Inside the System

Autodesk Green Building Studio—the Power Behind Autodesk Energy Analysis Features

Anyone interested in learning about Green Building Studio and how it is used for energy analysis by Revit my be interested in this post on the Building Performance Analysis blog.

Using Autodesk Vasari for Building Performance Analysis

There is an interesting and helpful blog post on the BPA Blog about how to use Autodesk Vasari for numerous types of Building Performance Analysis (BPA) including Energy Analysis, Solar Radiation, Sun Studies, Wind Tunnel (exterior airflow) and a few others. Check it out.

 

Advanced Energy Analysis with Green Building Studio DOE2 and EnergyPlus support

 

For those of you interested in Building Performance Analysis and working with eQuest or Energy Plus there is a very interesting blog post on the BPA Blog about using gbXML output from Autodesk^® Revit^®, Vasari^® or other authoring software and converting it to DOE2 or Energy Plus input files. By using Autodesk Green Building Studio (GBS) in that workflow you can use GBS to fill in a lot of the analysis input data that could take hours to complete by hand. You can find the blog post here.

 

 

Simple MEP System Traversal using the Revit API

For those of you with Revit MEP API development aspirations (or interest) - you may be interested in Jeremy's post on The Building Coder about using the Revit API to travers an MEP system. Take a look and tell us what you think.

 

Hey Autodesk! Revit's definition of Relative Roughness is upside down!

There have been a few inquiries lately regarding Revit's definition of Relative Roughness as documented here. 

Some eagle eyed users have noticed that this is the reciprocal of the defnition one may find commonly documented, such as here.

As the relative roughness factors into the computation of the friction factor, and the friction factor is used in the computation of pressure loss, the concern of these users has been that the computation of pressure loss may therefore be incorrect.

Fortunately, the developers had taken this into account in the computation of the friction factor.  For turbulent flow in piping, Revit's computation of the friction factor is based on the Colebrook equation as follows:

In both cases, computing f provides the same result.  So, even though Revit reports the friction factor as the reciprocal of how it may be defined elsewhere, you should find that the end result of the pressure loss on a given pipe segment is as expected.

We understand the concern of seeing large values of Relative Roughness  in the properties palette and pressure loss report, and we have logged this for consideration in future development.

How to export gbXML for only some spaces

"In my 3^rd party software tool, using gbXML, I can only import 50 spaces at once.  I wonder if it is possible to select the number of space or even the spaces itself for export process?"

Likely, the easiest way to deal with this is to (ab)use the Phases feature of Revit.  When you export gbXML, you have the option of which phase you are analyzing.  Since spaces exist only in the phase they are created, you can use this to control which spaces are exported.

For example, starting with a floor plan view configured for the Phase New Construction, you can create new phases and spaces for analysis. 

From the Manage tab of the ribbon, Phasing panel, click Phases, then define however many new phases you will need… in this case, I am just creating two new phases.

Then, duplicate the necessary view(s) to create new views for each phase.

Then, rename the views appropriately, and set the Phase property of the view accordingly.

Back in the original view, select the Spaces you want for a particular phase, and select Copy to Clipboard.

Next, open the appropriate phase specific view, and select Paste > Aligned to Same Place

Now, when you export to gbXML, you can select the specific phase (and thus spaces) you want to analyze.

Computing Per Phase Demand Loads

An update in Revit 2013 Update Release 1 makes this functionality possible... before attempting to replicate this on your own, please download and install the update.

For Revit MEP 2013, you can download here.

For Revit 2013 (part of the Design Suite), you can download here.

Near the end of this post, I suggested that it would be possible to compute the per-phase demand load by computing the ratio of Total Estimated Demand Current / Total Connected Current, then multiplying by each phase's current.  Lets take a look at the built-in parameters and how they are used to compute the Connected Current in two different ways, then how to find the demand ratio for each method, and then finally, how to apply that ratio to the per-phase currents.

 

(click image to enlarge)

In the attached image, if you based your panel sizing only on the Total Est Demand, you may resolve that a 200A panel and feeder is sufficient.  However, by seeing the load per phase, you may determine that the rating may need to be at least 230A.  Of course, balancing your panel will have an effect on ther per-phase loads. 

Panel Totals

Total Connected

This is a built-in parameter.

This is the total sum of all connected loads (VA)

 

Total Estimated Demand

This is a built-in parameter.

This is the sum of all loads after demand factors have been applied (VA)

 

Total Connected Current

This is a built-in parameter.

In the case of a 3-phase panel, this is computed as:

Total Connected / (Line-to-Line Voltage * sqrt(3))

 

Average 3-ph Connected Current

This is a Calculated Value in the schedule.

(Current Phase A + Current Phase B + Current Phase C) / 3

 

Total Estimated Demand Current

This is a built-in parameter.

In the case of a 3-phase panel, this is computed internally as:

Total Estimated Demand / (Line-to-Line Voltage * sqrt(3))

 

Demand Ratios

Demand To Total Ratio

This is a Calculated Value in the schedule.

