not.

So the calculation can, again, be done two ways, either as a straight line between the panel and all the devices or right angles. You also have the ability to set a fixed length, so if the calculated length we give you isn’t working out because you happen to know that it’s being run in a weird way or you’re going around the perimeter of the property or whatever’s going on, you can override it.

And this is probably the best…this is where you want to make those changes to account for the different lengths you might have. So if you really want to get an accurate calculation, you can kind of trace it out and figure out what the actual average length is, basically, for each device. And then enter that value here, and that’ll give you a fairly accurate voltage drop.

As an example of that, I’m going to jump over to AutoCAD for a moment if you’ll indulge me in using AutoCAD. This is our AutoCAD-based electrical BIM software, where I have…it’s a little easier to demonstrate this in AutoCAD because I’m much more familiar with how we do it in AutoCAD and we have slightly more sophisticated voltage drop calculations here.

So I’ve got the two example light fixture layouts. These are maybe some site lights with some long buns of wire. And so we’re connecting to this panel here. I set it up, so the panel’s the same distance to where we’re drawing the home run from. And so the question is, “All right, what’s the length for this circuit?”

And so you’ll see that, on this top one, you know, we’re going two different directions, so the average length is going to be shorter than this one here, where we’re actually looping around all of them in a single daisy chain. So that’s the kind of thing to do that, you know, to do a truly correct voltage drop, a very accurate one, you need to take these things into account.

Within Revit, we don’t have, necessarily, complete information to be able to do that all the time, so that’s why we use the average calculation. But here, you’ll see that, if we run our voltage drop calculation, if I select the Voltage Drop here, it’s 1.4% because it’s splitting the load. And if I calculate it here, where we’re going around, it’s 2.2%, so a significantly higher voltage drop.

So you know, how you are connecting the devices has an impact on voltage drop. With Revit, we don’t quite have all that information. So you know, if you have, you know, the large site plans and your site lighting wires, you at least want to take that into account. Make sure that the length that’s being used matches up with how it’s actually going to be wired in the field.

For the output, we again have a bunch of shared parameters that you can use to output these values. In this schedule and so on each panel, we have the maximum branch circuit voltage drop, which is this parameter being shown here.

So we look at all the branch circuits, and we just report the maximum value. And then we also have two parameters, one that shows you the circuit that that voltage drop corresponds to and also the wire size of that circuit. So we can show, you know, what the highest one is and what circuit it’s on. So you can then come in here and says, “Okay, these ones are above the 3%.

I need to go start resizing some wire on this panel to get down below that 3%. We also have a parameter that’s the total voltage drop. So that’s the feeder plus the branch circuit to give you your total. Whether you want to keep that less than 5%, and it looks like UF is higher than that. And again, we have parameters, so that you show those values without the warnings if you want those.

Those are shared parameters that are on each piece of equipment. We also have parameters that are on each circuit that will give you the voltage drop on that specific circuit, and that can be used in your panel schedule. So if I pull up a panel schedule here…where are my panel schedules?

There they are…and then pull LP2. So we have just a generic Revit panel schedule pulling values from Revit. We can modify that panel schedule template and add our voltage drop values to it. I’ll put in a new row and specify that we want to display the voltage drop as the value there, so all of our values are down here at the bottom.

We have a lot of options. We are looking for Voltage Drop. Ah, there it is. So there’s my voltage drop. Do a decent title, put in my borders, so that it looks kind of nice. And now, if I assign that template to this panel, now we have the voltage drop for each circuit listed.

So if you need to know the voltage drop at each specific circuit, probably the simplest way with Revit is to do it as part of your panel schedules, so you can see what your voltage drop is at each point there. All those values that we were showing in that voltage drop schedule could also be shown on your panel schedule, and that all can go up in the header. You can put a parameter that is something from electrical equipment, so we could show the voltage drop to this panel if we wanted to.

The feeder voltage drop, which is the total voltage drop to the panel, there’s the feeder without warnings, which is, you know, if you exceed 3%, we’re not going to tell you about it. We also have this Feeder 1 and Feeder 2. That’s the voltage drop on the wire just to the panel, so it ignores everything else.

So it’s the voltage drop of just the feeder, so if you want to see what the contribution of just that panel is, you can. The feeder, again, is the cumulative voltage drop basically from the utility or the main connection, so we can add that as a value to our panel schedule as well just using, again, standard Revit panel schedule, a functionality, and shared parameters to do that.

