forwarding for [email protected] ____________________Reply Separator____________________ Subject: RE: Using fuse to protect wire undersized for load? Author: Geoff Lister <[email protected]> List-Post: [email protected] Date: 9/14/00 10:37 AM
Lauren, I am not familiar with high current fuses, but I would expect their behaviour to be similar to those of lower value. A 50 Amp fuse is designed so that in normal service it will never blow at 50 amps. To allow for production spreads, most of the specs permit a fuse to withstand a continuous current of between 150% & 170% of their rating. I believe that UL248 permits a fuse to carry 200% rated current for up to 100 seconds before blowing. This would translate, in your application to a continuous current of up to 340 Amps, and 400 amps for up to 100 seconds. I am not familiar with the wire specifications that you mention, but it does look as though a thicker wire would be required. Regards, Geoff Lister ([email protected]) Senior Engineer Motion Media Technology Ltd. Horton Hall, Horton, Bristol, BS37 6QN, UK Voice direct +44 (0) 1454 338561 Voice switchboard +44 (0) 1454 313444 Fax +44 (0) 1454 313678 http://www.motion-media.com -----Original Message----- From: Crane, Lauren [mailto:[email protected]] Sent: 13 September 2000 21:00 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]
