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Posted by Jerry on December 12, 2006, 8:09 pm
# Fred # wrote:
> >
> > # 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?
No, they were regular squirrel cage induction 3-phase motors. The
onsite engineer called me at the beginning of the job to tell me they
had a major voltage drop problem. She later called and said they
either initially ordered the 460 volt motors or switched to correct the
voltage drop problem.
Electricians do not do engineering for good reasons, but in many
situations small contractors do a lot of design bid build and use
electrician math. This is especially true in some of the remote parts
of Alaska such as Nome, Kotzebue, and Barrow. I live in Alaska.
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