Calculating Voltage Drop in Medium Voltage Circuits

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meo1

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Location
Reading, PA
I need to calculate the voltage drop in a 13.8kV circuit. I know that I should take into account reactance in a medium voltage circuit but I am not sure how to do it. If I use the following formula from Mike Holt's "Voltage Drop Calculations" article on his website:

VD = (1.732 x K x I x L)/CM

I know that I need to adjust the K (Direct Current Constant) but I am not sure how to do it. According to the article, for a 600 volt circuit I would multiply K by and "Adjustment Factor" which is calculated by dividing the ac ohms-to-neutral impedance from NEC Chp 9 Table 9 by the dc resistance of NEC Chp 9 Table 8 for the conductor I am interested in using. Does this also apply to medium voltage calculations?
 
bdarnell

bdarnell

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Location
Indianapolis, IN
Occupation
Retired Engineer
I would say no, based on the fact that Table 9 is for 600 volt cables. You will need to get the cable data from your supplier for the cable that you are using.

I would start with that. How far are you running this circuit ? I've designed several customer owned MV systems and voltage drop wasn't an issue. Maybe mine weren't the length that you are working with.
 
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meo1

Member
Location
Reading, PA
The feeder is for an 8/10/13MVA transformer. I am running two(2) 500 MCM's per phase and the circuit is only about 600 feet long. It will be run in 5-inch conduits, cable tray, a trench, and finally a duct bank. I agree that voltage drop is not really a problem, however I like to know how to calculate voltage drop for a medium voltage cable. Thanks for the response.
 
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jtester

Senior Member
Location
Las Cruces N.M.
VD = (1.732 x K x I x L)/CM
Click to expand...

That approach won't work. Is the circuit overhead or underground? There are methods to determine the impedance of any circuit configuration, and then use it to determine voltage drop. The equations are available in the Westinghouse T&D Power Distribution Book, or send me a PM with your fax #, and I will fax the equation sheet for typical medium voltage calculations.

Jim T
 
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oliver100

Guest
VD = 1.73 x Z x I (the Ohm's law)

Z has 2 components

R is the resistance of the conductor
X is a the reactance

There are values for R and X per 1000 ft usually given by the manufacturers of the cables.

In your case (600ft run) the reactance is unsignificant and almost impossible to calculate.
 
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jtester

Senior Member
Location
Las Cruces N.M.
quote by oliver

Z has 2 components

R is the resistance of the conductor
X is a the reactance
Click to expand...

R and X are actually the resistance and reactance of the circuit, not the conductor. They includes phase, neutral, and also earth. These values are shown in manufacturers tables if you can find the proper configuration, or calculated if you don't, especially if you have overhead lines, where the values change significantly. In medium voltage work circuit configurations or shapes, are much more important than in low voltage design.

If you use the approximations available for 600 volts or below, they may work, or may not.

I agree that at 600' voltage drop will be insignificant.

Jim T
 
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