Date: Mon, 31 Aug 2009 16:55:57 -0500
Reply-To: Tom Hargrave <thargrav@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Hargrave <thargrav@HIWAAY.NET>
Subject: Re: Wire size question.
In-Reply-To: <4A9C410F.4020209@gmail.com>
Content-Type: text/plain; charset="us-ascii"
"Skin effect" only matters at high frequencies or high voltages, neither of
which exist on Vanagons and 8 gauge should be fine.
The last piece of the puzzle is something no-one discussed - connection
resistance. The best way to minimize connection resistance is to find a good
body ground, use "name brand" connectors & a "name brand" crimp tool
designed to squeeze the connector farrell onto the wire.
Thanks,
Tom Hargrave
256-656-1924
Our Web Sites:
www.kegkits.com
www.stir-plate.com
www.andyshotsauce.com
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
Rocket J Squirrel
Sent: Monday, August 31, 2009 4:31 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Wire size question.
The differences between your ampacity chart and the one I referred to
might be what one can expect stranded wire (which has less copper
cross-section) versus solid-core wire.
But the writer talked about how current is carried on the surface of the
wire and he right -- at high frequencies, I've never heard anyone describe
a "skin effect" at DC, so I am not certain whether that writer knows his
onions or not.
If in doubt, when looking at two ampacity charts, use the one that is more
conservative. Your chart shows 8 gauge capable of carrying 150A for "short
runs" at 12v and 46A at 110V, and the difference must be due either to
worrying about heating effects or from having used two different sources
of data, like household building codes for 110V AC wiring practices, and
some other source for 12VDC.
The chart I referenced shows 8 gauge as being suitable to carry 23 amps,
and voltage is not considered because watts, power, are not part of the
ampacity of the wire until you start to look at how many volts you will
lose when your current goes through that wire due to the resistance of the
wire. 20A flowing through 8 feet of 8 gauge is 20A -- the wire doesn't
know whether there is a 12V or a 110V source. You could have a 12 volt
bulb that wants 20A or a 110V bulb that wants 20A -- the wire only knows
how many amps are going through it and, based on the resistance of the
wire, how warm it will get.
Ampacity -- current carrying capability -- does not decrease as wire
length goes up. What does happen is that you start to lose more and more
volts as the wire length goes up. But without knowing how many volts you
are starting with and how much voltage loss you can tolerate at the other
end, there's simply no way for one chart to tell you what is an acceptable
wire diameter to use. That's back of envelope calculation time.
For example. Assume I have a 12V battery, and I want to power a device
that wants 12 volts, but can operate just fine down to 11.5 volts. Say
that device uses 10 amps to operate. And say that device is 100 feet from
the battery, and I am running a wire to it, and a return wire back, so my
overall run is 200 feet. Okay, maximum voltage loss I can tolerate is 0.5V
over 200'. The resistance of the wire, then, per Ohms law is R = E/I or
0.5/10 = R, 0.05 ohm.
4 gauge wire has a resistance of 0.25 ohm per 1,000 feet, or 0.05 ohm per
200 feet. It has an ampacity of 60A carrying current, so it can handle 10A
all day long. The total power loss in the wire will be P = E*I or 0.5 * 10
= 5 watts. 5 watts spread out over 200 feet of wire will raise its
temperature by . . . nothing. If I use skinnier wire more volts will be
lost, the wire will run warmer, and the device will not get the volts is
wants. If I use fatter wire, the device will get more than 11.5 volts, and
the wire will run cooler -- not that it was warm to start with, but my out
of pocket cost for the wire will go up.
You can change wires sizes as many times as you want over a run, as long
as the skinny bit, which is handling as much current as the fat bits, can
handle the current given its length and its contribution to the overall
resistive losses is calculated into the overall copper losses.
I was assuming that Don was going to use the engine body for the return
portion of the round trip. But even if not, 16 feet of 8 gauge wire will
still only lose 0.2v, the wattage loss (heating) of the wire will double
but be spread out over twice the wire, so spot temperatures will not
change (i.e., still immeasurable for our purposes).
I have no idea how them offroaders got their information, it's possible
that automobile manufactures or modifiers have developed rules of thumb
for things like this. Looks like a useful chart, anyway, as it is very
conservative. It makes hidden assumptions but I don't think you can go
wrong by following the chart.
But for me, the bottom line is that if I was running 8 feet of wire to
(and maybe 8' back from) a lamp that drew 20A, I'd not hesitate to use 8
gauge wire.
--
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
Bend, OR
KG6RCR
On 8/31/2009 1:00 PM neil N wrote:
> On Mon, Aug 31, 2009 at 12:42 PM, Rocket J
> Squirrel<camping.elliott@gmail.com> wrote:
>> 8 gauge wire has the ampacity to handle 20A. It's rated for 23A carrying
>> current, and fuses (melts, pretty much) at 472 amps. Chart here:
>>
>> http://www.interfacebus.com/Copper_Wire_AWG_SIze.html
>>
>> An 8 foot run of 8 gauge wire will have .005 ohm of resistance. At 20
amps
>> this will result in 0.1 volt less voltage getting to your lamps. Not
>> noticeable.
>>
>> So, 8 gauge can handle the job without getting hot 20 amps over 8 feet
>> will result in only 2 watts of heating.
>
>
>
> Ok, now I feel foolish.
>
> Are the wiring charts I just emailed to Don/list (shown below)
> incorrect, or am I just using them incorrectly?
>
> Bowing and deferring to the truly electrically knowledgeable,
>
> Neil.
>
>
>
> Neil wrote:
>
>
> This one shows #8 good to 150 amps:
>
> http://www.rowand.net/Shop/Tech/WireCapacityChart.htm
>
> This chart shows how ampicity of a wire decreases as length increases
>
> http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm
>
> It shows #8 wire is good up to 150 amps BUT that's up to max. 10' run
> (round trip)
>
>
>
>
