Inside the System

Importing gbXML results from CHAVC

If you are attempting to import the calculated results of Space and Zone Heating and Cooloing Load using gbXML, you are likely to run into a slight problem.  The result of which is that the values that are imported are about 9% of what the expected values should be.  The reason is due to how CHVAC is writing out the gbXML file; it boils down to an improper specification for the unit

According to the standard published at: http://www.gbxml.org/currentschema.php

Valid values for the load units are: BtuPerHour and Watt. 

(specifically:  )
<xsd:simpleType name="loadUnitEnum">
<xsd:restriction base="xsd:NMTOKEN">
  <xsd:enumeration value="BtuPerHour" />
  <xsd:enumeration value="Watt" />
  </xsd:restriction>
</xsd:simpleType>

However, in the gbXML file, what you may find is unit="Watts" insetead of unit="Watt"

Using a simple editor, like Notepad, you can do a search and replace for the unit="Watts" to correct the specification to Watt.  After doing so, you should get the expected results when importing into Revit.

If you are a user of CHVAC, and this issue is important to you, please contact Elite Software directly.

Why do I get different Voltage Drop results with Revit compared to the NEC?

The NEC states table 9 was compiled using the Neher–McGrath ac-resistance calculation method, and the values presented are both reliable and conservative.  The computation is not trivial, and is highly dependent on determining the conductor and insulating materials, and the ambient conditions.  At the bottom of the table, it lists some of the assumptions. 

Further analysis of the data between NEC and Revit shows that Revit is typically more conservative than the NEC.  The table below shows that for a Power Factor of 1.0, Revit is slightly less conservative (that is, Revit will report slightly less drop than using data from the NEC).  However, for power factors from slightly less than 1.0 down to 0.60, Revit will report greater drop (therefore, being even more conservative)... the delta between Revit and the NEC increases as the wire size increases, and as the power factor decreases.

As an example, in the worst case scenario for a 480V three phase circuit, 1000 MCM cable with 0.60 Power Factor, a maximum allowed feeder drop of 2% computed using the R value from Revit would result in a 1.71% drop compared to using the R value from the NEC.  (Our R’s are bigger).  Unless you know exactly the current in a feeder, and know exactly the ambient conditions, the best you can do is use an estimate, inheriting the assumptions such as those provided by the NEC or Revit.  Either way, the result is conservative.

To compute Voltage Drop, why does Revit use the formula VD=(L*R*I)/1000?

... "I’m used to using Table 9 in the NEC for R, and using the formula VD=F*(L*R*I)/1000, where F is 1.73 for three phase circuits, and 2 is for single phase circuits."

Referring to the question in the prior post, you can see that the F factor is already accounted for in the tabular data that Revit uses.  Thus, it is not necessary to multiply by such a factor.

Why does the voltage drop for single phase (single-pole) circuits in Revit seem to be about half what I’m calculating using the NEC R factor, and the formula 2*(L*R*I)/1000?

The 2 in the formula is used to compute Line-to-Line voltage drop.  For a single pole circuit, typically, you account for the Line-to-Neutral voltage drop, which is half of the Line-to-Line voltage drop (and thus, no 2 factor is necessary).  As stated in the documentation, the voltage drop for each circuit is reported as line-to-line for 2-pole (single phase) and 3-pole (three phase) circuits. The voltage drop is reported as line-to-neutral for 1-pole circuits.

Why does the Revit Wire Impedance Factor table have separate sections for Three Phase and Single Phase?

The table published in the documentation provides two sections simply a matter of convenience.  The data in the Single Phase section is the same as that in the Three Phase section, multiplied by the factor 2 / SQRT(3).  As an example, if you look in the first row for #14 in Steel Conduit at a power factor of 60, in the Three Phase section, the value is 3.322 and in the Single Phase section, the value is 3.836.  Multiplying the Three Phase value by the factor yields the value in the Single Phase section: 3.322 * 2 / SQRT(3) = 3.836.

