ࡱ> ܥhW e"܇xvxvv~v~v~v~v~~~~~~~4~~φC ԂԂԂԂԂ6 &Z\\\-Xjrφv~6ԂԂ66φ6v~v~Ԃ 6666v~Ԃv~ԂZ@[~~"v~v~v~v~6Z6$6Simplified Method for Calculating Short Circuit Currents for Low Voltage Systems  PRIVATE SRC="byline.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/byline1.bmp" \* MERGEFORMAT  Detailed short calculations require the collection of system data and the use of sophisticated computer programs. This paper presents a simplified method to calculate fault levels which can be used in some low voltage industrial systems operated at 120/208 and 480 volt. The method presented here is based on a number of charts which are dependent on a number of factors which contribute to the magnitude of the short circuit current. These charts can be used to provide approximate (but accurate) values for fault current duties on molded case circuit breakers.  PRIVATE SRC="One_Line.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/One_Line.bmp" \* MERGEFORMAT  Factors Affecting Short Circuit Values Short circuit current levels are a function of the following variables: The available short-circuit duty form the source (assumed here to be the electric utility, standby or emergency generators yield lower values than those shown) The transformer size (kVA) and impedance The size and length of the feeder Any motor load connected to system This document deals with some normally encountered situations in industrial power systems where the utility primary power being delivered to a main loadcenter transformer, which is located at or very close to the service entrance. This transformer then steps the voltage down to 480/277 V or 208/120V for distribution within the facility. For the purpose of illustration, three transformer sizes are considered here:150,500 and 1000 kVA. Also, for system voltages of 120/208 and 480 volts, it is reasonable to assume that the connected motor hp is approximately equal to 50% of the transformer kVA rating. How to Use the Curves The charts provided are for the conductors most commonly used on low voltage industrial feeders. For conductor sizes not shown, use the conversion table shown below to find an "equivalent" conductor. The charts presented here are for copper conductors. Similar curves can be derived for aluminum conductors.   PRIVATE MACROBUTTON HtmlDirect * StepProcedure1Obtain transformer kVA nameplate and system voltage2Push the correct botton in the Selection Table3Select the curve corresponding to the conductor size used. If your conductor size is different than the sizes shown, refer to the   PRIVATE HREF="#Conductor_Conversion_Table"MACROBUTTON HtmlResAnchor Conductor Conversion Table. If you are using n conductors in parallel, see Step 4.4Enter the chart along the horizontal axis with the distance (in feet) from the transformer to the fault Line. Draw a vertical line to intersect the appropriate conductor curve. Draw a horizontal line and read fault current. Note: For n parallel conductors, divide the distance to the fault by nConductor Conversion Table   PRIVATE
MACROBUTTON HtmlDirect * If you HaveThen Use these Equivalent ConductorsN#2/0 cablesN/2-500 MCM5-400 MCM cables4-750 MCM6-300 MCM cables4-750 MCM800 amp busway2-500 MCM1000 amp busway2-500 MCM1600 amp busway4-750 MCM Selection Table   PRIVATE
MACROBUTTON HtmlDirect * Transformer kVAVoltage w/o Motor ContributionWith Motor Contribution  PRIVATE
MACROBUTTON HtmlDirect *150  PRIVATE MACROBUTTON HtmlDirect *208  PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="150_208.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="blueball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/blueball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="150_208m.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="redball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/redball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *150  PRIVATE MACROBUTTON HtmlDirect *480  PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="150_480.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="blueball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/blueball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="150_480m.