Nate Holt's Blog

January 30, 2011

Circuit Builder – Creating an alternate electrical standards setting – AutoCAD Electrical

Filed under: Electrical, Tutorials — nateholt @ 12:11 am

AutoCAD Electrical ships with Circuit Builder’s “electrical standards” for wire sizing and breaker/fuse/switch ratings set to parallel some of those outlined in the National Electrical Code. Here is how to alter the settings or create a selectable, alternate electrical standards selection.

Circuit Builder’s “Wire Size Lookup” dialog is a tool that can link into an “electrical standard” and guide you as you select motor size, motor power wire sizing, and appropriate ratings of the main disconnecting means – whether some type of breaker, fusible switch, or just fuses.

The motor power wire sizing has a number of different de-rating factors selectable from the dialog. One of them is the “Fill Correction:” selection. The motor wire ampacity is reduced in steps when there are more than three power conductors grouped in the same conduit, raceway, or cable. The de-rating steps increase with increasing ranges of grouped power conductors.

Enemy: Heat

Internally generated heat is enemy and heat is the issue here. We’re talking about the “I squared R losses”, the power that is dissipated in a wire conductor in the form of heating of the wire itself. The more current-carrying wires that are grouped together, the warmer all of the conductors will become as a group, further raising the resistance of the conductors and potentially exceeding the conductor’s insulation temperature rating.  

So, an electrical code may require that the maximum amps that a conductor can safely carry, when it is all by itself or with no more that two other conductors, needs to be de-rated to a factor of 0.8 of the original when there are between four and six conductors grouped together in a conduit, cable, or raceway. And, if the number of power conductors is seven to nine, the de-rating needs to be 0.7 of the original.

Default “FILL” table

The ranges for “FILL” de-rating are defined in a table found in Circuit Builder’s Electrical Standards database file. The default table is called “FILL” and is shown below using AutoCAD Electrical’s built-in standards database editor (you can also view it using a copy of Microsoft Access).

The default table above yields these de-rating ranges:

  • 1-3:        1.0 ( no de-rating)
  • 4-6:        0.8
  • 7-9:        0.7
  • 10-20:   0.5
  • 21-30:   0.45
  • 31-40:   0.4
  • >40:       0.35

Creating an alternate standard that has a modified FILL table

Let’s say that you have some projects that need to follow a slightly different range of conductor groupings and de-rating factors, perhaps something that is closer to the UL 508A standard. Let’s say that the  alternate fill table needs to be more like this:

  • 1-3:      1.0 (no de-rating)
  • 4-6:      0.8
  • 7-24    0.7
  • 25-42  0.6
  • >42      0.5

We could just modify the existing table, but it might be better to create an alternate “UL508A” standard in Circuit Builder’s Electrical Standards database file. Here’s how to create a second standard as part of the standards database file.

1. Launch the built-in Access database editing tool as shown below.

2. The Electrical Standards Database Editor dialog opens. Expand the “Default” and “NEC” tree nodes to show all table names. Find the table named “OPT_NEC”. Right click and Copy / Paste this table. Name the new copy “OPT_UL508A” as shown below.



3.  Once you hit OK, the tree structure will update and add in the new “UL508A” entry as shown below. Expand it to display the copied table OPT_UL508A and click on it to open. Edit the two records shown and hit “Save”.

4. Make a copy of the “FILL_NEC” table. Name it “FILL_UL508A”.

 

5. Open up the copied/renamed FILL_UL508A table. Edit as shown below to give the altered ranges for grouped conductor counts and de-rating correction factors to parallel what might be found in the UL 508A standard. “Save” and “Close”.

 

Testing

Open up any project. Right click on the project name in the AutoCAD Electrical Project Manager and select it.

Now run Circuit Builder and start to “Configure” a motor circuit. On the “Motor Setup” right at the front end, hit the “Select Motor” as shown below and make some motor selection. This sets in motion the queries into the select standard in the Electrical Standards Database file.

Select any motor size in dialog above.

Now launch the “Wire Size Lookup” dialog (shown below). This is where we find out if our test is going to be a success or a failure. Turn on the “Fill correction” option. Hit the pop-list as shown. See if the five UL 508A grouping categories show up instead of the seven that show up for the NEC selection. Make some selections and confirm that the fill correction factor to the right of the pop list tracks the UL 508A ranges correctly.

That should do it.

If there are other things that you want to adjust per this alternate standard, copy the tables from the NEC version, rename with the UL508A suffix (replace the “NEC” suffix with “UL508A”), open, edit, and save.

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