3/4" Schedule 80 fill rate


Old 09-19-17, 10:13 AM
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3/4" Schedule 80 fill rate

I have a detached garage that already has a short (15 feet) section of 3/4" Schedule 80 PVC that is buried under concrete leading to the LB that enters the garage.

Based on the conduit fill charts I can only fit (3) #6 AWG in that conduit. The rest of the run to the main panel is 3/4" Schedule 40 which allows (4) #6 AWG conductors.

Because of that small section of Sch 80 I chose to pull 8/8/8/10 feeders to the garage and it was a piece of cake. So much that I am now wondering if I should pull 6/6/6/10 instead to maximize my potential at the subpanel.

If the fill chart says max of (3) #6 conductors, does that mean there is no room for the #10 AWG ground? Or does that just mean no room for a 4th #6 AWG?
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Old 09-19-17, 11:06 AM
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The charts are a quick guide line. If using THHN/THWN wire using the Southwire Mil size for #6 and #10 Cu, three #6 and one #10 will fit in 3/4 Sch 80 PVC. I get .1637 for the four wires and the NEC chart list 3/4" Sch 80 fill at 40% as .164. So it just does fit.

Edit: A moment of pause, I used the wrong size for the #10. Will recalculate.

Edit: The issue is South has two different insulation thicknesses on the #10 and one puts the total at .1637 and the other at .166 which is a very small amount over the .164. Bottom line I wouldn't worry about it. If you are using THHN/THWN I say go ahead with the 6-6-6-10 in the Sch 80.
Old 09-19-17, 02:30 PM
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It just dawned on me that the two size differences for the #10 is solid vs. stranded. The stranded is the larger.

Last edited by pattenp; 09-19-17 at 04:48 PM.
Old 09-19-17, 04:22 PM
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Using the NEC code book tables, the area available for conductors in 3/4" Sch 80 PVC is 0.164 (40% fill for over 2 wires)
#6 THHN/THWN wires are 0.0507
#10 THHN/THWN wires are 0.0211

0.0507 x 3 = 0.1521 + 0.0211 = 0.1732

You are 0.0092 over. IMO I would install the #6

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