# Multi-volt DC power supply ATX - Shared grounding current question

#### RogueRose

Joined Oct 10, 2014
375
My PSU is rated at 650w and has 22 18g ground wires. I'm setting up connection points for the seperate voltages & ground and am wondering how many wired I need for the ground. I'm curious how current works when there are separate voltages sharing a ground.

If I have 20A running on 12v and 15A on 5v and 10A on 3.3v, is the current capacity needed for the ground = 35A (sum total of amps of each V)? Since there were so many I thought it might have been something like amps = wattage/lowest voltage so in this case (240 + 75 + 33)/3.3

I've also found WIDELY varying #'s for max amp carrying load from 1.3A to 16A per ft of 18g and some don't mention strands. Wiki has it at 14A. http://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes

#### ErnieM

Joined Apr 24, 2011
8,296
The number of wires you need to use is not based on the capacity of the wires themselves, but what voltage drop you can accept.

One computer chassis I was just working on looses 0.2 volts from the PSU to the backplane over a short run of four #8 wires... But that has a 100 amp load on it. (don't do any computations with those numbers, they are all from memory.)

Obviously you want zero drop, but you can't get that short of using superconducting wires. The ATX supply regulates the voltage inside of itself, making you live with whatever drop occurs in the wiring. More wires means less of a drop.

You can either start by adding one wire at a time until the load side sees the correct voltage, or bite the bullet early and use them all right from the start.

#### RogueRose

Joined Oct 10, 2014
375
The number of wires you need to use is not based on the capacity of the wires themselves, but what voltage drop you can accept.

One computer chassis I was just working on looses 0.2 volts from the PSU to the backplane over a short run of four #8 wires... But that has a 100 amp load on it. (don't do any computations with those numbers, they are all from memory.)

Obviously you want zero drop, but you can't get that short of using superconducting wires. The ATX supply regulates the voltage inside of itself, making you live with whatever drop occurs in the wiring. More wires means less of a drop.

You can either start by adding one wire at a time until the load side sees the correct voltage, or bite the bullet early and use them all right from the start.

Thank you for clearing that up! So a short 6" run shouldn't matter to much and doesn't need nearly so many wires.

#### ErnieM

Joined Apr 24, 2011
8,296
Thank you for clearing that up! So a short 6" run shouldn't matter to much and doesn't need nearly so many wires.
No it matters so much and you may need every single wire in the cable and then some.

Joined Jul 18, 2013
25,227
Keep in mind when using PC P.S. that the common is also generally connected to Earth ground.
Max.