There are TWO issues here. One - current carrying ability over a long period of time, two - current carrying ability over a short period of time. For extended use, basic good engineering practices say, the trace has to be able to carry the current for which it's fused. As if it were solid wire. Look up AMAPACITY in the Electrical Engineer's Handbook. Find the current you need at the max. temp you'll experience in the Underwriter's Laboratories column. Cross over to the gauge wire for it. Look up the SQUARE MILS of the wire. From your known trace thickness calculate your width. Short time stressing for wires should be under a second. Fuses typically react well under a second. Be careful if you are doing trace calculations for Primary voltage carrying traces. Doug McKean ADC Video Systems [log in to unmask] ______________________________ Reply Separator _________________________________ Subject: des re: hot trace width Author: [log in to unmask] at internet-mail Date: 4/8/96 4:10 PM Andy Pedersen asked about trace with to current capacity requirements. In addition to Kevin L. Seaman's variables (conductor width, thickness and current) the board layout has an impact also, in a flex design, The test folks caused a short (get to blame them _this_ time ;) ) that put 4 amps on a .013 trace. Where the trace was near other copper, the board survived, but de-laminated in more open areas. (and we still don't realy know how _long_ it was shorted) (The trace is still connected & not shorted to any near by traces) In "Electromechanical Design" by Ronald A Walsh, pg 255, 6.6.10 "Fusing Time-current for copper connections" he quotes an equation by I.M. Onderdonk: (let's see if I get this right..) 33(I/A)**2 * S = log10(((Tm-Ta)/234+Ta)+1) I=A * ((log(((Tm-Ta)/234+Ta)+1))/(33 * S)) **.5 I=current, Ampers, A=conductor area, cir mils, S=time current applied, seconds, Tm=melting point of copper in dec C, Ta= ambient Temp in deg C. Has any one used this? I haven't tested it out ( we try to keep the smoke _in_ the parts don't you know ;) ). Bill Gaines AeroJet (for $) IHPVA (for speed & fun) IEPS (for info) SCCA (for trophys) [log in to unmask]