Total Estimated Demand Current / Total Connected Current

 

Demand To Average Ratio

This is a Calculated Value in the schedule.

Total Estimated Demand Current / ((Current Phase A + Current Phase B + Current Phase C) / (3)) 

Per Phase Demand Loads

These are computed for each phase, replacing X with A, B, and C.

Demand phase load based on Demand To Total Ratio

These are Calculated Values in the schedule.

Current Phase X * (Total Estimated Demand Current / Total Connected Current)

 

Demand phase load based on Demand To Average Ratio

These are Calculated Values in the schedule.

Current Phase X * (Total Estimated Demand Current / ((Current Phase A + Current Phase B + Current Phase C) / (3)))

 

How does Revit Compute Per Phase Currents?

Let’s take a look at how Revit computes the current on each phase in various scenarios, and see how this compares to a current meter.

277V/1ph/10A load on a 480/277v 3-phase panel

The load for the circuit in VA shows 2770 VA (10A * 277V).  Additionally, the Total Load shows 2770 VA, and the Total Amps as 10A.  The current on the circuit wire is 10A, and the current on the ‘bus’, as indicated by the Total Amps is also 10A.. as is the current on the feeder.  This is shown diagramatically below.

In this case, the voltage phase-to-neutral on that bus/phase is 277V, and thus, the current going through it is 10A... since the current isn't divided onto multiple paths, and since the voltage is constant, the current is constant.

When the phase column values are shown as Current, the 10A load is shown for the circuit.  Nothing else has changed, so the Total Load is still 2770 VA, and the Total Amps is 10A. 

One anomaly you can see is that the Total Conn: shows 3.33 A.  Why is this?  This value is simply taking the Total Conn Load (2770 VA) and dividing by the 3-phase voltage (480*sqrt(3)), which is quite common in engineering practice.  2770 VA / 480*sqrt(3) = 3.33A.  This is one of the reasons that the per-phase totals show the way they do, it is sort of a reality check.  In reality, the load on the bus is 10A, not 3.3A.  If one were dealing with 10x larger numbers, say 100A vs. 33.3A, one would need to select a panel with busses rated at 100A… no less (i.e., 60A).  Otherwise, the bus would be under-rated.  Additionally, the wires would need to be sized for 100A, not something smaller, like 40A as might be interpreted if one only looked at the Total Load / 3-phase voltage.  Typically, of course, one is dealing with balanced loads, where the disparity between the current per phase and the total current are closer to the same.

480V/1ph/10A load on a 480/277v 3-phase panel

The load for each phase of the circuit shows 2400 VA, for a total of 4800 VA. (10A * 480V = 4800 VA).  Additionally, the Total Load shows 2400 VA per phase, with a Total Conn Load of 4800 VA.  In this case, again, the total amps on each phase is 10A, even though the load has shifted from 2770 VA on one phase to 2400 VA on two phases.  Again, the current on the circuit wire is 10A, and the current on the ‘bus’, as indicated by the Total Amps is also 10A.  The only thing that can change the current is if the voltage changes.  In this case, the voltage phase-to-phase on the busses is 480, and thus, the current going through it is 10A.

When the phase column values are shown as Current, the 10A load is shown for the circuit as expected.  Nothing else has changed, so the Total Load is still 4800 VA, and the Total Amps is 10A per phase.

The Total Conn: is computed as 4800 VA / 480*sqrt(3) V = 5.77A.  Again, the reality check of the per-phase loads confirms that in the case the loads were 10x, we need to size the bus of the panel and feeder wires based on the per-phase load of 100A, not a Total Conn: load of 57.7A.

480V/3ph/10A load on a 480/277v 3-phase panel

For a 3-phase balanced load, the load on all three phases are the same.  In this case, 2770 VA per phase.  There is still 10A on each wire, and 10A on each bus/phase.  Further, the total connected load is 10A.

Showing the per-phase loads in Current (amps) reveals the same results.  All phases of the circuit have 10A.

The Total Conn: is computed 8310 VA / 480*sqrt(3) V = 10.00A.  In this balanced case, it doesn’t really matter if we refer to the per-phase load or the total connected load.  Since in the balanced Current Phase A = Current Phase B = Current Phase C = Total Current.

Unbalanced loads on a 480/277v 3-phase panel

In this scenario, I am starting with the phase column values showing in units of Amps.  This is where things may not ‘add-up’, but there is a very logical explanation derived from the discussion above, and he theory behind 3-phase power.

In this case, we have the same three loads, but in a different configuration, with them all connected at the same time:

• Circuit 2,4,6: 480V/3ph/10A load
• Circuit 8: 277V/1ph/10A load
• Circuit 10,12:  480V/1ph/10A load

As you can see, Phase B and Phase C each show two loads at 10A each…. However, the phase B and C Total Aps is 18.66A!  How can this be?  Let’s take a look at the per-phase loads in units of VA:

Here, we can more clearly see that the loads aren’t simply 10A, but rather are made up of different types of loads at varying voltages.  As such, the math behind it is a bit more complicated than simply dividing by the phase-to-neutral or 3-phase-to-phase voltages. 

