Re: Voltage Drop revisited
I thought you might be interested in the method I promote at IPL.
THE CONCEPT
The concept of voltage drop is very basic. Regardless of the formula that you use, it is a modification of E = IR. The formula that is given in the National Electrical Code Handbook is:
Vd = 2RLI / 1000
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- <font size="2" face="Verdana, Helvetica, sans-serif">Vd = voltage drop (in volts)</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">R = resistance of the conductor per 1000 feet</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">I = amperes</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">2 = both ways, going out to the load and back to the source</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">L = length of the run</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">1000 = gets rid of the 1000 feet on the conductor resistance</font>
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In order to change the formula to 3 phase, you must multiply by the Sin of 60 degrees, i.e. Sq. Rt. 3/2. This changes the formula to:
Vd =
Sq.Rt.3RLI/1000
The concept and formulae above are based on direct current, not alternating current. If calculations must be accurate, reactance must be accounted for as well as the power factor of the load and the resistance of the circuit to alternating current. The question is, how important is the accuracy of your calculations?
When doing load calculations, you are making a ?best guess? as to the expected load and as to the future power factor. As long as the method used is not too cumbersome and as accurate as your needs, your method is fine. The above formula fits this criteria and may be used for most of the voltage drop calculations that the IPL project engineer will need to perform. Unless the run involves a significant amount of primary, open wire secondary, or metallic conduit, reactance will present no problem. Converting kW to kVA will take care of the problem of power factor since voltage drop is based on kVA amperes.
Remember this was written for PL engineers and we are guessing at the actual demand expected for a (or several) service.