ࡱ > ܥh W e " ܇ x v x v v~ v~ v~ v~ v~ ~ ~ ~ ~ ~ ~ 4 ~ ~ φ C Ԃ Ԃ Ԃ Ԃ Ԃ 6 & Z \ \ \ - X j r φ v~ 6 Ԃ Ԃ 6 6 φ 6 v~ v~ Ԃ 6 6 6 6 v~ Ԃ v~ Ԃ Z @[~ ~ " v~ v~ v~ v~ 6 Z 6 $ 6 Simplified 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 *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
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