Assign that template to this, and so the voltage drop to the panel is 0.52%. The final place that we can display the voltage drop is in the one line diagram. So I’m going to export all this information out to our one line diagram, and then, when we do our one line diagram, one of the labels that we have available to us is the Voltage Drop.

So if you want to display your voltage drop on a feeder at any point than the one line, you can do that. This will take just a moment to do the export, and then we’ll see that on the one line in AutoCAD. Open up my Revit one line project, and we’ll generate this one line.

And none of these lines panels include the voltage drop by default, so we’ll use the label Insert/Modify command.

Select this panel, and the Voltage Drop is listed here. It includes both the total to the panel and then the feeder, the voltage drop on just that feeder. I don’t think this…yeah, this panel doesn’t have any load on it, so there’s no voltage drop on that feeder. That’s why the 0% there. We can do that Total Voltage Drop label there. So you have the ability to do the voltage drop for your panels on the one line diagram if you want to document it there.

So you have a number of different ways that you can document it in a schedule…on a panel schedule in the one line diagram. That is voltage drop for you in Revit. Are there any questions on that? There we go. “I was curious. If you change parameters on a circuit, then move the circuit to a different breaker, do the changes follow?”

And that actually is a really good question. I had not thought through the implications of that. So in Revit, with our software, you use the Circuit Edit to, you know, set all of these values here, and I believe Revit has some ability to move circuits around.

And I believe the question is, okay, if we do the moving…you know, if we take this circuit here and move it up to number 1 to that space…will this new conductor size follow? I don’t actually know because I’ve never tried it before. Let’s see if we can do that and see what happens. And now you’re stretching my ability to actually use Revit.

Let’s see if I can find the command. Where would I put that if I was in charge of writing Revit? If anyone knows off-hand…oh, there they are. Let’s see. “Move the Selected Circuit to the Slot Above.”

All right, let’s see. If we move this circuit up, so we push the circuit up. We’ve swapped 3 and 1. I’m hoping that it pulls our parameters along. And yes, it does, which is nice, so it’s actually taking that same circuit, just renumbering it. And so our parameters are connect properly, and everything follows along.

So I see the answer in the chat box is, “Yay!” I also am excited that that works because I did not want to have to write code to get around that. But they implemented that in a nice way, so that our parameters follow along nicely So if you do use these commands to move your circuits around, the settings in Design Master will follow along with it.

“Can you show me how to get to the panel again?” The panel editing, yes. So on the DM Electrical tab, that’s where all the Design Master commands live, and so Panel Edit is where you’re going to make all your changes to your panel’s feeder sizes and things like that.

So you run Panel Edit, and you can select your panel here and then make your changes on the right-hand side. The Panel Edit is slightly aware of where you are looking at a schedule, it’s going to, by default, pull that panel up when you press Panel Edit. So we’re looking at LP2.

It pulls up LP2 as the active one. And the same way if you are looking at a Plan view and you have something selected. When you run Panel Edit, we will have that one be selected. Our software does require you to kind of change your workflow where changes previously you might have made in the properties or in your schedule, you need to come into our panel on our Circuit Edit.

So we did add that, you know, awareness to where you are, so that…more for, say, if you want to change a circuit breaker size or a wire size, that when you run Circuit Edit, we actually are paying attention to where you are and make it relatively easy to find that circuit to change the value. So with that, I think we will be done for the day, so thank you, everyone, for attending.

Appreciate you coming out for the training. If you have questions or need help in the future, feel free to contact us by email or phone. We’ll see you in a couple of weeks, again, for more one line diagram and more Revit-related training. Thank you, and hope everyone has a good day.

Related Articles

Filter by
Post Page
Electrical RT Training Electrical RT General Electrical Electrical Training One-Line Diagram Training One-Line Diagrams RT One-Line Diagrams Electrical Customization Electrical RT Customization
Sort by

Leave a Reply


Electrical for Revit

Electrical for AutoCAD

HVAC for AutoCAD

Photometrics for AutoCAD


Contact Support

Install & Update Instructions

Weekly Training

Knowledge Base

User Manuals


Release Notes


Wish List

Remote Support



Customer Stories





Electrical for Revit

Electrical for AutoCAD

HVAC for AutoCAD

Photometrics for AutoCAD

Free Trial

Electrical for Revit

Electrical for AutoCAD

HVAC for AutoCAD

Photometrics for AutoCAD

©2001-2018, Design Master Software, Inc