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I have a PC PSU opened up. The yellow wires are 20AWG, 12V. According to the AWG table, a single 20AWG wire can deliver 1,5A Max on power transmission, so if I were to power up a device that needs 2.5A, I would need to bundle 2 of those yellow wires together so I can deliever 3A (over my device max draw) on the socket.

Am I understanding this correctly?

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  • \$\begingroup\$ Can you link to the table you are using? As far as wires in parallel, yeah, two wires doubles the capacity. Also, you need to double the GND wires, too. And how long are the wires? \$\endgroup\$ – mkeith Sep 18 '16 at 17:14
  • \$\begingroup\$ This is table I'm using: powerstream.com/Wire_Size.htm The wires are fairly short, 15cm/5,9inches \$\endgroup\$ – arave Sep 18 '16 at 17:16
  • \$\begingroup\$ The same table says "chassis wiring ... 20AWG ... 11 Amps" so don't worry about 2.5A over a 6 inch length. DO calculate the voltage drop over a foot - 10.15 milliohms, 2.5A, so about 0.026V ... fine unless your device is absurdly sensitive to PSU voltage. \$\endgroup\$ – Brian Drummond Sep 18 '16 at 17:36
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Am I understanding this correctly?

Two wires of half the cross sectional area at DC don't quite give you double the current rating. Take for example AWG 24 (84 ohms per km) and AWG 21 (42 ohms per km).

AWG 24 allows a maximum amps for power transmission of 0.577 amps whereas AWG 21 (close to double the cross sectional area) allows 1.2 amps. 2 lots of AWG 24 only allows a current of 2 x 0.577 = 1.154 amps. Additionally you would need to stop one wire heating the other one up so this means two lots of AWG 24 would need to be spaced apart a little bit.

This makes it somewhat inconvenient of course.

However, I would think that going from 1.5 A to 2.5 A by using 2 lots of AWG 20 would be OK and, of course, if the current passing were a high frequency superimposed on DC then you might be significantly better off given the skin effect seen by AC currents.

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  • \$\begingroup\$ that was a perfect example, I understand now how it works. Thanks a lot! \$\endgroup\$ – arave Sep 18 '16 at 18:23
  • \$\begingroup\$ Curious. At least here in Britain, larger CSA wires actually carry less current per square unit than smaller ones in terms of ratings. The reason being there is less radiating surface (which increases proportional to the radius) per square unit (which increases as the square of radius). So two wires spaced apart can carry more current than one wire of double the CSA, by absolute rating. In practice it depends anyway what temperature you can tolerate. You would derate in a hot environment for instance, and uprate for running through a freezer area. \$\endgroup\$ – Ian Bland Sep 18 '16 at 18:36
  • \$\begingroup\$ Also, Andy, you ought to add a very strong recommendation not to compute to 3 decimal places for this sort of problem. Just because the table uses it, I'm certain you know that that is an inappropriate level of precision. \$\endgroup\$ – WhatRoughBeast Sep 18 '16 at 19:08
  • \$\begingroup\$ @IanBland The figures quoted are for insulated cables I presumed (although having now checked it doesn't say) so, effectively there is more thermal insulation on two AWG 24 wires than 1 AWG 21 wire. I've seen another website that quotes exactly the same numbers. \$\endgroup\$ – Andy aka Sep 18 '16 at 19:25
  • \$\begingroup\$ @WhatRoughBeast thank you - you've saved me that job. \$\endgroup\$ – Andy aka Sep 18 '16 at 19:26
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Ampacity for transmission is based on temperature rise. - For short lengths you can exceed this, but you may be more interested in % voltage drop which is a function of both length and current.

  • First decide what voltage drop in connectors and wires is tolerable. (e.g. 1%)
  • Then determine length of cable x2 for (supply+return)
  • Then compute max resistance R=Delta V/I
  • e.g. 0.12V/3A= 400 mOhms then divide by length to get Ohms per unit length and choose proper wire gauge or bundle.

ref http://www.powerstream.com/Wire_Size.htm

For 1m length cable 0.4 Ohm/m reads AWG29 as smallest size. So if you are ok with 1% loss and 3A is peak and not average. So AWG20 is great for short lengths.

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