NEMA BU 1.2-2002(R2008) Application Information for Busway Rated 600 Volts or Less.pdf

NEMA Standards Publication BU 1.2-2002 (R2008) Application Information for Busway Rated 600 Volts or Less Published by National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, Virginia 22209 www.nema.org © Copyright 2008 by the National Electrical Manufacturers Association. All rights including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literary and Artistic Works, and the International and Pan American Copyright Conventions. NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. 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BU 1.2-2002 (R2008) Page i CONTENTS Page Foreword ii Section 1 SCOPE1 Section 2 REFERENCED STANDARDS2 Section 3 RESISTANCE, REACTANCE, AND IMPEDANCE3 3.1Method to Determine Resistance, Reactance, and Impedance3 3.1.1Readings Taken During the Temperature-Rise Test3 3.1.2Calculate the Average Phase-to-Neutral Impedance Z3 3.1.3Calculate for Each Individual Phase .4 Section 4 VOLTAGE DROP6 4.1Voltage Drop Ratings 6 4.2Voltage Drop Test for Three-Phase BuswaysGeneral 6 4.3Calculation of Three-Phase Voltage Drop and Voltage Drop Deviation .6 4.3.1Average Phase-to-Phase Voltage Drop6 4.3.2Phase-to-Phase Voltage Drop (VD) for Each Phase7 4.3.3The VDavg Calculated in Paragraph 4.3.2 .7 4.3.4The Percent Voltage Drop Deviation Per 100 Feet 7 4.4All Voltage Drops and Deviations Indicated in Section 4.3.7 4.5The Voltage Drop of the Busway.7 4.6All Preceding Voltage Drop Formulas.8 Section 5 RESISTANCE WELDING APPLICATION9 5.1General9 5.2Current Carrying Requirements 9 5.2.1Group of Welders10 5.2.2Single-Phase Distribution Systems.10 5.2.3Three-Phase Distribution Systems .10 5.3Voltage Drop Requirements10 5.3.1General11 5.3.2Determine Total During-weld kVA for Voltage Drop Calculations 11 5.3.3Determine Total During-weld Current for Voltage Drop Calculations.11 5.3.4Determine the Welder Multiplier for Voltage Drop Calculations .12 5.3.5Determine the Voltage Drop12 5.4Example Of Determining Proper Busway For Resistance Welder Application.12 5.4.1Example of Current Carrying Requirement Calculations12 5.4.2Example of Voltage Drop Requirement Calculations12 5.5Summary .13 Table 5-1 DUTY CYCLE MULTIPLIERS.9 Figure 3-1 METER CONNECTIONS 4 © Copyright 2008 by the National Electrical Manufacturers Association. BU 1.2-2002 (R2008) Page ii Foreword This Standards Publication is intended to provide a basis of common understanding within the electrical community. The purpose of this Standards Publication is to provide a guide of practical application information for busway rated 600 volts or less. User needs have been considered throughout the development of this publication. Proposed or recommended revisions should be submitted to: Vice President, Engineering Department National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, Virginia 22209 This Standards Publication was developed by the LVDE 04 Busway Product Group of the LVDE Section. Approval of the publication does not necessarily imply that all members voted for its approval or participated in its development. At the time it was approved, the Group/Section was composed of the following members: GE Industrial SystemsPlainville, CT Siemens Energy thus reasonable assumptions should be made for these varying quantities and then used for the following determinations. To determine the busway current carrying capacity required, it is necessary to convert the intermittent welder loads to an equivalent continuous load or effective kVA. If the during-weld kVA demand and the duty cycle for a welder are known, the effective kVA can be obtained by multiplying the during-weld kVA demand by the square root of the duty cycle divided by 10. The duty cycle is the percentage of the time during which the welder is loaded. For simplicity sake, multipliers for various duty cycles are listed in Table 5.1. Based upon the welders duty cycle, the proper multiplier is chosen. This multiplier times the during-weld kVA demand