TEST PREP - NEW2011 Table 310.15 B 3 A

[Basslee]

Hello all,

I am new to the Mike Holt Forums. I am a 2nd year apprentice with IBEW local 20 in DFW. I am studying for a test i have soon and i am confused on how to use this table.

My study question is:

What is the maximum current allowed on a #10 THE when in a conduit with five other current carrying #10 THW's and two bare #10 grounding conductors? (all are copper conductors)(this is a total of 8 conductors in the conduit).

The correct answer is 28 amps. But i am confused on how to arrive at this answer. I understand that the table adjusts 70% for 7-9 conductors. But if table 310.15 b 16 gives a 35 amp ampactiy for #10 THW, why do i arrive at 24.5 amps for the adjustment?

My calculation is

70% of 35 amps. 35*.7= 24.5

I don't get it. Please help!!!!

 

[augie47]

Note your adjustment factor addresses current carrying conductors. The questions tells you your #10 accompanies 5 other ccc's for a total of 6.
The multiplier for 6 ccc is 80%. The 75? rating of THW of 35 amps X 80%=28.
The other info is superfluous. Common "trick" with questions.

 

[Dennis Alwon]

I agree with GUS. The equipment grounds count for pipe fill but not for de-rating purposes since they do not carry current