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Change how wire is auto-sized or allow override for engineering judgement

Change how wire is auto-sized or allow override for engineering judgement

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; http://bit.ly/2rEXLFq 

7 Comments
James_Kerfoot
Enthusiast

Completely agree! I would opt for the ability to override the built-in wire size using an instance-based checkbox in the circuit's properties. There are just too many situations where there can't be a hard link between a certain breaker size and a certain wire size... Motors w/o a VFD, Voltage Drop (since Revit does it all wrong), Neher-Mcgrath heat calcs... etc. We need more freedom on this, or we will never use it. We don't use Revit's wire sizing now, and I don't see that EVER changing unless Revit changes.

jkerfoot
Community Visitor

This is extremely necessary. As an Engineer Revit's default wire sizing is useless unless we have the ability to override for the many variations that existing underground heat calculations (https://en.wikipedia.org/wiki/Neher%E2%80%93McGrath_method) and motor circuit sizing prevent us from using this feature now.

adam.jw
Advocate

Agreed 1000%

magno.vinicius.silva
Contributor

Mandatory for any MEP platform.

Especially when we are talking about complex engineering solutions like hospitals and other industries.

BradJ3Y65
Contributor

I agree with this as well. Another use case is with HVAC equipment. The conductor sizing and OCPD are not related. It is possible to have a 10AWG wire on a 50A breaker. There needs to be an opportunity to disassociate the wire and breaker sizing if the load type is specific. Specifically, 440.4(B) would allow a unit with an MCA of 31A and a MOP of 50A to have 10 AWG on a 50A Breaker @ 75C. No matter what logic is applied, this would not be computed.

In New Zealand the wire size and rating also depend on the containment, including if the cable runs underground or through insulated construction.

starry.cui
Autodesk

Greetings,

 

as an Autodesk Revit Experience Designer focusing on electrical features, we're seeking your input to better understand user needs and potential improvements regarding electrical conductors.  We believe that your insights can greatly contribute to our ongoing efforts. We kindly invite you to participate in our brief 2-5 minute survey. Thank you for your time.

 

Many thanks,

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