Rounding NEC Values / Conduit Sizing

[turtlepokerman]

Good Morning Everyone,

My question deals with rounding in the Informative Annex C. Below is my example:

Let's say I am running 2 parallel 3/C 350kcmil THHW cables in a EMT conduit. So I would do the following calculation to determine the conduit size:

Input Data:

350kcmil THHW Cable = .5958 in² (Conductor Size per Chapter 9 Table 5)

Calculation:

.5958 x 6 = 3.5748 in² (Conductor Total Area)According to Table 4 of Chapter 12 our conduit size for over 2 wires should be a trade size of 3.5".

This value does not correspond however to Informative Annex C Table C.1

Which indicates that I could place 6-350kcmil THHW cables in a 3" conduit.

Which value should I use (3" emt conduit or 3.5" emt conduit)?

Am I allowed rounding in my calculations, if so how much?

 

[Dennis Alwon]

Before the 2014 there were errors in Annex C. You need to use the calculation method using Tables 4 and 5

 

[Dennis Alwon]

Actually the table is correct. Look at Chapter 9 note 7. If the decimal is smaller than .8 you can round down

 

[turtlepokerman]

Actually the table is correct. Look at Chapter 9 note 7. If the decimal is smaller than .8 you can round down

Perfect. I never saw that before, it looks like it is new to 2014. Thanks for responding.

 

[Dennis Alwon]

Perfect. I never saw that before, it looks like it is new to 2014. Thanks for responding.

It is new in the sense that it was added to Annex C but it was there in Chapter 9 Tables Note 7 in past codes

 

[Timbert]

Actually the table is correct. Look at Chapter 9 note 7. If the decimal is smaller than .8 you can round down

:slaphead:Great leave it to the NEC to change rounding rules that we learned in second grade.

The NFPA should stick to rules concerning grounding not rounding.:rant:

 

[Dennis Alwon]

:slaphead:Great leave it to the NEC to change rounding rules that we learned in second grade.

The NFPA should stick to rules concerning grounding not rounding.:rant:

Not sure why they used .8 instead of .5 but that rule has been there for awhile. I forget about it often.

 

[don_resqcapt19]

:slaphead:Great leave it to the NEC to change rounding rules that we learned in second grade.

The NFPA should stick to rules concerning grounding not rounding.:rant:

The round up from a x.8 to the next whole number is an increase in the wire fill over the percentage that is permitted by Chapter 9, Table 1. I assume that the code making panel felt that a round up from x.5 could result in an excessive overfill of the raceway.

Note that you can only use the "round up" to the next whole number where all of the conductors in the raceway are the same size.