[Bglug] DC Current and Wire Size

Brad Rodriguez brad at bradrodriguez.com
Sat Dec 5 13:48:08 EST 2015


On Sat, 5 Dec 2015 12:04:07 -0500
William Park <opengeometry at yahoo.ca> wrote:

> Hmm, so DC current is bi-directional on the wire, also?  I thought only
> AC current does that. :-)

No, Peter was referring to the fact that there's a wire carrying current
to the load, and then a second wire returning current back from the load,
and both wires contribute to the total voltage drop.  So if the load is
10 feet from the source, the current is passing through 20 feet of wire.

> Long ago, I was curious (and still am) how much current USB cables can
> carry.  USB wires are 26, 28, or 30 AWG.  Ethernet is 24AWG wires.
> Rule of thumb to remember:
>     - 24AWG is rated for 500mA, and 
>     - USB specs says 500mA max.
> So, I don't know how 5V/2A USB chargers passed certification, when USB
> cables can't do 2A.

"Rated for" depends on how much heat you can have the wire dissipate.
120 volt wires built into wooden walls are more conservatively rated
than short wires at low voltage hanging in open air.  Looking at this
table <http://www.powerstream.com/Wire_Size.htm> I see that 26 AWG wire
is 40.81 ohms per 1000 feet, so a six foot length would be roughly 0.25
ohm.  Times two for the return conductor, and that's 0.5 ohm, giving a 1
volt drop at 2 amps (and thus two watts of heat to dissipate).  I don't
know what the high-current USB specs are, but I vaguely recall that the
standard 500 mA spec allows about 0.75 volt drop.

- Brad
-- 
brad at bradrodriguez.com




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