Voltage Drop Formula

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charlie b

charlie b

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We are having a wonderful time with a plans reviewer who is questioning, well, everything. We have to explain the kitchen sink to him. In a recent face-to-face meeting between him and my boss, he produced a very old copy of a code book (perhaps NEC, I am not certain, as I was not there at the time). He pointed out a formula for voltage drop, and it included the constant value "21.6." He asked my boss to explain that value (what it meant, where it came from, how it can be verified, etc.). Now we are trying to reproduce that formula. We can't find any method of calculating voltage drop that uses that value. The fact that we are using a different formula to calculate VD for this project does not seem to have an impact on this plans reviewer. Can anyone give me a clue on what to look for?
 
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Mouser

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The exact K can be found by multiplying the CM by the Ohms per ft (Table 8 Chapter 9).

Example for Aluminum:

#1 = 83690 x .000253 = 21.17, we usually figure 21.2 in the voltage drop formula.

Another Aluminum example:

#12 = 6530 x .00318 = 20.76, so there are some variances depending on the wire size.

To the OP I just did this to a few wire sizes but did not come up with 21.6 but I am sure there is a wire that may come close.
 
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mwm1752

mwm1752

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Mouser said:
The exact K can be found by multiplying the CM by the Ohms per ft (Table 8 Chapter 9).

Example for Aluminum:

#1 = 83690 x .000253 = 21.17, we usually figure 21.2 in the voltage drop formula.

Another Aluminum example:

#12 = 6530 x .00318 = 20.76, so there are some variances depending on the wire size.

To the OP I just did this to a few wire sizes but did not come up with 21.6 but I am sure there is a wire that may come close.
Click to expand...

Mouser has the correct answer --
I always used the KISS rule K= 12 for CU & 21=K for AL
 
jumper

jumper

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Found it!!!.....I hope.

conductor resistivity. Conductor resistance increases with temperature. Hence, the resistivity constant employed in the calculation should be the value corresponding to the operating temperature of the conductors. The resistivity constant for copper conductor voltage drop calculations is 12.9 ohms at 90°C and 10.8 ohms at 60°C. The resistivity constant for aluminum is 21.2 ohms at 90°C and 17 ohms at 60°C.
Click to expand...


http://ecmweb.com/content/calculating-voltage-drop-power-distribution-systems


Edit: I think that quote may be in error. I think it should be 75 degrees, not 90 degrees.
 
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Dennis Alwon

Dennis Alwon

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jumper

jumper

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Dennis Alwon said:
Good searching Derek but I thought Charlie was looking to explain the 21.6... I may be wrong.. Personally I think your find should be enough to satisfy the reviewer. The older 21.6 may have been gotten from older aluminum wire where the resistance was different????
Click to expand...

2 x 10.8 = 21.6, the 2 is the total circuit length (L) in a VD formula.
 
charlie b

charlie b

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jumper said:
It looks like an old value for the resistance of copper at 75 degrees, 2 x 10.8. I believe we use 12.9 now.
Click to expand...
That is exactly what I was looking for. Many thanks. I mentioned that the plans reviewer was looking at an old code book. The reference you found, the one that used 10.8 as th K factor for copper, was published in 1921!

 
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