determines the effective kVA. Table 5-1 DUTY CYCLE MULTIPLIERS Percent Duty Cycle Multiplier 50 0.71 40 0.63 30 0.55 25 0.50 20 0.45 15 0.39 10 0.32 7.5 0.27 5 or less 0.22 © Copyright 2008 by the National Electrical Manufacturers Association. BU 1.2-2002 (R2008) Page 10 If the during-weld kVA demand is unknown, it can be assumed to be 70 percent of the welder secondary short-circuit kVA. If both the during-weld kVA and the duty cycle are unknown, the effective kVA can be assumed to be 70 % of the nameplate kVA rating for seam and automatic welders and 50 percent of the nameplate kVA for manually operated welders other than seam. Nameplate kVA rating is defined as the maximum load that can be imposed on the welding machine transformer at a 50 % duty cycle. 5.2.1 Group of Welders It has been found by actual measurement that the total effective kVA of a group of welders is equal to the effective kVA of the largest welder plus 60 % of the sum of the effective kVA of the remaining welders. Once the total effective kVA has been determined, the busway current carrying requirement can be easily calculated as follows: 5.2.2 Single-Phase Distribution Systems (Total Effective kVA) x 1000 (Busway Current carrying requirement) = (Line to Line Voltage) 5.2.3 Three-Phase Distribution Systems (Total Effective kVA) x 1000 (Busway Current carrying requirement) = (Line to Line Voltage) x 3 5.3 VOLTAGE DROP REQUIREMENTS To assure consistently good welds, the overall voltage drop in a distribution system should be limited to 10 percent. In some instances this may be excessive; therefore, specific permissible voltage drop information should be obtained whenever possible. The overall 10% value includes voltage drop in the primary distribution system, the distribution transformers, and the secondary distribution system. The voltage drop in the primary distribution system can be obtained from the power company provided the maximum kVA demand and the power factor of the largest welder is furnished. The voltage drop in the distribution transformer can be calculated from the formula: (Voltage drop Percent) = (During-weld kVA) x (Transformer Impedance Percent) (Transformer kVA Rating) Voltage drop curves for busway can be used as a basis for determining the voltage drop in the secondary distribution system. It is general practice to permit 2 % voltage drop in the primary distribution system, 5 % in the distribution transformer, and the remaining 3 % in the secondary distribution system. © Copyright 2008 by the National Electrical Manufacturers Association. BU 1.2-2002 (R2008) Page 11 5.3.1 General Voltage drop for welder circuits can be determined in the same way as for conventional circuits except that it must be based on a welder multiplier factor which equates to the total during-weld current divided by the busway current rating. 5.3.2 Determine Total During-weld kVA for Voltage Drop Calculations Large welders are sometimes interlocked to prevent excessive voltage drop caused by the possibility of simultaneous firing. In such cases, it is necessary to consider only the largest of the interlocked welders in calculating voltage drop. a) Total the nameplate kVA ratings of all large production or butt welders, excluding interlocked welders. b) Total the nameplate kVA ratings of all other non-interlocked welders. c) Record the nameplate kVA rating of the largest of any interlocked welders. The during-weld kVA can be assumed to be approximately 4 times the nameplate kVA rating for large projection or butt welders and 2 1/2 times the nameplate kVA rating for other types. 1) Multiply the total from “a” above by 4. 2) Multiply the total from “b” above by 2-1/2. 3) Multiply the number from “c” above (if any) by either 4 or 2-1/2 as applicable. 