Hi Brian:
Wire ampacity ratings are based on the thermal characteristics of the wire insulation. The insulation will fail due to temperature long before the copper will melt. Wire temperature ratings are marked on the surface of the wire insulation. Temperature ratings range from 60 C to over 300 C. Wire temperature ratings are based on a single conductor in free air. As John Woodgate has pointed out, when wires are bundled together into a cable, all of the wires contribute to the temperature rise (and limit the thermal dissipation of individual wires), so each individual wire must be derated accordingly. In a cable, this derating can be very significant. Thermal degradation is a function of absolute temperature, not temperature rise (as has been specified in some safety standards). Thermal degradation is also a function of time at a high temperature. If a fault is likely to persist, then the insulation temperature rating should be based on the fault-condition temperature. If the fault current is not likely to persist (for example, due to a fuse or circuit-breaker operation), then the temperature rating may be based on the normal- condition temperature. Insulations do not fail instantaneously when the temperature exceeds the rated temperature. Wire terminations and connectors have contact resistance, which is almost always higher than the incremental resistance of the wire. Overheating is more likely at these points than the wire itself. (However, beware of coiled wire which acts like a bundle and can easily overheat even though the current is below rated ampacity of the wire.) Best regards, Richard Nute Product Safety Consultant Vancouver, Washington, USA > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf > Of Kunde, Brian > Sent: Tuesday, August 02, 2011 5:36 AM > To: [email protected] > Subject: RE: Electrical design challenge > > > Richard, > > You bring up a point which I have debated many times in the > past. Since the wire can handle more current in a fault > condition than it is rated, the only real way of knowing is > to do an overload test as you suggested? Slowly increase the > output load while measuring the wire temperature until the > PSU shuts down or the wire overheats. Since this would > simulate a fault condition, do we use 150ºC (calculated for a > 40ºC ambient) as our upper limit for the wire temp? Is this > how it is done? > > When we have done this test in the past we generally find out > the wire can handle much more current than we thought, but > connectors tend to overheat first, so don't forget to check > the connectors. > > It is too bad that the world isn't designed so you can take a > 10A output, run it through a 10A wire, a 10A connector, and a > 10 amp load and not have to worry about what happens in a > fault condition. > > The Other Brian > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
