Confused on Annec C table C.8A

I have the following question from the sample test:

If we have a trade size 2 rigid metal conduit and we want
to install three THHN compact conductors, what is the largest
compact conductor permitted to be installed?

Table C.8A says 300 kcmil

Answer key says 350 kcmil

Please help and verify.

Thanks
Coty
 

Dennis Alwon

Moderator
Staff member
I think there are errors in the compact tables.

Use Table 5 in Chapter 9

Compact 350kcm conductor with THHn has a cir mil of .4656
1.3968 X 3 = 1.3968 cm

Table 4 for RMC shows 2" at 1.363 for over 2 conductors

If we look at art 220.5 (B) we are allowed to drop the anything below .5 so the 1.3968 or 1.4 becomes 1 which is less than the cm for 2" RMC with over 2 conductors-- 1.363
 
still can not find

still can not find

I am on page 70-820 looking at THHN, trade size 2, following it down to the number 3, and it crosses with 300 kcmil.
 

don_resqcapt19

Moderator
Staff member
...
If we look at art 220.5 (B) we are allowed to drop the anything below .5 so the 1.3968 or 1.4 becomes 1 which is less than the cm for 2" RMC with over 2 conductors-- 1.363
That rule does not apply to wire fill calculations. It is in Article 220 and only applies within the scope of Article 220.
220.1 Scope. This article provides requirements for calculating branch circuit, feeder, and service loads. Part I provides for general
requirements for calculation methods. Part II provides calculation methods for branch-circuit loads. Parts III and IV provide calculation methods for feeders and services. Part V provides calculation methods for farms. ...
220.5(B) Fractions of an Ampere. Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.
 

Dennis Alwon

Moderator
Staff member
That rule does not apply to wire fill calculations. It is in Article 220 and only applies within the scope of Article 220.

You are correct Don and I should know I did those Tables in Annex C-- Notes to the table #7 says we can drop down when the results are smaller than .8 not .5
 

don_resqcapt19

Moderator
Staff member
You are correct Don and I should know I did those Tables in Annex C-- Notes to the table #7 says we can drop down when the results are smaller than .8 not .5
Actually it says you can "round up" where the fraction is .8 or larger. For example if the calculation says 4.8 conductors, Note 7 would permit you to install 5 conductors.
 

Dennis Alwon

Moderator
Staff member
Yes, I have no idea what I am doing right now-- cooking, eating and typing without thinking. So we have 1.363/.4656 and we have 2.9 conductors which means we can use 3--- thanks for keeping me straight
 
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