conduit sizing for multiple wire sizes

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bohl1076

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I'm an apprentice wireman only 2 and a half months and have been attending an apprenticeship school. I'm doing a report for school, and I'm not sure if I'm intimidated by the code book or just not understanding how to read it. the report is suppose to be on finding a "formula" for how many conductors are allowed conduit, including the possibility of different sized wires. is there a formula that's simple enough for me to understand and use when I'm on the job site? and that i can explain in front of class?
:confused:
 
Re: conduit sizing for multiple wire sizes

Unfortunately, the conduit fill calculations for different size conductors requires some math.
But it is the same math that is used to develop Annex C, for wires of the same size) in your code book.
Go to yahoo or google and search for
conduit fill
and you will find lots of web sites that walk you through the process.
 
Re: conduit sizing for multiple wire sizes

Chapter 9 table 5 of the code book states the square inch area for each type of conductor. Simply add together the sq. inch volume for all of the conductors needed in the pipe.

Chapter 9 table 1 will tell you what percentage of pipe fill you are allowed for the number of conductors you will have in the pipe. Note: A conduit 24" or less is considered a nipple and can be filled to 60%, reguardless of the number of conductors.

Finally, look at chapter 9 table 4. This will give the available sq. inches for each size and type of conduit. The charts even give the available volume for different conuit fill pecentages.
 
Re: conduit sizing for multiple wire sizes

Originally posted by bohl1076:
I'm an apprentice wireman only 2 and a half months and have been attending an apprenticeship school. I'm doing a report for school, and I'm not sure if I'm intimidated by the code book or just not understanding how to read it.
Click to expand...
Two and a half months are you haven't mastered the NEC yet? What's up with that? :)

is there a formula that's simple enough for me to understand and use when I'm on the job site? and that i can explain in front of class?
:confused:
Click to expand...
As Magoo mentioned, it is only addition. Once you can visualize what the numbers represent, then it will be smooth sailing.
Table4.jpg


Essentially, think of the numbers as limits. This table is telling you:
</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">A 1" EMT is .864 square inches in area.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">You can only use a portion of that area, because overfilling the conduit damages the conductors, traps the heat against the inner conductors, and makes the conductors hard (if not impossible) not to pull through the conduit.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">The table gives you the limits for various circumstances (1 wire, 2, more than 2, and nipples).</font>
<font size="2" face="Verdana, Helvetica, sans-serif">So if there are three conductors in a 1" EMT, then you can only fill it 40% of the way, to .346 square inches.

Edit:
Table5.jpg

So, looking at Table 5, on the far right column you have different values for the size of each conductor.

#12 THHN is .0133 square inches in area.

If you wanted to install 10 #12 THHN's in a 1" EMT, you'd have...

.0133 x 10 = .133 square inches of conductors.
And .364 square inches available per Table 4.

.364 - .133 = .231 square inches still free to use in the conduit.

Different types of conductors just get added in.

Let's say you want to add eight #10's into the same conduit you were installing the #12's in.
.0133 x 10 = .133 #12 THHN's
.0211 x 8 = .1688 #10 THHN's
.1688 + .133 = .3188 square inches.

.364 - .3188 = .0452 square inches to spare in the conduit.

[ October 01, 2005, 11:33 PM: Message edited by: georgestolz ]
 
Re: conduit sizing for multiple wire sizes

thanks you guys. you have definitely made it easier to understand and visualize. I'm not as nervous to get up in front of class now!! :cool:
 
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