1. ## Condit fill calculations

I'm confused on the tables and annex c part of the nec.

Say I have the following....

All THHN in a schedule 80 pvc
#12 x10
#10 x5
#8 x3

Since I have over 2 wires in one conduit do I calculate the cross sectional area of the conduit at 40% and then add up cross sectional area of each wire and add them up and see what size the Will fit into?

2. First start on Table 5 ch9
find THHN row and use Approximate area square inches.
then add the total of all of the conductors of your example.

Then go to raceway properties Table 4 of ch9
Find rigid PVC conduit
scroll down the 40% column until you find a number that is higher than the total area that you calculated.
then look to the left of that row you will see what conduit is needed.

Added: Based on my calculation you need a 1.25" conduit

3. That's pretty much what I was thinking. Thank you for the clarification. Does the annex c take the 40% into consideration when it states the maximum number of conductors in any given raceway?

4. Originally Posted by crtemp
That's pretty much what I was thinking. Thank you for the clarification. Does the annex c take the 40% into consideration when it states the maximum number of conductors in any given raceway?
A

Annex C is only used when all the conductors are the same size with the same insulation. It is based on the table for 1 wire (53%), 2 wires(31%) or over 2 wires (60%)

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Originally Posted by Dennis Alwon
A

Annex C is only used when all the conductors are the same size with the same insulation. It is based on the table for 1 wire (53%), 2 wires(31%) or over 2 wires (40%)
Corrected.

Essentially, it represents about 70% of the conduit diameter filled with cable, if you were to pack the cables as tightly as possible. Since two circles have a lousy packing factor in a circle, the area fill percentage is significantly less for 2 wires. 1 circle obviously has a 100% packing factor in a circle, so its area fill percentage is high. Then when you look at 3+ circles in a circle, the packing factor takes off to a steady cruising value between 80% & 90%.

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