Conduit Sizing

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ch_0124

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I am an electrical wire pulling foreman for an industrial contractor out of Steamboat Springs Colorado. I have what may seem to be a stupid question but i know a lot of people that haven't a clue on how to size a conduit. I believe that i myself can size a conduit with no problem as long as its single conductor but when it comes to a multiconductor i'm not sure i'm doing it right. When sizing conduit with multiconductors, chapter nine says to treat it as a single conductor. Would you just measure the diameter of the cable and convert it to square inches and then use the tables in chapter nine or would you find the circumference of the cable? I am so tired of trying to pull cables into conduits that were sized by wadding up cables and seeing if they will fit into a conduit because they don't know or don't care about the 40% fill the code recommends.
 
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ch_0124 said:
Would you just measure the diameter of the cable and convert it to square inches and then use the tables in chapter nine or would you find the circumference of the cable?
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You would find the diameter of the cable at its widest ellipse, and turn that into the cross-sectional area. Pi x Radius x Radius.
 
Volta said:
You would find the diameter of the cable at its widest ellipse, and turn that into the cross-sectional area. Pi x Radius x Radius.
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Umm...no I don't think so. You use the inches squared and divide that into the 40% figure for the type of conduit used if more than 3 current carrying conductors and then round that number down to the mearest whole number and that is your max. # of conductors. And I don't think the NEC allows cable assemblies to be pulled into conduit.
 
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au contraire...you can install multi-cond in pipe

try the nec but today there are plenty of online conduit programs

done it forever...

gw
 
steelersman said:
I know I can't. That's all I can say.
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Well, I tell you what...... neither can I!
heehaw.gif
 
Note 9 in the Chapter 9 tables says
"A multi conductor cable of two or more conductors shall be treated as a single conductor for calculating percentage fill area. For cables that have
elliptical cross section, the cross section area calculation shall be based on using the major diameter of the ellipse as a circle diameter".

So when calculating the 40% fill, use the maximum diameter of the cable
to determine the area required.
 
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Had to double check the math . . .
I think that we will find the area of the cable by multiplying Pi times the radius squared.
The radius will be half the diameter at the widest axis.
Use any wire from Table 5 as an example, say the first one, RFH-2 18AWG:
diam .136 in x 0.5 = 0.068 = Radius
Radius squared = .004624 x PI 3.14159 = 0.0145" cross-sectional area.
 
steelersman said:
I've always known conductors to be round and to not have an ellipse. I don't think cables are allowed to be installed in conduit. Although I have a feeling someone will chime in and say au contraire. :)
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I am going to agree just because it is my new years resolution to agree with everyone. This is painful. The new warm and fuzzy cow.
 
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