NEC General Question - Table 9 of Chapter 9

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dnguyen5

Member
Location
United States
Hi all,

First time poster here. I'm reviewing for the Electrical Power PE exam and a voltage drop question puzzled me. When referring to Chapter 9, Table 9, I'm not sure why there are two numbers inside the box. For clarification in Table 9 Chapter 9, when finding the zeffective at 0.85pf for a 250-awg uncoated copper wire in an aluminum conduit, the Zeff is 0.07ohms, but I'm not sure what the number above it (i.e. 0.230) represents and how it may play a role in a voltage drop calculation. Rather than be ignorant, I come for your insight.

Thanks for your help in advance.

Best,
Dan
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I think the neumerator is ohms to neutral per kilometer and the denominator is ohms to neutral per 1000 feet. This is given in the top part of the table
 

MasterTheNEC

CEO and President of Electrical Code Academy, Inc.
Location
McKinney, Texas
Occupation
CEO
Which makes sense when 1 kilometer = 3,280' this the increased EffectiveZ....we all for the most part are more familiar with the "FEET" breakdown when doing these calculations...if that makes sense.
 

ozman

Member
Location
Texas
Occupation
Electrical
Hey guys thanks for opening this thread on uncoated vs coated. I have a question that’s been driving me insane!
“Two 12 AWG stranded uncoated copper conductors are run 100 feet; the current through the circuit is 20 amperes. What is the power loss of the conductors?”
I used 2 methods which give me similar answers, I just want to knw why it’s so similar.
The first method I used was the “vd=2RLI” which gave me 158.4 watts
Then I used the “vd=2KIL” method which gave me exactly 158.
Now the answer on the practice exam is 158.
Why is the 2KIL method more accurate than the 2RLI?
I’m sorry I knw it may be something so simple that I should knw by now but, it just bothers me. Im an apprentice wrking on jm license. Thanks
 
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