This question started as a design specific question but is (now) more of a general question going forward onto other PCB product designs...

For now, I am designing an I/O PCB and want to mix supply voltage (+24V) and signal I/O onto one Molex (36-pin) type header. I plan to use 12 pins (non-adjacent) for +24V/0V, these will power (2-3mA/device) 12ea prox-type (PNP) sensors. The lion's share of the load will be 6 pins (adjacent) for +24V/0V that will power 3 external 2A contactors; duty-cycle TBD.

The datasheet does not offer any current/thermal/loaded-pins graphs but the general current rating (per pin) is 3A. My research is telling me that rating connectors for current/thermal is very subjective in terms of the datasheet's current rating per pin. Current density, plating, exposure to open-air, PCB/plug conductor size, ambient air temp and the list goes on; all play into the over-all power/thermal rating of any connector.

That said, no one-size-fits-all and to provide all of the details and worst-case scenarios could be exhaustive and perhaps this type of question is too general for ESE. I spent a reasonable amount of time looking for some technical guide-lines for rating connectors but have yet to find a one-stop-shop in an effort to demonstrate and design in due-diligence.

Finally, here is my question. How do I rate/de-rate connectors for ampacity?

  • \$\begingroup\$ I've had some luck with asking manufacturers directly for any derating or environmental testing they've done on specific parts. \$\endgroup\$
    – Bort
    Jun 5, 2018 at 16:09
  • \$\begingroup\$ Do you mean that if an individual contact is rated for 3 A, how much current can hold 3 of them in a row? Is this a concern? \$\endgroup\$ Jun 6, 2018 at 0:27
  • \$\begingroup\$ multiple pins (i.e. more than one +24/RETURN pair per load) to share the 3x (2A) stuff would seem reasonable to me \$\endgroup\$
    – Pete W
    Jan 4, 2021 at 21:13

1 Answer 1


Molex makes a vast selection of connectors each which have current ratings. So I’m not going to pick one out for you. Rather, I’ll give you a general design philosophy on connectors, but Molex does have a current ratings if you can find the specification on every connecter or contact.

But the so called “Molex 4 pin HDD connectors” in PC’s were rated for 10A with separate return contacts for +5V and +12V .

Generally look for this type of pin in cylindrical sleeve socket for high currents and not simply bifurcated (split or double point ) contacts that have small surface area contact. High wetting current contacts >2A are never gold plated while those for <=2A are generally gold plated with varying quality for gold thickness.

The same is true for all Relay contacts except they use a silver palladium alloy while connectors are generally plated with lower cost metal alloys such as nickel or zinc over steel , brass or beryllium copper for substrate.


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