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]
