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In general how do you decide what trace width to use?

I prepared a PCB layout with the trace width mainly 0.4064 mm and couple of them are 0.3048 mm. power ratings are 5V , 12V at max 0.4A. What would be the smallest trace width that would be fine with that ratings?

I am going to have a trace for a power unit. I will connect the outlet supply (220-240V), what trace width would you recommend for it?

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There are IPC standard calculations for trace widths based upon current draw. (IPC-2221)

These take into account freq, temperature rise, what layers of the board the traces are and many other factors.

There are many web calculators out there - http://circuitcalculator.com/wordpress/?p=25/

My absolute favorite tool for calculating many of the variables in pcb design is a free tool from Saturn PCB. It is free and I am not associated with them in any way.

This tool is great and will answer all your questions from above.

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    \$\begingroup\$ I should also note that you may want to look at what your board house can manufacture at a nominal cost as far as trace widths go. Normally i think that 5mil is the smallest before you start incurring extra costs. But I am sure this varies from fab house to fab house. I try to use 7mil as my smallest default trace. \$\endgroup\$
    – EE_PCB
    Jul 2, 2013 at 16:47

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