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Posted by # Fred # on December 12, 2006, 7:19 pm
>
> # Fred # wrote:
>> What is your source for the cable impedance R+jX values? Do you use a
>> default power factor or is it user defined?
>
> The voltage drop calculator uses the equation:
> vd=2*K*L*I/CMA
> Where K = 12.9 for copper and 21.2 for Aluminum
> K is the resistance in ohms of one foot of a one mil conductor at 75
> degrees C.
> L is one way distance from supply to load in feet
> I is current in amperes
> CMA is circular mil area of conductor
> vd is voltage drop in volts
> This is a a very conservative calculation since the load is assumed to
> be the same as the OCPD value and the conductor is assumed to be at 75
> degrees C.
> This is the standard method of calculating voltage drop used by
> electricians for the last 50 years and appears in journeyman
> electrician tests for several states.
> Using the impedance method and knowing the precise load can be more
> accurate, but voltage drop is a best guess for the most part anyway.
The more exact formula incorporating impedance and power factor is no more
complicated than the above equation but the system power factor maybe a
little tricky to get - measurement, calculation or assumption. When
conductors get larger, the Z (impedance) will be significantly larger than
the R (dc resistance) so you expect a significant different between the
equations that incorporate only R values and equation with Z values. The
different in voltage drop is even more so between a 60Hz system and a 400Hz
system like an aircraft generator.
> At the $300 million Healy Power Plant Job in Alaska the engineers
> simply specified 460 volt motors to be used on a 480 volt system and
> that solved all their voltage drop problems.
>
You mean 460V synchronous motors?
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