Voltage Loss Calculation for Long Cable Run
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Re: Voltage Loss Calculation for Long Cable Run
Here's an online calculator
http://www.lv-dcsi.com/calc.htm
Here's an online calculator
http://www.lv-dcsi.com/calc.htm
physis
Senior Member
Re: Voltage Loss Calculation for Long Cable Run
You have to use current and resitance in the calculation because your looking for voltage (voltage drop).
V = I x R (That's the basic Ohm's law form for voltage not the formula you want)
Here is a formula for single phase voltage drop:
Vd = (2 x K x L x I) / cmil
K is the resistivity of the conductor material.
Vd = voltage drop
K = 12.9 (for copper) or 21.2 (for aluminum)
L = conductor length (one direction)
I = current
cmil = circular mil area of the conductor (chapter 9 table 8 NEC 2002)
to find the percentage of voltage drop divide Vd by the source voltage.
You have to use current and resitance in the calculation because your looking for voltage (voltage drop).
V = I x R (That's the basic Ohm's law form for voltage not the formula you want)
Here is a formula for single phase voltage drop:
Vd = (2 x K x L x I) / cmil
K is the resistivity of the conductor material.
Vd = voltage drop
K = 12.9 (for copper) or 21.2 (for aluminum)
L = conductor length (one direction)
I = current
cmil = circular mil area of the conductor (chapter 9 table 8 NEC 2002)
to find the percentage of voltage drop divide Vd by the source voltage.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Voltage Loss Calculation for Long Cable Run
quote by Sam
V = I x R (That's the basic Ohm's law form for voltage not the formula you want)
.
I've seen your equation for many years and never was comfortable with it. It seems to be an approximation that is correct only in certain instances.
If you rearrange your equation you can come up with Z=2K/MCM where Z is in ohms per 1000'.
Now check what you get with this equation against Table 8 or 9.
For example #12Cu 6530cm or 6.53MCM 25.9/6.53=3.95 ohms/1000'. This doesn't match either Table 8 or Table 9 very well.
If I try 500 MCM 25.9/500=.0518 ohms/1000'. This is closer when you consider Z and not R, but it is still off.
What is your experience with this vs Ohms Law?
Jim T
quote by Sam
V = I x R (That's the basic Ohm's law form for voltage not the formula you want)
.
I've seen your equation for many years and never was comfortable with it. It seems to be an approximation that is correct only in certain instances.
If you rearrange your equation you can come up with Z=2K/MCM where Z is in ohms per 1000'.
Now check what you get with this equation against Table 8 or 9.
For example #12Cu 6530cm or 6.53MCM 25.9/6.53=3.95 ohms/1000'. This doesn't match either Table 8 or Table 9 very well.
If I try 500 MCM 25.9/500=.0518 ohms/1000'. This is closer when you consider Z and not R, but it is still off.
What is your experience with this vs Ohms Law?
Jim T
physis
Senior Member
Re: Voltage Loss Calculation for Long Cable Run
It's also been my experience that it doesn't match the Ohm's law IR=V method. I prefer that too. I use this formula on the forum because it seems to be an industry standard that's used a lot for tests and a lot of times that's the reason for the question. Plus, it's pretty close anyway.
I'm sure you know that you can find the error in "K". Try this value:
for 12 AWG solid
K = 13.1253
___________________________________________________________________
Edit: And you're going to find as much error in the manufacturing of conductors.
[ February 25, 2005, 02:46 PM: Message edited by: physis ]
It's also been my experience that it doesn't match the Ohm's law IR=V method. I prefer that too. I use this formula on the forum because it seems to be an industry standard that's used a lot for tests and a lot of times that's the reason for the question. Plus, it's pretty close anyway.
I'm sure you know that you can find the error in "K". Try this value:
for 12 AWG solid
K = 13.1253
___________________________________________________________________
Edit: And you're going to find as much error in the manufacturing of conductors.
[ February 25, 2005, 02:46 PM: Message edited by: physis ]
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Voltage Loss Calculation for Long Cable Run
The "K" equation gives a linear relationship between conductor area and conductor resistance or impedance. If you believe in skin effect, the relationship ought to be linear between conductor diameter and conductor resistance. Stranded conductors have more "skin" than solid ones,s so the actual relationship seems to be tough to accurately determine. I think that is the basic problem for me.
Jim T
The "K" equation gives a linear relationship between conductor area and conductor resistance or impedance. If you believe in skin effect, the relationship ought to be linear between conductor diameter and conductor resistance. Stranded conductors have more "skin" than solid ones,s so the actual relationship seems to be tough to accurately determine. I think that is the basic problem for me.
Jim T
physis
Senior Member
Re: Voltage Loss Calculation for Long Cable Run
K is based on the resistance of a 1 mil diameter conductor 1 foot long.
I actually shouldn't blame K itself.
For K to be accurate it has to be taken from the specific conductor you're using.
But when you take a generalized value for K and extend it to represent any conductor you don't get an accurate resistance value. There are other factors that effect the final resistance of a conductor that are not accounted for.
You can see in table 8 that even insulation changes the resistance.
____________________________________________________________________
Edit:
That would only be true in the other world where everything isn't a complex shmushed up heap of a bunch of stuff.
[ February 25, 2005, 04:51 PM: Message edited by: physis ]
K is based on the resistance of a 1 mil diameter conductor 1 foot long.
I actually shouldn't blame K itself.
For K to be accurate it has to be taken from the specific conductor you're using.
But when you take a generalized value for K and extend it to represent any conductor you don't get an accurate resistance value. There are other factors that effect the final resistance of a conductor that are not accounted for.
You can see in table 8 that even insulation changes the resistance.
____________________________________________________________________
Edit:
Hah. You're an engineer. You should know better than that.
That would only be true in the other world where everything isn't a complex shmushed up heap of a bunch of stuff.
[ February 25, 2005, 04:51 PM: Message edited by: physis ]
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Voltage Loss Calculation for Long Cable Run
quote by Sam
That would only be true in the other world where everything isn't a complex shmushed up heap of a bunch of stuff.
My hat is off to you. I couldn't have put it better.
Jim T
quote by Sam
That would only be true in the other world where everything isn't a complex shmushed up heap of a bunch of stuff.
My hat is off to you. I couldn't have put it better.
Jim T
Re: Voltage Loss Calculation for Long Cable Run
Guys
I see this caculation all the time on this forum.
I want to list the following information:
R = Resistance per unit length
C = 1.02 for stranded and 1.00 for solid conductors
K = volumn resistivity in ohms-cmil/ft
10.575 for copper at 25C
A = cross-sectional area in cmil
R = C x (K/A)x 1000 = ohms/1000 ft
K varies with temperature and K = 12.9 at 75C.
If the conductor is in a conditioned area, the temp will not be 75C and if K = 12.9 is used the VD caculation will larger that it should be. It will give a safety factor if needed. The variance in values may be due to the temperature used in the caculation.
Guys
I see this caculation all the time on this forum.
I want to list the following information:
R = Resistance per unit length
C = 1.02 for stranded and 1.00 for solid conductors
K = volumn resistivity in ohms-cmil/ft
10.575 for copper at 25C
A = cross-sectional area in cmil
R = C x (K/A)x 1000 = ohms/1000 ft
K varies with temperature and K = 12.9 at 75C.
If the conductor is in a conditioned area, the temp will not be 75C and if K = 12.9 is used the VD caculation will larger that it should be. It will give a safety factor if needed. The variance in values may be due to the temperature used in the caculation.
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