Upgrade Your Project from 2012 and Get Loopy!

Revit 2013 introduces a new Arrow Style for Loops, though it may not be immediately obvious how to use it in an upgraded project.

1. On the Manage tab > Settings panel > Additional Settings > select Arrowheads
2. Select any of the types, and click Duplicate...
3. Name the duplicate Loop, or something suitable, and then click OK twice.
4. Now, select a tag with a leader in a view (such as the Pipe Size Tag).
5. On the Properties Palette, click Edit Type.
6. Click Duplicate...
7. Name the new type 'Pipe Size Tag with Loop', and click OK.
8. Set the Leader Arrowhead to Loop (or whatever you named it in step 3.)
9. Click OK.

Your pipe tag now has a loop leader.

How to Identify Uncircuited Connections

You can quickly find uncircuited elements using System Browser.  Under the Unassigned > Electrical section, you can find connections that are not circuited.

Under the Electrical section, you can find connections that are circuited, but not connected to a panel... these will have the circuit indication of <unnamed>.

Connections that are circuited will show the circuit number.

You can select any connection in System Browser, right click, and select Show to 'zoom' to the associated object in the model.

If you want to visually identify this in the model canvas as you are working more dynamically, you can use View Filters to color these conditions.

On the ribbon View tab, Graphics panel, click Filters.

I've defined three filters for three conditions as shown below.

Uncircuited

Circuited

No Panel

With these filters defined, you can now apply them to a view as shown below.

The resulting view will look something like that shown below.

The System Browser can reliably be used to find uncircuited connections, however, since only the primary electrical connector in the familly is tracked by the Panel / Circuit properties of the instance, the View Filter may not accurately show the circuited color state of all connectors for elements with more than one connector. 

Happy circuiting!

Using the Spot Elevation Tool to Tag Bottom and Top of Pipe in Revit

You can use the spot elevation tool to tag the Top Of Pipe (TOP) and Bottom Of Pipe (BOP).  With a little configuration of the types, you can annotate the top and bottom of the pipe elevations with an appropriate prefix.

 

Adding Dimensions to Pipes in 3D Views in Revit

You can add dimensions to pipes in 3D views.  By creating reference planes for your pipe annotations and adjusting your workplane, you can place the dimension lines adjacent to the pipes.  By constraining the reference planes to the pipes, you can keep the dimensions aligned with the pipes, even if the pipes move.

  

Default Load Values

I was recently asked where the default values for lighting loads and power loads come from.  These default values are found on Spaces when you inspect them using the Heating and Cooling Loads tool, and when you directly inspect the Space in the model. 

The short answer is that they come from the settings in Manage > MEP Settings > Building/Space Type Settings.

By way of an example, if the Space has its Space Type set to <Building>:

The Building Type is Specified in H/C loads dialog (or in Manage > Project Information > Energy Settings)

The Building Type has a set of properties in the Building/Space Type Settings window.

These settings are referenced when you are running a load, or checking things such as the Electrical Loads settings on a Space.

To help visualize, this eye chart may be helpful.

Constraining a Family to Disallow 'Invalid' Connector Sizes

Recently, I was asked how one might constrain a connector width in an instance parameter to be less than the width of solid it is hosted on.  In the specific case, there are two solids, one representing the main body of the equipment, and the duct 'collar' on which the connector is hosted.  The duct size must match the collar, and the collar must stay smaller than the equipment.

The formula below to control the collar size works just fine... however, when you select the family instance in the project, you can use the blue numerical control to set the connector width to something larger than the geometry... as you can see from the formulae, there is are no constraints on the actual duct connector sizes.

The problem is one of validation, not constraints/dimensions.  As you can see in the revised parameters below, there are a couple of checks to make sure the duct size stays smaller than the section size.  If the duct is modified to be bigger than the body, we force the collar length to 0, which Revit will not allow, and a error to the user will force them to undo the invalid setting.

How would you approach this problem?