Practical Electronics/PCB Layout/Trace Current Capacity
Because PCB traces have resistance, they will heat up as current flows through them. As any heat buildup is continuously being removed by the air and the PCB substrate, the trace will heat up to a point where the heating effect of the current is balanced by the cooling. The amount of resistive heating is governed by:
- The width of the track
- The thickness of the copper
- The resistivity of the copper (generally fixed - this page deals with a "standard" value only).
This page gives information on the maximum current that can be passed through a trace of a certain width and thickness in order that the temperature does not rise above certain levels.
The information presented here is for guidance only. Please exercise caution when calculating current limits. No liability is taken for any damage caused by the information presented below being incorrect. Always leave a comfortable margin of error.
Also please note that this information does NOT take into account things like PCB composition, air flow, humidity, air pressure, and so does not necesarily give accurate results for your application. It is not advised to extrapolate the data beyond that given.
The graphs that follow are based on the equations below:
For external traces:
For internal traces:
- A is the cross-sectional area of the trace, in units of...;
- Imax is the maximum current allowable in the trace in order not to exceed ΔT, in units of...
The values for internal traces are lower because the PCB substrate is a good thermal insulator, so the energy is not dissipated as fast.
The equations are based on
Current vs. Trace Width
This section contains graphs that show the maximum current allowable for given trace thicknesses. The maximum currents are given for changes in temperature of 10, 15, 20, 30, 50, 100 and 110 degrees Celcius.
It is common on commercial consumer through-hole PCB's (typically PSU's) to leave the solder mask off PCB traces in order to have the traces coated with solder during the wave soldering process. This coating of the traces with solder increases the current handling capability of the trace, effectively "for free" compared to to higher cost of thicker copper substrate. The solder coating thickness can vary widely, so the exact reduction in resistance cannot be reliably calculated. But empirical testing shows roughly a 50% reduction in trace resistance. 
- ANSI Trace Width Calculator An on-line calculator for determining trace widths to the ANSI guidelines.
- "EEVBlog #317 - PCB Tinning Mythbusting"