Calculating diamter of wire that will "blow" if current exceeds 1A 1. You need to design a 1.0 A fuse that "blows" if the current exceeds 1.0 A. The fuse material in your stockroom melts at a current density of 500A/cm^2. 2. J = I/A 3. J = 500A/m^2 = 50000A/m^2 = 1.0A / (pi*r^2) r = sqrt[1/50000*pi] = 0.00252 m diamter = 2*r = 0.005 m = 5.0mm That is incorrect (answer in mm).