Sizing Conductors for < 3% Voltage Drop; The 'K' value 3

[workn26]

Hello everyone,

Here, I'm using the circular mills method as opposed to the resistive method:
To find the Voltage Drop of a single-phase ckt. I'd use: VD=2*R*L*I/CM
In order to size a conductor properly while staying within the 3% voltage drop recommendations I'd use: CM=2*K*L*I/VD
VD=voltage drop
L=length of conductor in ft.;one-way
CM=circular mills from Chap.9, Table 8
I=load current
R=the resistive value from Chap.9,Table 8 or 9
K=The D/C constant for the resistance of a 1000-mill conductor that's 1000 ft. long at 75 degrees C.

My question is with the K value. I've seen four different sets of values noted. The values are different for copper and aluminum.
The two sets that seem most accurate are:
cu-12.9
alu-21.2
...and
cu-12
alu-18

An electrician on a different forum stated that he uses cu-7 & alu-11. I'm not too sure about that.
I've been using the first set thinking that keeping the value slightly higher is playing it safe.

Any takers on this one?


I'd appreciate it

Dan

 

[tommom]

12.9 looks closest for copper. R x CM/1000=k .

This formula is okay for most work. However, if it is critical to be very close, or for medium or high-voltage work, I would use the formula associated with the ac resistance of the wire, which for low-voltage cables is in Table 9 of NEC Ch.9.

 

[waross]

Further to tommom's post;
Your method is valid for DC circuits.
Hard drawn copper has a different resistivity than soft copper. That is the source of the two figures, 12, and 12.9
For AC double check the voltage drop tables which include factors for skin effect and reactance .
yours

 

[wareagle]

The value of K is direcly determined by temerature. The resistance of a conductor is R = (K/A) x 1000 ohms/1000ft.
K = volume of resistivity in ohms-cmil/ft
A = cross sectional area in cmil

K = 10.371 at 20C for CU
K = 17.002 at 20C for AL
K = 10.575 at 25C for CU
K = 17.345 at 25C for AL
K = 12.9 at 75C for CU
K = 21.2 at 75C for AL

 

[waross]

Hi wareagle
I'm on the road and away from my library. Possibly you could check in a text that is less theoretically oriented and find a value for hard drawn copper at the various temperatures.
respectfully