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I'm designing a PCB that is acting as a pass-thru between a power supply and another device. Worst-case draw of the device is 12A @ 12V. The PCB will have a power in connector for the supply, and a power-out connector to power the connected device. I'm using traces on the PCB to route this power, so I don't have to have a separate wiring harness.

The PCB also has a few low-draw components on it that will draw off the 12V.

If the two power connectors are directly connected with large traces capable of handling the current, do I need to worry about the smaller traces that still technically route power from supply to device (albeit through much smaller traces and multiple other components)?

I'm assuming that the fat traces will have very low resistance as compared to the rest of the circuit and therefore most of the current will follow that path... but wanted to double check my thinking.

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  • \$\begingroup\$ Is this DC or AC, and if AC, what frequency? You're generally right but there's some frequency-dependent nuance. \$\endgroup\$ Commented Apr 24, 2025 at 1:26

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The main thing to worry about with PCB traces and current is the temperature rise - you need to make sure that at whatever your ambient temperature is, the PCB trace stays at a safe temperature (safe here meaning that none of the solder/glue/silkscreen/soldermask etc get damaged).

The temperature rise is dictated by the power dissipated by the trace, and the thermal resistance of the trace - the thermal resistance is a function of the size, but depends on a few other things - the important thing is that it's generally proportional to the square root of the width.

The overall power dissipation is easy to calculate - its just V=IR and P=IV. From this you can get P=V^2/R (importantly, V here is the voltage across the PCB, NOT the voltage that the trace is at (i.e. not 12V)). You can see then that the power dissipated is directly inversely proportional to the resistance of the trace, which is directly proportional to the width of the trace.

Finally, we can put this together and say that as the width goes up, the ability of the trace to dissipate heat goes up slower than the amount of heat disippated. This means that if your big trace is ok, then your smaller traces will be ok as well (as long as there are no dramatic changes in width along the smaller traces, and assuming its not AC).

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