1978 SAE 300 duty cycle ??

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I am courious to know what the approximate duty cycle would be at 350 amps.  The nameplate states 60% at 300 amps but duty cycles are not linear so, does anyone have any idea what the duty cycle could be at 350 amps?  I need to know because I have a job involving air arcing and plan to use 5/16 carbons at 350 amps.  Thank you very much.
Reply:I make it about 19%
Reply:Thank you transit.  I have no knowledge to dispute your claim of 19%.  Can you fill me in as to how you came to that number?  I am not trying to be a jackass, just courious about the math you used. thank you again.
Reply:This is the only site that I found with a sensible answer after I derived the same formula. Welder Duty Cycle Calculations, and Thermal Stresshttp://ezinearticles.com/?Welder-Dut...ess&id=2414619I started with the premise that the heat in a circuit is proportional to the current squared. Im= maximum current.Iw= working current. Qm= maximum heatQw= working heatDC= Duty Cycle[Im X Im]  is proportional to the heat QmQw = DC x Qm orQw/Qm = DCSubstituting:Therefore  [Iw X Iw] / [Im X Im] = Duty Cycle. This number is less then one.Part IGiven 60% Duty Cycle @ 300 amps. What is the maximum current. Solving the expression for Im:Im =Iw/ square root of DC 300/0.7746 = 387.3 amps.Part IIAt 350 amps what is the Duty Cycle?Using  [Iw X Iw] / [Im X Im] = Duty Cycle. [350 x 350] / [389.3 x 387.3] = 0.8166 or 81.66% however that’s not what we’re looking for. We’re looking for the difference from the maximum.Therefore 100% - 81.66% = 18.33% we’ll round up to 19% or just under 2 minuetsLast edited by transit; 08-03-2009 at 07:43 PM.
Reply:Originally Posted by transitThis is the only site that I found with a sensible answer after I derived the same formula. Welder Duty Cycle Calculations, and Thermal Stresshttp://ezinearticles.com/?Welder-Dut...ess&id=2414619I started with the premise that the heat in a circuit is proportional to the current squared. Im= maximum current.Iw= working current. Qm= maximum heatQw= working heatDC= Duty Cycle[Im X Im]  is proportional to the heat QmQw = DC x Qm orQw/Qm = DCSubstituting:Therefore  [Iw X Iw] / [Im X Im] = Duty Cycle. This number is less then one.Part IGiven 60% Duty Cycle @ 300 amps. What is the maximum current. Solving the expression for Im:Im =Iw/ square root of DC 300/0.7746 = 387.3 amps.Part IIAt 350 amps what is the Duty Cycle?Using  [Iw X Iw] / [Im X Im] = Duty Cycle. [350 x 350] / [389.3 x 387.3] = 0.8166 or 81.66% however that’s not what we’re looking for. We’re looking for the difference from the maximum.Therefore 100% - 81.66% = 18.33% we’ll round up to 19% or just under 2 minuets
Reply:Transit, thank you very much for your time in this problem.  Honestly, I was hoping the duty cycle for 350 amps would be a little more than 2 out of 10 minutes.  The max on the dial of the machine is 445 amps although I 've never had the machine load banked- I can only assume the max would approximate that of on the dial (445 amp).  I am not a math guy and wont pretend to be.  Thanks again.