NEC Ch.9 Table C.11 Error?

RookiEE

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Hawaii
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Electrical Engineer
Hi all,

I was doing some calculations to get familiar with Ch.9 of NEC regarding conduit fill and found the following, if someone can help confirm that I did it right.

Q: 4#3 RHW and 1#8 RHW GND in PVC Sch 40, find the minimum size of PVC Sch 40.

Per Table C.11, 4#3 RHW can fit in 1-1/4''C PVC Sch 40. The problem is when you use Table 1, 4, & 5 to find the cross-section area of 1-1/4''C PVC Sch 40 for 40% fill is 0.581. But when you add up the cross-section area of the 4#3 RHW cables (Ignore the #8 ground wire and use the RHW without *) = 4 x 0.1521 = 0.6084 which is bigger than 0.581 (the cross-section area of 1-1/4''C PVC Sch 40). Does that mean Table C.11 is wrong on 4#3 RHW cables can fit in 1-1/4''C PVC Sch 40 conduit?

Thanks
 
Yeah, you use the approximate area of Table 5, no need to calculate the cross-section using the diameter.
 
So from Table C11 RHW with covering says 4-#3 conductors in a 1¼". I agree with you that number is incorrect based on the 0.581. These tables have been known to have inaccuracies and they are not part of the enforceable code.
 
This would be a good but somewhat devious test question where 99.9% would just look at the table for the answer and not do the calculation as you've done. Those tables are known to have errors and the correct answer comes from the calculation not the table.

Looking at the table again for 1" @ 53% fill you're allowed one conductor then you move up to 1¼" @ 40% fill you're allowed 4 conductors. That looks like there is something not right about the conductor numbers.
 
@RookiEE Where did you get the .1521

I get .1134 for rhw without outer jacket
If you go to Table C11 for RHW it says 4-#3 in a 1¼" raceway. That is for RHW with outer covering. RHW* is without outer covering which is 5-#3 in a 1¼" raceway.
For #3 AWG:
RHW= .1521
RHW*=.1134
 
If you go to Table C11 for RHW it says 4-#3 in a 1¼" raceway. That is for RHW with outer covering. RHW* is without outer covering which is 5-#3 in a 1¼" raceway.
For #3 AWG:
RHW= .1521
RHW*=.1134
Isn't that what I said. RHW* + .1134
5 x .1134= .567

! 1/4 Pvc = .581


Table C11 state 5 #3 in 1 1/4" Pvc so where is the issue
 
Oh, I see....okay
The issue is rounding up
.58 rounds to .6 which is allowed by the notes 7 to the tables, imo

4x .1521= .6084 = .6
 
I think Dennis is talking about the "Notes to Tables" at the begining of chapter 9, where note 7 says:

"When calculating the maximum number of conductors or cables permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the maximum number of conductors permitted when the calculation results in a decimal greater than or equal to 0.8."

So 1-1/4" SCH 40 PVC (0.581) would be how many whole #3 RHW wires?

0.581 / 0.1521 =3.82 of the #3 RHW wires. So by note 7, you would be allowed to round up to install 4 of the #3 RHW wires.


** Since there is an EGC the OP needs to use a larger conduit size **
 
I think Dennis is talking about the "Notes to Tables" at the begining of chapter 9, where note 7 says:

"When calculating the maximum number of conductors or cables permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the maximum number of conductors permitted when the calculation results in a decimal greater than or equal to 0.8."

So 1-1/4" SCH 40 PVC (0.581) would be how many whole #3 RHW wires?

0.581 / 0.1521 =3.82 of the #3 RHW wires. So by note 7, you would be allowed to round up to install 4 of the #3 RHW wires.


** Since there is an EGC the OP needs to use a larger conduit size **
Makes sense. I didn't have the codebook so I didn't read the notes.
 
Yeah, I was referring to the RHW with outer covering.

I didn't know the rounding note existed. So per the note, you would round up the number you got from (conduit fill area / total cable cross-section area of same conductors). So there is nothing wrong with the table for the 4#3 RHW.

Looking at the notes, this rounding only applies when you size conductor of same size and not a mix of different size conductors.

Thank you everyone for the feedback.
 
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