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Tips from the Pros
__________________
Terry Carroll
IBEW member since 1999. Terry works as a foreman for a larger electrical firm in Halifax. He is known as a man who gets the job done.
Maximum Number of Conductors in a Conduit
Having the right amount of conductors in a conduit is important for a few reasons. One being the protection of the conductor insulation. If there are too many wires, the temperature in the conduit will increase and the insulation will be subjected to higher temperatures than they're rated for. This will cause damage to the insulation of the conductor over a period of time, and cause either faulty wiring, a fire, or a short.

Another reason this is important is because of ampacity. If there is more heat in the conduit - because there are too many wires - the excess heat causes additional resistance that the power must travel through. This all adds up to reasons that there are codes that need to be observed, and practiced, in order to keep your installation safe from fire.

Section 12 of the CEC deals with topic. (12-1014).

Table 6 is only to be used if the conductors are all the same size. So, since we don't usually have the leisure of pulling all of the same wire size - they came up with Table 8, 9, and 10.

Table 8 is a percentage of allowable fill for a conduit. Table 9 is the cross sectional area of conduit and tubing. Table 10 is the dimensions of cable for calculating conduit and tubing fill.

Now, to put it in a non-confusing way.

The order you must follow in order to get the correct calculation is 10, 8, then 9. So why didn't they renumber their charts to go in the proper order of operation? If you flip the pages more, it'll wear out the book, and you'll have to get another one - that's why.

Let's say you have 10 - #12 guage, 10 - #10 guage and 4 - #8 guage wires that need to be piped into an electrical panel. How do you figure it out? First go to Table 10 in the CEC, and look for the wire you are using. We'll use RW90XLPE "jacketed" - 600 Volt as an example below.


Qty
Guage Wire
Area mm2
Total Area mm2
10
#12
16.75
167.5
10
#10
21.45
214.5
4
#8
35.77
143.08
-
-
Total
525.08


You have a number to reference now (525.08). So go to table 8. It says if you have three conductor sizes you must use 40% fill. That one was easy.

Now go to table 9. You must use the 40% column and find a number that is the same or greater than your number. It is 525.40. The trade size conduit it gives is 41. Which is 1 1/2" conduit. If you are confused about the trade sizes, go to xxvii in your code book (2006) and it'll give you the conversion.

So in an attempt to give you some practice, I've given an exercise below to get your hands dirty. When you've completed your answers you can check this page for your answer key - to see if you're on the right track.

Question #1
Qty
Guage Wire
Area mm2
Total Area mm2
22
#14
-
-
20
#12
-
-
8
#10
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-

Question #2
Qty
Guage Wire
Area mm2
Total Area mm2
6
#8
-
-
3
#6
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-

Question #3
Qty
Guage Wire
Area mm2
Total Area mm2
15
#14
-
-
10
#12
-
-
10
#10
-
-
4
#8
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-

Question #4
Qty
Guage Wire
Area mm2
Total Area mm2
42
#12
-
-
16
#10
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-

Question #5
Qty
Guage Wire
Area mm2
Total Area mm2
15
#12
-
-
6
#10
-
-
3
#8
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-

Question #6
Qty
Guage Wire
Area mm2
Total Area mm2
4
#4
-
-
10
#10
-
-
18
#18
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-

Question #7
Qty
Guage Wire
Area mm2
Total Area mm2
5
#16
-
-
10
#8
-
-
20
#10
-
-
30
#12
-
-
-
Total
-
Cross Sectional Area Needed
-
Conduit Size
-