Find the correct wire size for a load or the load for a selected wire size


I have seen a few of your posters over at alt.engineering.electrical asking questions about sizing conductors. I have written two very useful 2005 NEC compliant web page calculators that do what you are looking for and they are out of the beta stage and in the alpha stage. In other words, known bugs are fixed. It has taken me about 10 years on and off and about 2000 hours to write these programs. I am a licensed electrician in two states and a licensed electrical administrator in the state of Alaska and have a degree in Mathematics. The programs are free to use. The programs are at: http://www.electrician2.com/calculators/wireocpd_ver_1.html http://www.electrician2.com/calculators/wireocpd_ver_1_reverse.htm
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What is your source for the cable impedance R+jX values? Do you use a default power factor or is it user defined?
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# Fred # wrote:

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. 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.
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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.

You mean 460V synchronous motors?
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# Fred # wrote:

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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