Sizing Smaller Conduit bodies

[Geraldselectric]

I get confused when sizing conduit bodies. If I am correct, there are different rules for conductors of #6 and smaller and #4 and larger. When dealing with #6 and smaller, I understand that the cross sectional area of the conduit body has to be atleast twice the cross sectional area of the conductor from table 4? Here is a test question for smaller conductors:

Step 1: Find cross sectional area of 1" Emt: Looking up 1" Emt in Chap. 9, Table 4 at 100% fill I found the cross sectional area to be .864 sq. in.

Step 2: Calculate size of LB conduit body: I mulitplied Step 1 .864 times 2 and the size of the conduit body would be 1.728 sq. in.

This is where I get confused. Question does not explain, but states that the LB conduit body has a 28" sq. in. capacity. Can anyone tell me how this was determined by results of step 2 or is this possibly a stamped standard volume on the conduit body?

Step 3: Find number of #12 THHN conductors allowed in LB: I took the unknown 28" divided by 2.25 (314.16b for #12) and got 12.44. So, 12-#12 are allowed in LB.

Step 4: Find #12 THHN conductors allowed in 1" EMT: Chap 9, table 4 @40% fill gives me .346 sq. in. and Chap 9, table 5 gives me .0133 sq. in. for #12 THHN. I divide .346 sq. in. by .0133 sq. in and get 26.02 sq. in. So, 26 - #12 THHN could fit in the 1" EMT.

I want to learn a better method to size these conduit bodies. Does anyone know if the volume is much the same between for the same size between different manufacturers? What normally gets sized first, the conduit or conduit body? Is it as simple as if I have a 1" conduit, that Im going to select a 1" conduit body?

 

[barbeer]

I personally would say ONLY size your conduit, not the c.b.! I would think the manufacturer took conduit size into accordance when designing/fabricating the piece.

As far as I know- the C.I. marking on a conduit body is there only to tell you if a splice can be made or not (if no C.I. marking you are not supposed to splice inside)?

I think it is safe to say, use 1" EMT, use 1" LB............

 

[Dennis Alwon]

This is where I get confused. Question does not explain, but states that the LB conduit body has a 28" sq. in. capacity. Can anyone tell me how this was determined by results of step 2 or is this possibly a stamped standard volume on the conduit body?

Sq. in is not a measure of Volume but of area. If an LB measures 7" long by
4" wide the area is 28 sq. in the volumne would entail the depth of the LB. So if the LB was 3" deep the Volume would be 28*3=84 cu. in

 

[Dennis Alwon]

Step 3: Find number of #12 THHN conductors allowed in LB: I took the unknown 28" divided by 2.25 (314.16b for #12) and got 12.44. So, 12-#12 are allowed in LB.

The problem, to be more precise is you are using sq.in and cu. in. the 2.25 for # 12 is cu. in., and the 28 you are using is sq. inches. If the LB is 2 inches deep--just a guess then the cu. in is 56. 56/2.25 is 25.

You can see this is about the same as your conduit fill. I was guessing on the depth.

 

[dezwitinc]

Sizing Conduit Bodies

Sizing Conduit Bodies

All conduit bodies are not rated the same as the conduit they attach to.
We recently had a situation where the inspector required cbs larger than the conduit.(I don't remember the exact details but will investigate and post the results).
Check the cb and make sure that it is rated for what you are installing.
It is very costly to remove the conductors and replace the fitting.

 

[mdshunk]

Cable assemblies, like URD, have a min bend radius. Sometimes, this will make a pipe sized conduit body too small to accomodate this min bend radius.

 

[Charles R. Miller]

An in-depth series on box-fill calculations and pull- and junction-box calculations ran from July 2004 through February 2006 in Electrical Contractor magazine. I?ve included links to the four parts covering conduit bodies. Electrical Contractor Magazine?s web site only has the text . . . not the illustrations. This is unfortunate because a picture is worth a thousand words.

Code In Focus, March, 2005
Box-Fill Calculations, Part IX
http://www.ecmag.com/index.cfm?fa=article&articleID=6202

Code In Focus, April, 2005
Box-Fill Calculations, Part X
http://www.ecmag.com/index.cfm?fa=article&articleID=6247

Code In Focus, May, 2005
Box-Fill Calculations, Part XI
http://www.ecmag.com/index.cfm?fa=article&articleID=6273

Code In Focus, June, 2005
Box-Fill Calculations, Part XII
http://www.ecmag.com/index.cfm?fa=article&articleID=6303

Charles

 

[infinity]

Charles,

I remember reading that series of articles. Very well written. And I agree that the illustrations made a big difference in understanding the requirements. Were you the author?

 

[Charles R. Miller]

Thank you for the compliment Trevor.

Yes, I write the Code In Focus column every month for Electrical Contractor Magazine. I have been writing the column for the last seven and a half years. Although I?m not an artist, I also draw all the illustrations for my column (and books.) It is a lot easier to remember code requirements if we can visualize a drawing instead of memorizing words.

Thanks again,
Charles