Date: Mon, 31 Aug 2009 15:21:51 -0700
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Re: Wire size question.
In-Reply-To: <181101ca2a85$d2355d10$76a01730$@net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Tom's right. I did obliquely mention the importance of the connection:
"The butt splice I can't comment on. If it's marginal - high resistance -
then it could be that hot spot you don't want to see. So use a good beefy
one."
Tom's answer is more to the point: Use good quality splicing parts and the
proper tools.
BTW, someone earlier suggested using a "VOM" (i.e. a "volt ohm meter," a
pretty old school name for a multimeter) set to amps to measure the
current in your line. Be aware that most multimeters have a dedicated
connector on them for measuring current and are generally limited to, and
fused for, 10A. The fuse is usually a physically tiny one that you'll
never find a replacement for, ever, and that fuse is gonna pop if you
stick it into a current-carrying line willy-nilly.
I am proud to say that every multimeter I own has blown fuses. Do as I
say, son, I say, do as I say, not as I do.
(I do so enjoy it when a thread addresses something I can contribute to.
I'm completely out of my element when the discussion turns to tires,
engines, transmissions, suspension bits, CV joints, and wherever they hid
the windshield washing fluid container I can never remember where it is.)
--
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 2:55 PM Tom Hargrave wrote:
> "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)
>>
>>
>>
>>
>
>
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