Doug, I agree that Belden's catalogue is an excellent guide. However, you should be aware that it is conservative. When I called Belden, they said they have to be conservative because they don't know the boundary conditions.
Heat is removed from wire by convection and radiation along the length. and by conduction through the end terminations. Since Belden's tables provide advice on various conditions of convection, all kinds of termination, and a variety of lengths, their numbers are not exact. My experience is that it is much more practical to "build one and measure it" than to rely on simple equations. Only the military can afford full-bore computerized thermal modelling, and that's not quick or easy. Pete > Hi Richard, > > I don't claim to be an expert but have you tried the > back of Belden's catalogue to see if it compares? > > ***** Begin Quoted Material ***** > > Current Degree Rise above ambient > Amps 10 C 35C > > 17 10 AWG 14 AWG > > ----- > > No. of Conductors Factors > > 1 1.6 > 2-3 1.0 > 4-5 0.8 > 6-15 0.7 > 16-30 0.5 > > > ***** End Quoted Material ***** > > The decrease in factors is a result of external wires > thermally insulating interior wires of a wire bundle. > > Regards, Doug > > > At 01:47 PM 11/3/98 +0000, Richard Steele wrote: > >Dear All, > > > >I'm trying to make sure that some conductors carrying 230V r.m.s in an > >ambient temperature of 70oC have a large enough conductor and the > >correct insulation to withstand a short circuit current of 17A for 30 > >minutes. > > > >Does anyone know which formulae are appropriate? > > > >I have tried the calculations in BS7454 : 1991 "Calculations of > >thermally permissible short-circuit currents, taking into account > >non-adiabatic heating effects" and the results don't seem to make sense. > > > >So here are the parameters > > > >Max current = 17A > >Start temp - 20oC > >Conductor size - 1.5mm2 > >Time period - 30 mins > >Conductors not bunched > > > >I'm trying to find out what the final PVC insulation temperature will > >be, all the catalogues that I have only seem to give the current at 20 > >or 30oC. > > > >The results I have are: > >Adiabatic - 23291.48oC > >Non-adiabatic - 34.48oC > > > >The results are supposed to give the final temperature. For the > >non-adiabatic result, is a 14oC rise what you would expect or is this > >the > >whole rise 20 + 34.48 = 54.48oC? > > > > > >Are there any wire experts out there? > > > >Regards > > > >Richard > > > >Attachment Converted: "D:\Documents\Web Docs\vcard7.vcf" > > > ---------------------------------------------------------------------------- Peter Van Raalte | Email: [email protected] Compliance Engineer | V-Band Corporation | Phone: 914-789-5117 565 Taxter Road | Elmsford NY 10523 | FAX: 914-347-3432 ---------------------------------------------------------------------------