If we look simply at the 3-phase load total of 15880 VA / 480*sqrt(3) we get the reported 19.10A, consistent with everything sated so far.

If we were to remove 370VA from Phase A, we would have Phase A = Phase B = Phase C = 5170 VA.  Then, take the total load divided by the voltage: 5170 VA * 3 / 480*sqrt(3) = 18.66A.  Not-so-coincidentally, this is the current we see on Phase B and Phase C in the schedule, and is what we would measure on the feeders to the panel in an installation.

Further, if we took this difference of 370VA, and divided it by the unbalanced load (which in this cases is at 277V/1p), we would have 370VA / 277A = 1.34A.  Not coincidentally, if we add this 1.34A to the balanced three-phase-current of 18.66A on Phase A, we get 20.00A on Phase A… again, the current one would measure on the Phase A feeder.

Since the load is not balanced, naturally, the current on Phase A will be slightly larger than the reported Total Conn current across all three phases… similarly, Phase B and C are slightly smaller. 

In a more realistic scenario, of course, there would be de-rating on the loads, so the value you use to size your bus bars and feeders is not solely dependent on the Total Connected current.  For most practical purposes, where systems are almost balanced, one can use the Total Est. Demand current as the basis of sizing the panel bus bars and feeders.  However, in the case you have an unbalanced system  Revit provides the detail on a per-phase basis to show you how the connected load and current on the phases tabulate.  If you only looked at the Total Est Demand, there are scenarios where you’d get into trouble, and therefore, in some cases, the additional detail is necessary.

In a real installation, you are un-likely to really see the design load or current, or even the voltage.  There will be voltage drop, which will vary per phase depending on the loading, and of course, the loads per phase will vary depending on the type of load and their utilization.

However, all the same theory presented here will apply.

Here we have the readout from a 3-phase current meter (the numbers in the black circles correspond to numbers in the next image).  As you can see on the top row, right column, the voltage is 93.697, though, due to unbalanced loads and other variations, the voltage varies on each phase.  However, if you average the per-phase voltages, you get precisely this value. (In Revit, we don’t have the ability to account for voltage fluctuations on per phase… but, typically, designers don’t do this anyway.)

As you can see in the green VA row, the total VA is a sum of the VA from the three phases.

If you take the VA, and divide by the voltage in the same column, you get the current shown in that column (e.g., for phase A: 89.231 VA / 95.626 V = 0.9331 A).

If you average the values in the blue row (average of 0.9331, 0.8483, and 0.9884) you get the reported 3-phase current value:  0.9233 A.  This is almost the same value as taking the total VA, and divide by the voltage * 3 (this is showing the line-to-neutral average, and thus, we need an extra sqrt(3)), you will get very close to the reported 3-phase current:

260.29 VA / (93.697 V * sqrt(3) * sqrt(3) ) = 0.9260 A (a difference of less than 0.3%)

So, how do we test this against what Revit reports as the per-phase current and the total current?  Quite simply, create a device with an unbalanced load as shown above, and connect it to a 162.29 / 93.70 V/3Ph panel (93.70 * sqrt(3) = 162.29), and check out the panel schedule.

The top image is from the Revit Panel Schedule… the bottom portion is computed in Excel to show step by step where the values come from.  The values here have been rounded since Revit only stores two decimal places on power (VA) and voltage (V), so the results will vary a bit from the meter photo.

Let’s see where these numbers come from.

(1) shows the original entered loads.  The spreadsheet shows how the load is divided across each phase and what voltage.  There are three parts to this load, a 3-phase L-L part, a 1-phase L-L part (phases A and C), and a 1-phase L-L part (phase C).  The current from each load part is tabulated in the column marked (2).  The currents are then shown on the appropriate phases, and summed (3).

(4) shows the total VA load divided by the 3-phase voltage.  (5) shows the average across all phases.  These numbers are slightly different because a portion of the load is at 1-phase L-L.   Since the voltage across each phase in the meter photo varies, the results aren’t exactly the same.

However, if the loads were balanced, and the voltage was consistent, which is the ideal design scenario, the following truism holds.

Current Phase A = Current Phase B = Current Phase C = Total Current

Or, alternatively for the unbalanced condition:

Total Current = Average (Current Phase A , Current Phase B , Current Phase C)

Revit shows the ‘Total Conn’ as VA divided by the 3-phase voltage, since this is how most engineers compute simplify the calculation.  It is also very easy to compute the Average across the phases.  And, in the event you need it, you also have the connected current per phase.

Knowing what the demand per phase (after demand factor values are applied) is a bit more complex because internally Revit isn’t keeping track of the load type per phase (and most engineers don’t either).  However, you could estimate the values by computing the ratio between the Total Est Demand and the Total Conn (or the Average 3-ph Conn Cur).. then, multiply this ratio by the per-phase current.

If you happen to have any photos of such a meter at 480/277 (instead of my example at 162/94), I would be happy to update this example.

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.