4) Sum the total of 1, 2 and 3. Total kVA of all non-interlocked large production or butt x 4 Total kVA of all other non-interlocked x 2.5 Largest Interlocked kVA x (4 or 2.5 as applicable) _ Total During-weld kVA This is the total during-weld kVA for Voltage Drop Calculations 5.3.3 Determine Total During-weld Current for Voltage Drop Calculations Multiply the total during-weld kVA (see 5.3.2) by 1000. Divide by the line to line system voltage times the square root of 3. (Total during-weld kVA) x 1000 (Total During-weld Current) = (Line to Line Voltage) x 3 This is the total during-weld current for Voltage Drop Calculations. © Copyright 2008 by the National Electrical Manufacturers Association. BU 1.2-2002 (R2008) Page 12 5.3.4 Determine the Welder Multiplier for Voltage Drop Calculations Divide the total during-weld current (see 5.3.3) by the proposed busway current rating. (Total during-weld current) (Welder Multiplier Factor) = (Busway Current Rating) This is the welder multiplier factor for Voltage Drop Calculations. 5.3.5 Determine the Voltage Drop Determine the voltage drop of the proposed busway from the manufacturers data for the appropriate power factor and distance the same as for conventional circuits. Multiply this voltage drop by the welder multiplier factor (see 5.3.4). 5.4 EXAMPLE OF DETERMINING PROPER BUSWAY FOR RESISTANCE WELDER APPLICATION It is desired to determine the minimum size busway that will meet current carrying and voltage drop requirements for an industrial plant with 440-volt, 3-phase, 3-wire service. The busway is to supply the following group of welders which are balanced on the phases and evenly distributed along a 200 foot feeder run: (1) 300 kVA butt, (1) 175 kVA butt, (1) 150 kVA seam, (4) 100 kVA spot, (5) 50 kVA spot, (10) 5 kVA spot. The welders are manually operated and the 300 and 175 kVA welders are interlocked to prevent their firing simultaneously. Power factor of the welders is given as 40 % and permissible voltage drop in the feeder duct is 3 percent. Specific information regarding during-weld kVA and duty cycles is not available. 5.4.1 Example of Current Carrying Requirement Calculations a) Effective kVA of largest welder 300 x 50% = 150 kVA. b) Effective kVA of seam welder 150 x 70% = 105 kVA. c) Effective kVA of remaining welders 700 x 50% = 350 kVA excluding the interlocked 175 kVA welder. d) Total effective kVA 150 + (105 + 350) x 60% = 423 kVA. e) Equivalent continuous current: amp555 3 1000 440 kVA423 Thus, 600-amp low-impedance busway will meet the current carrying requirement. 5.4.2 Example of Voltage Drop Requirement Calculations a) Total nameplate kVA of butt welders-300 kVA excluding the interlocked 175 kVA welder. b) Total nameplate kVA of remaining welders-850 kVA. c) During-weld kVA of butt welders 4 x 300 = 1200 kVA. d) During-weld kVA of remaining welders: 2 1/2 x 850 = 2125 kVA. e) During-weld kVA is 1200 + 2125 = 3325 kVA. f) Three-phase during-weld current: © Copyright 2008 by the National Electrical Manufacturers Association. BU 1.2-2002 (R2008) Page 13 amp4370 3440 1000kVA3325 For example, using the voltage drop calculations shown in section 4. At 40 % power factor the voltage drop per 100 feet of 600 ampere low impedance busway carrying rated load would be about 2.7 volts. Since the load is distributed, use half this value. Voltage drop for feeder system is: volts6.19feet200 feet100 volts7.2 2 1 A600 A4370 %.5.4or 440 6.19 is drop voltage Percent This exceeds the permissible voltage drop of 3 %, and it will be necessary to go to a larger size busway. An 800 ampere low impedance busway would have a voltage drop of 3.3 %. Because of the conservative nature of the assumptions made, this would be the logical choice. 5.5 SUMMARY Since it is difficult to obtain specific information concerning the operation of welders (particularly in new installations) and to determine accurately the possibilities for simultaneous firing of the welders, exact solutions to problems of distribution systems for resistance welders are not feasible. In the example, it was stated that the load was balanced and distributed. In actuality, it is extremely difficult to balance the load, and distribution may be far from uniform. In the case of unevenly distributed loads, it may be necessary