Coated vs. Uncoated Conductor Properties

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brannen

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Working through the "Electrical Fundamentals" book I am working on a problem which states, "What is the voltage drop of two 12AWG conductors supplying a 16A load, located 100ft from the power supply?"

Referring to Table 8 "Conductor Properties" I see that 12 AWG can be Qty 1 or 7 so I'm going with 1, and Copper can be Uncoated 1.93Ohm/kFT or 2.01Ohm/kFT.

My question is, on the exam, if the question doesn't specify coated vs. uncoated, should I go with Coated? Coated fits the problem in the book with 2.01Ohm/kFT = .201Ohm/100FTx2 = .4Ohm resistance * 16A = 6.4V.

Thanks.
 
charlie

charlie

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Re: Coated vs. Uncoated Conductor Properties

Uncoated is bare wire, coated is tinned wire. The diameter of the conductor is the same after tinning so the diameter of the copper is reduced about .002" to make up for the addition of the tinning. This increases the resistance of the conductor. I would assume that the question is referring to solid uncoated wire for #10 and below unless the question states otherwise.

The difference between 1 and 7 is 1 strand is solid wire and 7 strands is stranded copper wire. I assume you are using 2RLI/1000 to calculate with. After making the assumption of the formula, the answer I get is 6.176 volts for solid wire or 6.336 volts for stranded wire. If you are using the exact value of K, your answer will likely be a little different. :D
 
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brannen

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Re: Coated vs. Uncoated Conductor Properties

Hey Charlie, thanks for your reply.

I'm a real newbie, so I appreciate your patience.

I wrote the question verbatim from Mike Holt's book.

Yeah, I figure it's solid wire also, since it doesn't state otherwise.

What does this mean, "2RLI/1000"

I'm using Table 8 in the 2002 code, Chapter 9 Tables. So, the resistance is 1.93Ohm/kFt. = .193Ohm/100Ft x 2 = .386 Ohms for 200 Ft. * 16A = 6.18V.

However, in the book, he writes:
Evd = I x R
Evd = 16A x 0.4 Ohm.

So, I guess he took the Coated value of 2.01 Ohm/kFT.

Just wondering, on the NEC exam, if I should go with coated or uncoated when there's a question like this?

Thanks again. :)
 
charlie

charlie

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Location
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Re: Coated vs. Uncoated Conductor Properties

What does this mean, "2RLI/1000"
Click to expand...
The concept of voltage drop is very basic. Regardless of the formula that you use, it is a modification of E = IR. The formula that is given in the NEC Handbook is:

Vd = 2RLI / 1000

Vd = voltage drop (in volts)

R = resistance of the conductor per 1000 feet

I = amperes

2 = both ways, going out to the load and back to the source

L = length of the run

1000 = gets rid of the 1000 feet on the conductor resistance

In order to change the formula to 3 phase, you must multiply by the Sin of 60?, i.e. Sq Rt 3/2. This changes the formula to:

Vd = Sq Rt 3RLI/1000

The concept and formulae above are based on direct current, not alternating current. If calculations must be accurate, reactance must be accounted for as well as the power factor of the load and the resistance of the circuit to alternating current. The question is, how important is the accuracy of your calculations?

The formula that Mike Holt uses is with the exact value of K.
Just wondering, on the NEC exam, if I should go with coated or uncoated when there's a question like this?
Click to expand...
In my opinion, the uncoated wire should be used unless specified otherwise in the question. :D
 
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brannen

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Location
Charlotte, NC
Re: Coated vs. Uncoated Conductor Properties

Thanks for that explanation, Charlie. :)

Given that the book is "Basic Electrical Theory", I don't think the calculations have to be THAT exact. ;)
 
charlie

charlie

Senior Member
Location
Indianapolis
Re: Coated vs. Uncoated Conductor Properties

George, it can be coated since we are talking about the conductor only and the insulation is not germane to this discussion. :D
 
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