It's useful to keep a trace calculator handy too, eg

http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator
/

When drawing parts for a library it's important to know what
width traces are likely to be connected to it, particularly power
components like relays, switches and connectors that need to
carry more than signal currents. Then later, when the part is
placed on the board, you can use the restrict layers to keep
low current/low voltage lines (like relay coil traces) out of the
high current/high voltage areas


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