Lauren, To add to Peter's comments below, another relevant factor might be the required observation of termination temperatures and conductor ampacity and therefore selection per NEC art. 110-14(c). Without knowing more of the product nature or implementation, the requirement may yet add further argument to increasing conductor size beyond 2/0. My 2 cents and not that of my employer. Kaz Gawrzyjal , P. Eng. Sr. Product Safety Engineer ---------------------------------------------- Sanmina Canada ULC Wireless Development Centre 2924 11 Street NE Calgary, Alberta Canada, T2E 7L7 tel: 403-769-4805 (ESN 758) fax: 403-769-4813 (ESN 758) e-mail: [email protected] <mailto:[email protected]> , [email protected] <mailto:[email protected]>
-----Original Message----- From: Tarver, Peter [SC1:9031:EXCH] Sent: Wednesday, September 13, 2000 5:02 PM To: [email protected] Subject: RE: Using fuse to protect wire undersized for load? Lauren - I'm not familiar with NFPA79, so I'll answer only with regard to NFPA70 (the US NEC). The 125% issue in the NEC is for two items: attachment plugs sizing and circuit breaker sizing (1/1.25 = 0.80), both relative to load current. Attachment plugs for equipment that contain motors must be rated for not less than 125% of the steady state operating current. General purpose circuit breakers need to be sized at 80% of steady state operating current for all branch circuits, unless marked for 100% load rating when used as a switch for lighting circuits (IIRC, this marking is "SWD"). Fuses, on the other hand, are useable up to 100% of their marked rating. Wiring need must be sized according to the applicable load current and overcurrent protection provided according to the wire size and load current. The above, of course, can be used outside these ratings, if under engineering supervision and acceptable to the Authority Having Jurisdiction. Use at lesser currents than rated is just fine for wiring, fuses, circuit breakers and attachment plugs. It's difficult to comment further, without knowing more detail about the product, since speculation can get far afield from your specific case. BTW, I hope your client is aware of Sections 400-7 and 400-8 of the NEC, as well as the notes to Table 400-5(b). I would have normally expected to use the tables in Article 310 for this. Regards, Peter L. Tarver, PE [email protected] -----Original Message----- From: Crane, Lauren 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]
