SPECIFIC RESISTANCE (K) COPPER

[dereckbc]

In my line of work, I perform a lot of voltage drop calculations with a spreadsheet. In that spreadsheet I have a set value for K of 11.1. I see a lot of post using the same formula but 12.9 is stated for K, which I believe comes from NEC. As I look at various reference material such as ANSI/IEEE, Ugly’s, ASTM, equipment manufacture, etc I see values ranging from 10.7 to 13.1 for K of copper.

Since I am a data collection nut, I have measured numerous high current circuits, and reversed engineered the value of K and have found it ranges from 10.7 to 11.5. So I choose to use 11.1 as an average. DC currents are on the low end, and AC on the high end, naturally.

My question is why does the NEC use such a high number for K of copper? It seems to conservative, and would cause to oversize most of the time.

 

[iwire]

Re: SPECIFIC RESISTANCE (K) COPPER

Hi Dereck, I think you are aware voltage drop calcs are not a strong point for me so this is really a question.

Wouldn't the use of 11.1 for K result in conductors that are to small about half the time as your real world measurements ranged from 10.7 to 11.5?

If you are setting a standard to go buy wouldn't you want one that always results in large enough conductors?

Or it is an evil move by the copper industries to sell more product.

Bob

 

[roger]

Re: SPECIFIC RESISTANCE (K) COPPER

Hello Dereck, I think the reason for the generalization of a high value (12.9) vs "true or exact K values" is per the words in your last sentence, and BTW your observation is true.

? It seems to conservative, and would cause to oversize most of the time.

Training literature and standards or codes would want to oversize here which is against the grain when we say "codes are minimum.

Roger

 

[bob]

Re: SPECIFIC RESISTANCE (K) COPPER

Dereck
I looked in several books that I keep on hand and
found the following:
Southwire Power Cable manual list the volume
resistivity of copper at 20C = 10.371, 25C = 10.571 and in another book it lists copper at
12.57 at 50C all ohms-cmil/ft. If you know the resistance R1 and Temp1(initial temp)then
R1 = DC resistance
R2 = R1[1+a1(T2-T1)] T1 = 25C and T2 new temp
where a1 = 0.00385 at 25C for copper
and a1 = .00395 for aluminum
or an equation most used is
R2 = [{234+T2}/(234+T1)] for copper
R2 = [{228+T2)/{228+t1)] for al

[ August 11, 2003, 11:14 PM: Message edited by: bob ]

 

[dereckbc]

Re: SPECIFIC RESISTANCE (K) COPPER

IWIRE BOB, when performing my calculations I use 80 percent of the breaker rating. Since loads rarely approach 80 percent, 11.1 is overkill. The range I have measured I believe is due to tempurature.

I like the conspiracy theory.


[ August 11, 2003, 09:13 PM: Message edited by: dereckbc ]

 

[rb]

Re: SPECIFIC RESISTANCE (K) COPPER

The information Bob provided can also be found in Table 8 of Chapter 9 of the NEC. K can be found by multiplying the resistance by the circular mils and then dividing by 1,000. The temperature correction formula can be found in the notes at the bottom of the table along with the the temperature coefficients of resistance for copper and aluminum.