Max Current Through Broken Strands

Max Current Through Broken Strands

This calculator uses equations developed by Preece and Onderdonk to estimate the amount of current that will cause a copper wire to melt or burn out. With this calculator you can explore the maximum current of wires with broken strands.

Wire information



Wire Gauge

___ gauge (with __ strands where __ strands are broken)

Wire Diameter

___ mils

Wire Area

___ cmils

Area per Strand

___ cmils

Area of Conducting Strands

___ cmils

Preese Current

___ Amps (A rough estimate of the amount of current needed to melt the wire)

Onderdonk Current

___ Amps (An estimate of the amount of current that will melt the wire in about __ seconds)

Max Current

___ Amps


  • 1 mil = 0.001 inches
  • 1 cmil (circular mil) = the area of a circle with 1 mil diameter = 785.4x10-9 square inches
  • The area (A) in circular mils of a wire with diameter (d) mils is: A = d2
  • Ambient temperature is assumed to be 25°C
  • The melting point of copper is assumed to be 1084.62°C
  • Heat lost in the connections to the wire is neglected (thermal end effects are ignored)
  • Only heat lost to the air is considered
  • This model works well for wire gauges less than 20 when 0.75 inches of strands are exposed to the current
  • If broken wire strands act as heat sinks (due to small separation distance between broken strands) the fuse current will not be much lower than copper wire without any broken strands.

References: The Preece and Onderdonk equations for fusible metals were found in the Standard Handbook for Electrical Engineers, 15th Edition 2007, Ed. by H. Wayne Beaty and Donald G. Fink. McGraw Hill 4-25