IPC FTW

specifically IPC-2152 - Standard for Determining Current Carrying Capacity in Printed Board Design

Ampacity and fusing currents

Breadboard wires are typically single-strand 24 AWG wire. Current flowing through these small wires causes them to heat up (like all resistances do); too much current and the wires melt — specifically, the insulation melts.

Use the data from Wikibooks: Engineering Tables/American Wire Gauge to compute the following quantities:

  1. Resistivity of copper in Ω·m

  2. Conductivity of copper in (Ω·m)-1

  3. Current density in A/m2 at the Preese fusing current of a 24 AWG wire

  4. Current density in A/m2 at the Preese fusing current of a 12 AWG wire

You might expect that because a 12 AWG wire is about 16× the cross-sectional area of a 24 AWG wire[1], then the fusing current would also increase by about 16× — fusing at the same current density flux.

  • Why does the fusing current density change?



Think first, then click for an explanation

Melting wires has to do with heat transfer.

Power absorbed in the wire heats it, increasing the wire’s temperature. Eventually, the temperature will reach the melting point of copper (1084 °C) and …​ game over.

However at the same time, heat is lost to the environment via radiation and convection, counteracting the temperature rise. When the system reaches steady-state, the electrical power absorbed will equal the heat power lost at some elevated temperature.

Both convection and radiation depend on (the wire’s) surface area (\(2\pi r h\)), but, as we learned in Lesson 10, power is absorbed per volume (\(\pi r^2 h\)).

The ratio of volume to surface area (or cross-section area to circumference in 2-D) is proportional to diameter, meaning that the power absorbed increases faster than the “cooling power” as the wire diameter increases. This effect makes for higher temperatures for larger wires at the same current density.

A similar volume to surface area effect was involved in the construction technique for the Hoover Dam. It would have taken about 125 years to cool off from the heat generated from the concrete curing process without active cooling. See The Story of Hoover Dam - Concrete.

1. The area doubles for every 3 AWG decrease, approximately.