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="redball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/redball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *500  PRIVATE MACROBUTTON HtmlDirect *208  PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="500_208.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="blueball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/blueball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="500_208m.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="redball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/redball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *500  PRIVATE MACROBUTTON HtmlDirect *480  PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="500_480.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="blueball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/blueball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="500_480m.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="redball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/redball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *1000  PRIVATE MACROBUTTON HtmlDirect *208  PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="1000208.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="blueball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/blueball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="1000208m.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="redball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/redball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *1000  PRIVATE MACROBUTTON HtmlDirect *480  PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="1000480.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="blueball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/blueball.bmp" \* MERGEFORMAT   PRIVATE MACROBUTTON HtmlDirect *  PRIVATE HREF="1000480m.htm"MACROBUTTON HtmlResAnchor  PRIVATE SRC="redball.gif" MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/redball.bmp" \* MERGEFORMAT  Source:   PRIVATE HREF="mailto:abisamra@ix.netcom.com"MACROBUTTON HtmlResAnchor Nicholas Abi-Samra Westinghouse Electric Corporation (714) 372-5640   PRIVATE HREF="http://www.earthlink.net/~abisamra/index.html"MACROBUTTON HtmlResAnchor  PRIVATE SRC="home2.gif" ALT="Back to Home Page" BORDER=0 ALIGN=middle MACROBUTTON HtmlResImg INCLUDEPICTURE "C:/WINDOWS/TEMP/wiaa98/home2.bmp" \* MERGEFORMAT    PRIVATE HREF="http://www.earthlink.net/~abisamra/index.html"MACROBUTTON HtmlResAnchor Back to Power Engineering Notes Page /= h        .........)()() :t<1G"    I C II(I!)19BJRZcks{{skcZRJB91)!!)19BJRZcks{{skcZRJB91)!!)19BJRZcks{  !!!""###$$$%%&&&'''(()))***++,,,---..///000112223334455566677788999:::;;;<<===>>>????@@@AAABBBCCDDDEEEFFFGGGHHHIIJJJKKKLLLMMMNNNOOOPPPQQQRRRSSSTTTUUUVVVWWWXXXYYYZZZZ[[[\\\]]]^^^^___```aaabbbccccdddeeefffgggghhhiiijjjkkkllllmmmnnnoooopppqqqrrrrssstttuuuuvvvwwwxxxxyyyzzz{{{{|||}}}}}~~~~  !!!""###$$$%%&&&'''(()))***++,,,---..///000112223334455566677788999:::;;;<<===>>>????@@@AAABBBCCDDDEEEFFFGGGHHHIIJJJKKKLLLMMMNNNOOOPPPQQQRRRSSSTTTUUUVVVWWWXXXYYYZZZZ[[[\\\]]]^^^^___```aaabbbccccdddeeefffgggghhhiiijjjkkkllllmmmnnnoooopppqqqrrrrssstttuuuuvvvwwwxxxxyyyzzz{{{{|||}}}}}~~~~  !!!""###$$%%%&&&''((()))**+++,,,---..///000112223334455566677788999:::;;;<<===>>>????@@@AAABBBCCDDDEEEFFFGGGHHHIIJJJKKKLLLMMMNNNOOOPPPQQQRRRSSSTTTUUUVVVWWWXXXYYYZZZZ[[[\\\]]]^^^^___```aaabbbccccdddeeefffgggghhhiiijjjkkkllllmmmnnnoooopppqqqrrrrssstttuuuuvvvwwwxxxxyyyzzz{{{{|||}}}}}~~~~:)(  `nFMU"lDf"31J@`Ow^n}"Df33GG\\pp)Rz3"J1`@wO^n}Ҍ"Df3Mf"+3<DMUh:|Wu3Pm33MMff3M"f+3<DMUh|:Wu"31J@`Ow^n}"Df33MMff:Wu3P(m7ETbq3Mf̙ٳ3Uw--DD[[qq3M&f3@MYfs:Wu33MMff:Wu3&M3f@MYfs:WuȒׯ:     C (@Ȥkkksss{{{RRRJBBBB{cJ9B{99111))1)1s!s!kc!ZZJJՀժ@@UUUUUUUZU+U+UJ1U+U+c!UUUUUUUcJkBUUUUUUUUUUUUUUUUUUUUՀUժUUUUUUUUUU++U+++{1UUUUUcUU{UUՀժUU@++U++++UUUUUUUUUUՀժUU++U++++UUUUUUUՀՀUՀՀՀ֌ժժUժժժժ@@@UՀժ!(clƽ{   AA pAAAA AAA AA AA A Бɼ:rr     C (Ջ~0f ^VQTLCEBBCb|bjAK:{{;;231cJ))J12)0s"s!kc ZZJJUՀժUU++U++++UUUUUUUՀՀUՀՀՀ֌ժժUժժժժ@@@UՀժ!(clƽ{ !&&! 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