Common ground gauge for multiple circuits, and amp rating of 7-pin terminals?

To determine gauge for a common ground, is it just adding the amps for each circuit that's using that ground? So a common ground for three 12v DC 20amp circuits would need to handle 60 amps minimum? I've always ran same size or larger gauge as power, but thats on a single circuit.

Also, anyone have a guess as to the amp ratings of those DIY 7-pin trailer plugs? I'm trying to plan a quick disconnect for a removable power station setup, three 20amp circuits and ideally a single common ground. I was thinking a 7-pin connector would do it if the terminals are rated high enough.

 

what's the anticipated draw? will all 3 20A circuits be drawing 20A 100% from the moment they're plugged in? are they large motors with a big initial inductive draw?

 

They are for USB outlets, 12v sockets, lights etc. 20A is just what the wire gauge and length will safely handle. Even if I have everything running at max output I won't see over 40A.

Circuit 1 is for 12v sockets for fridge and inverter;
Fridge is a max of 5 amps
Inverter is a max of 12.5 amps

Circuit 2 is USB outlets;
USB outlets are a max output of 5.1 amps each, or 10.2 total

Circuit 3 is lights and USB outlets in truck bed;
USB outlets are a max output of 5.1 amps each, or 10.2 total
Lights are .5 amps total

 

and all of those things will be used 100% of the time at their full rating?

 

Probably not, no. But better to plan for worse case scenario IMO.

 

If that is how you want to plan the "duty cycle" then you need a 40A rated wire run.

Pretty handy calculator to help you get it right.
https://www.wirebarn.com/Wire-Size-Calculator-_ep_41.html