forwarding for [email protected]
____________________Reply Separator____________________ Subject: RE: Using fuse to protect wire undersized for load? Author: [email protected] List-Post: [email protected] Date: 9/14/00 9:28 AM The way I understand it they used a 4:1 step down transformer between the input supply and the 0.26 ohm load. The SCR based circuit is there to chopped the input waveform feeded to the transformer allowing to modulate the actual loading of the circuit. Now we have 120V divide by 4 which is 30V apply to the 0.26ohm load giving 115.4A into the load. This is the worst case load current. It thus seems that a 300A rated cable will meet the 125% load requirement. The modulation introduces by the SCR based circuit will only reduce the voltage appearing across the load and will just reduce the amount of current draw by the 0.26ohm load. Opinion express here are my own and do not necessarily reflect those of my employer. Daniel Sicard Compliance Engineer / Ingénieur Certification Marconi Communications - Optical Network Corp Tel: 514-685-1737 Ext. 4631 Fax: 514-822-4077 E-mail: mailto:[email protected] Web: http://www.marconi.com Jim Eichner <[email protected]> on 09/13/2000 04:34:28 PM Please respond to Jim Eichner <[email protected]> To: "'EMC-PSTC - forum'" <[email protected]> cc: (bcc: Daniel Sicard/MAIN/MC1) Subject: RE: Using fuse to protect wire undersized for load? Either I'm missing something, or you won't be able to run the load, so code issues are a moot point. You point out the "equivalent" fusing is 200A and you have a 462A load. You will blow the fuse as soon as you power up, every time! In the general case, you are right: the code does not consider basing your wire size on your fuse size. At least not in that order. It tells you the wire ampacity has to be 125% of the continuous load, and the fuse or breaker has to be the next size up from that ampacity. Regards, Jim Eichner Sr. Regulatory Compliance Engineer Xantrex Technology Inc., Mobile Markets [email protected] Any opinions expressed are those of my invisible friend, who really exists. Honest. -----Original Message----- From: Crane, Lauren [mailto:[email protected]] Sent: Wednesday, September 13, 2000 1:00 PM To: [email protected] Subject: Using fuse to protect wire undersized for load? Colleagues, I have an internal client who is proposing a design. In this design, a resistive heating load is being supplied by conductors that are far below the 125% of load recommended/required by the NEC or NFPA-79 (ref 1994 sect 15.5.3). The client argues that it is okay that the supply conductors are far below because they are fused at a level that will protect them. .26 ohm load driven at 120 volts max Full Load Amps calculates to 462 amps. 125% = 577 amps. 60degC wire size should be 700 mcm per NFPA-79 1994 table 11. Client proposes using 2/0 90degC wire because drive circuit is an SCR controller fused to 50 amps feeding a 4:1 step down transformer. They rationalized max current (given 50A fuses) is 200 amps on the secondary. 2/0 90degC wire is rated to 300 amps per NEC table 400-5b. It seems clear to me that the codes say the wire should be sized to the load. But the codes don't seem to say that wires SHOULD NOT be sized to thier over current protection (regardless of the load). Does anyone have a pro or con rational for undersizing the conductors to a load provided they are protected correctly by a fuse or breaker? Thanks in advance. Lauren Crane (my own opinions, not my employeer's) * Product Design Safety & Compliance Manager * Ion Beam and Thermal Processing Systems * Axcelis Technologies Inc. * 108 Cherry Hill Dr. * Beverly, MA 01915 * 978.921-9745 [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
