Here is what I wrote there:
Revit, in some cases, cannot properly calculate voltage drop due to how it automatically assigns wire size. Here is a brief description on this issue from the user guide of the Electrical Productivity Pack from Cad Technology Center that I helped to create.
- The method that Revit uses to assign wire sizes is to first select the rating (OPD) for the circuit, then Revit hard links the wire size to the rating of the circuit. This is unworkable, because it does not allow for adjustments for wire sizes independent from the OPD rating. It is not possible to increase wire size for voltage drop without changing the OPD rating, nor is it possible to increase the OPD rating for motor starting without also increasing the wire size.
- Revit is backwards, the wire size should be selected based on the load first. Then the OPD should be selected for correct ampacity. NEC (National Electric Code in USA) may allow next standard OPD size to be used without increasing the wire size. However, this is not always permitted; i.e. a 53 A load may use conductors rated at 55 A and OPD at 60 A; but a 58 A load would require the next larger size conductor and perhaps the same 60 A OPD. Revit does not allow this flexibility.
I am glad we can edit circuit path in 2018, this is a big step in solving the issue of voltage drop and wire size. I talk about this in my 'What's New in Revit 2018" article for AECbytes.
In addition to what I had posted above... wire sizes are “hard coded” to the ampacity of the upstream OPD. Revit calls this “Rating”. Revit will automatically select the wire size defined in Manage > MEP Settings > Electrical Settings > Wire Sizes (see image below); ampacity equal to or the next size larger than the “Rating” (OPD) of the circuit.
Until this limitation is addressed, we are forced to totally ignore Revit's wire sizes and use a Key Schedule to manually document this in the electrical equipment schedule. It is not possible to partially use this information in schedules as we cannot override this information.
I previously touched on this limitation in my AECbytes article: What's New in Revit 2018.
I do not believe this is just a problem in the United States. But even if it is, we need to be able to control this to create code compliant electrical designs... like how stairs have two fundamental ways in which Revit can calculate the tread and riser height; 1) Max. riser & Min. Tread and 2) Stair Calculator dialog (I have never used the latter method).
Fun stuff! Thanks for reading.