Voltage Drop

[David Kelley]

Please explain why you use 2 in the formula 2X12.9XDXI/CM....I understand the 2XD for the circuit length, but in your explanation of Voltage Drop, there is not eplanation of why 2X12.9 or 2XI.......Please help!!!

 

[bob]

Please explain why you use 2 in the formula 2X12.9XDXI/CM....I understand the 2XD for the circuit length, but in your explanation of Voltage Drop, there is not explanation of why 2X12.9 or 2XI.......Please help!!!

Are you referring to another post regarding the equation? The 2 is in the equation is to account for the return distance of the conductor as you say.
The equation may be written as 2xDx12.9xI/CM if that helps.

 

[drbond24]

12.9 is the value of 'K' which is a constant having to do with the resistance of copper (it is 21.2 for aluminum according to my Ugly's manual). D is one-way length of the circuit in feet, and I is current.

The 2 is only in there once and it accounts for the return path as you say. Multiplication is associative so it doesn't matter where the 2 is applied so long as its there. Writing it next to the K or the I doesn't make any difference as long as you understand it is there for the D.

Hope that was a little more clear than mud. Brackish, perhaps...

 

[coulter]

Serious questions - cause I really don't know - and am somewhat curious:

1. What is the advantage of using this formula with a K value instead of using Ch 9, T8 or T9?

2. For K= 12.9, what the copper temperature?

carl

 

[drbond24]

1. Tables 8 and 9 list resistances for specific wire sizes which vary because you are talking about different amounts of copper. K is a constant for all copper and is defined as the resistance in ohms of one circular mil foot of conductor. Look at Table 8 and take CM * ohms / foot (must divide by 1000) and see what you get. 12.9 every time.

2. 75 C

 

[haskindm]

drbond24,
Actually the 12.9 is an approximation of the K value. You will get about 12.9 (actually 12.8832) when you use your formula for 1/0 uncoated copper. The exact K will vary. For #12 stranded it is 12.929, for solid it is 12.6029. So 12.9 is an approximation that is "close enough". If you are looking for exact voltage drop use table 8. The approximation is especially close enough since many electricians do not consider voltage drop at all!

 

[bob]

drbond24,
Actually the 12.9 is an approximation of the K value. You will get about 12.9 (actually 12.8832) when you use your formula for 1/0 uncoated copper. The exact K will vary. For #12 stranded it is 12.929, for solid it is 12.6029. So 12.9 is an approximation that is "close enough". If you are looking for exact voltage drop use table 8. The approximation is especially close enough since many electricians do not consider voltage drop at all!

12.9 is the value of K at 75C. K = 10.2 at t = 20C and 10.5 at t = 25C