Voltage Drop and Green - AutoCAD Electrical

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Posted March 31, 2008

by Nate Holt
This tutorial reviews some basic tools that might help you group wire conductors into a conduit or wire-way run and then size it appropriately.

For example, the reporting tool lets you generate a simple project-wide conduit report.

Say the schematic shows a 50Hp motor, tied back to terminals in motor control center through a safety switch. Elsewhere in the schematic is an aux contact from this safety switch tied in to the control circuit. The three phase wiring is drawn on layer which probably means black wire conductors, size 6AWG, insulation type.

How can AutoCAD Electrical help us calculate the appropriate conduit or wire-way size and ultimately give us a basic conduit/wiring report?

Setup

There is some initial setup required. This involves editing a couple ASCII text files – one to map each wire layer name on your drawings to a conductor diameter value. The other support file lists trade-size conduit and wire-way maximum fill cross-section values based upon your local electrical code.

1. Wire diameter mapping file

This is an ASCII text file, default name is default.wdw and an initial copy of this might be found in your AutoCAD Electrical "user" folder. Here is a modified version of this file set up for our example drawing set:

Each line of data is delimited into a maximum of three pieces (semi-colon delimited). The first gives the wire layer name, the second gives a text string that can be substituted for the layer name in various wire reports and for the color/gauge label command. The third entry gives the wire’s actual diameter value.

This file is used in other places in AutoCAD Electrical, but for conduit/wire-way sizing, the first and third data elements are what we are focusing on. You may already have a file like this on your system (in the "user" folder) but it may be lacking the third entry in each line. You’ll need to add in this conductor diameter data to enable AutoCAD Electrical to do any fill calculations. (Note: like many of the customizable support files in AutoCAD Electrical, this file can be made "project-specific" by copying it to a name "<project name>.wdw")

So, in the motor example above, we have three conductors on layer BLK_6_XHHW. From the above table, AutoCAD Electrical now knows that each conductor’s diameter is 0.274 (pulled from manufacturer data or perhaps from an NEC Electrical Code table).

2. Conduit / Wire-way Size Table

This second file’s default name is default.ww1 and is shown here (may be on your system under folder c:\program files\autodesk\acade 200x\support\) .

Each line is broken into two parts by a semi-colon delimiter. The first part gives the conduit or wire-way size and the second piece gives the maximum allowable cross-section fill. So, in the example file above, a ¾ inch conduit is 100% full when the sum of the conductor cross-sections reaches a value of 0.21.

Running the Conduit tagging/sizing utility

A small, 5-button toolbar should show up somewhere on your screen.

Okay, we’re ready to go.

Let’s create a new drawing and add it to our project. This will be our conduit / interconnection diagram. Select the “Insert Footprint (Schematic List)” command on the first pull-down of the panel toolbar. Insert generic representations of the motor and the motor’s disconnect switch.

Now draw a line or polyline between them to represent the conduit (or wire-way).

Next, select the right-hand button on the conduit toolbar. This will make sure that the project’s scratch database is up-to-date with schematic wire connection data. Now pick the left-hand button on the conduit toolbar. Pick on the line that represents the conduit and one more pick to identify where the little conduit marker tag is to be. Before you exit the command, it prompts to you select the device or devices that this conduit is going to run to. Pick on the motor symbol.

The Big Dialog

This Conduit Label dialog now pops open.

Two key things initially show up. First, in the bottom right hand part of the dialog is a list of all schematic wires that AutoCAD Electrical found going to motor MOT211.

And in the middle left-hand column is a listing off all the conduit entries you’ve set up in default.ww1 with the fill calculated for the sum of all of the listed wiring.

It looks like a ¾” conduit is large enough to handle the three # wires. But wait, let’s say we always run a green ground wire with all motor wiring. We may not show it on our schematics but it is in our installation specification for the contractor. Back at the dialog, select the “Spares” button and add in a #8 Green XHHW (highlight in the left-hand list and hit the “Add->” button to move a copy to the right-hand list).

Select OK. Note that the conduit fill window updated to take into account the extra added wire, and now a ¾” conduit is not quite large enough. Need to specify 1”. So, highlight the 1” entry and hit OK.

And here is our first conduit tag label. The wire information is stored right on the tag as invisible data.

Now let’s draw in a home run from this DS211A disconnect plus add in a second motor from our schematic. Here’s what we might lay out before attaching tags and sizing the conduits.

Now let’s do the run starting at the lower disconnect switch heading back to MCC. Pick the Insert/Edit Conduit Tag command again (left-most toolbar button) and pick on the line. Then pick on the DS211A representation. The big dialog displays all wires that tie into this disconnect switch. This includes two wires from a auxiliary contact in the control circuit and the three wires we already have going to the motor. Subtract out the direct to motor wires by doing a CTRL-select and hitting “Remove^”.

Add in the spare ground wire and look at the conduit fill values. Looks like 1” should be large enough for this segment.

Continuing on with the second motor...

And now, the final home run from the bottom tee-intersection back to MCC01A. This conduit segment will need all wires that show up in “C2” and in “C5”. So run the conduit tag insert and pick right on these two existing tags ( ! ). That’s it.

Conduit Report

To finish off, pick the 4^th toolbar button and run a report. You may not get it to format exactly the way you’d like, but all the data is there. You can save it to an Excel spreadsheet and massage it as needed and then pop it back into your drawing as a table.

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