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


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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

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