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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Sizing Copper Traces
There are complicated curves available for copper versus temperature rise of PCB traces in the now-obsolete standard MIL-STD-275E. These curves have also found their way into more recent standards like IPC-2221 and IPC-2222. Engineers often try to create elaborate curve fit equations to match these curves. But the truth is the earlier curves can be easily approximated by simple linear rules as follows.
The required cross-sectional area of an external trace is approximately
1.  37 mils2 per Amp of current for 10°C rise in temperature (recommended).
2.  25 mils2 per Amp of current for 20°C rise in temperature (aggressive, but OK).
3.  18 mils2 per Amp of current for 30°C rise in temperature (usually not recommended).

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