This is exactly the reason I created the ATX Adapter board, you just plug it in and turn on.
Yes you absolutely need to ground PIN 16 (green wire) for the PSU to work.
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LoginGives good information for all those wires
I would always use more than one wire just to lower the resistance even over the foot of wire from the PSU.
One will work - four or more 12v wires and ground wires would be better and safer.
Why Safer?
Wire size should be 18guage but seems to be trending on newer supplies to 20 guage, now an 18 gauge wire is rated somewhere up to 15A for chassis wiring and lower for any length and modern 400w/500w supplies will output 25A plus on the 12v, more than enough to overheat wiring and be a fire risk if only one wire is used and it gets shorted out.
yeah i know that the PSU should shutdown but....
Low voltage - high current is stuff that us aussies are used to.