Electrical Current Calculations

[shockley.jd]

I have not seen this addressed anywhere else, but have noticed that Revit's electrical current calculations are incorrect and have been since at least 2013. I have made custom panel schedules with my own calculations to make it show up right, but since no-one else seems to have noticed and it hasn't been fixed in this many versions, I'm starting to wonder if I just don't understand the parameters or am missing a setting somewhere.

I created a schedule based on a dummy project (Revit 2020) with 120V circuits that I set up as an example, image attached. The first column is the Apparent Load Phase A parameter for each circuit, second is Apparent Current Phase A, straight from Revit. The third column is a calculated parameter with the formula: Apparent Load Phase A / 120V, which gives the actual current for the load. Notice that five of them match Revit's number, but the rest are off by varying amounts. My thought was that maybe Revit is using another number for the voltage, like 115V or 110V, so I added a fourth column that backs out what voltage Revit would be using to come up with the current it's calculating, using the formula: Apparent Load Phase A / Apparent Current Phase A. This gives varying voltages that seem to have some sort of pattern, but I can't figure out anything.

Has anyone noticed this or figured out what the issue is?

[David_Robison]

Current calculations are one place where Revit is surprisingly sophisticated. Those numbers might actually be right.

Take a look at this article. See if it explains the calculations you are seeing for the loads you have defined.

https://inside-the-system.typepad.com/my_weblog/2012/07/how-does-revit-compute-per-phase-currents.ht...

If it doesn't, post your sample project. There might be settings that explain the numbers. It also might be an error in Revit. Seeing the project will help sort that out.

[timlove]

I have not had this issue.  It could be something in the families that you are using or in some of the electrical setting.  We did spend a considerable amount of time to configure these to report everything correctly for our engineers.  I would need to see more information to help troubleshoot, and I think most of the community here would be in the same situation.

[s.borello]

Let's see that sample project!

[stever66]

Although that link went into a lot of detail, I didn’t even see power factor mentioned.  If some of your loads have a power factor other than 1, that will also affect the calculations.  So the calculations are even more complex than what that article explains.

So short answer to the original question- Although engineers normally just approximate the total loads by adding all the currents (Or VA’s) for each phase, that’s really just an approximation (but usually a pretty good one).   Revit is probably doing a more exact calculation. 

if you are getting a total of something like 100, and Revit is getting 95 or so, I wouldn’t worry about it.

But if the numbers are way off, something is probably wrong.

[shockley.jd]

@David_Robison thanks for the article, it helps but I had to do some more thinking on this to figure it out. I attached a panel schedule (Panel B, I'll get to Panel D later) that more clearly shows what my question was. I was able to come up with the Revit numbers for this panel using the calculations below. The numbers do appear more accurate than just dividing each phase kVA by the phase-N voltage, which is what our Excel schedules do. The currents should be calculated separately for balanced load and for unbalanced loads connected line-line. The L-L single-phase currents are 30-deg out of phase with the L-N voltage, so these would be kVA/(L-N voltage * cos 30). Let me know if you agree with these calcs.

In the attached schedule for Panel B, there are 3 single-phase loads of 10A each connected line-line. These show up as 10*208/2 = 1.04kVA on each phase of the schedule, as expected. Phase C only has one load of 1.04kVA, which is balanced by the same loads on phases A & B, so the phase C current is just 1.04/.12 = 8.7A, rounded to 9A. This is the balanced load for the panel. Phases A & B have balanced and unbalanced load. The unbalanced load is connected L-L. The current on phase B is 8.7A (balanced load) + 1.04/(.12*cos 30) (unbalanced load) = 18.67A, rounded to 19A. Phase A is similar, as 8.7A (balanced load) + 2*1.04/(.12*cos 30) (unbalanced load) = 20A.

That said, I also attached a schedule for Panel D, which has an additional 1.04kVA connected L-N on phase C, so that phases B & C are balanced load only at 2.08/.12 = 17.33A. Revit now seems to treat the unbalanced 1.04kVA load on Phase A as L-N: 2.08/.12 (balanced load) + 1.04/.12 (unbalanced load) = 26A, where I would expect it to be 2.08/.12 (balanced load) + 1.04/(.12 * cos 30) (unbalanced load) = 27.3A. Not sure I follow this, as the unbalanced load is clearly connected L-L.

@stever66 since we're using kVA, power factor doesn't come into play. We set all of our families to a power factor of 1.0 since we always work in kVA.