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Date:         Mon, 31 Aug 2009 12:42:49 -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.
Comments: To: Don Hanson <dhanson928@GMAIL.COM>
In-Reply-To:  <6bc66ccf0908311127i4db275a2n263a84a6d83e2da0@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

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.

But you can always use fatter wire.

If you connect a fat wire to a small wire and dump a ton of current down that wire then the part with the highest resistance in the shortest length will indeed get the hottest. Like say you replaced a foot of that 8 gauge wire with something really skinny, like 43 gauge wire that fuses at one amp, you'd have made for yourself a DIY 1 amp fuse. That part would melt for sure.

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.

For fusing, incandescent lamps draw a lot of current momentarily upon turn-on. This higher inrush current drops to normal current so quickly that your wiring is not at risk, though the fuse might be. I'm not familiar with automotive fuses, just electronics ones which come in fast blow and slow blow types. A 30 amp fast blow fuse would adequately protect the battery from a short if there was a fault in the wiring to the lamp, but might pop when the lamp was first turned on. I'd use a slow blow, m'self, but as I say, I don't know if auto fuses comes in those kinds of flavors. David Beierl knows more about this, I bet.

-- 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 11:27 AM Don Hanson wrote:

> I have a spotlight on the roof of my van and like to use it, when safe, to > light far down the road or along the shoulders. Sometimes for setting up > camp after we're stopped. Right now, I am only using it for short periods > because it's wired into my interior celing light circuit, which I am > assuming isn't adequate for a prolonged big draw accessory like that > spotlight...20amps, maybe. The light came with a short supply wire that is > quite small gauge...Maybe #8...Which seems small to me, but I am an > electrical dunce. Right now I've simply run a wire from my over the drivers > door interior light across the top of the windshield to the > spotlight..Works, but I worry about the draw and I want it on my secondary > battery rather than the starting batt. > So, I am planning on taking a wire from my Aux battery (under the drivers > seat) fuse block (inside the undersink cabinet) up to the spotlight. I > would like to be correct with the wiring. The run will be about 6-8' from > the fuse block up to the pigtail that comes right out of the light. I plan > on crimping the supply line with a butt splice. My question(s) are: What > size wire should I run up to the light and won't that small wire pigtail > that came on the Unity spotlight get hot if I use it? When you take a big > wire for a long run then hook into a short smaller wire does the small wire > become the hot spot in the whole run? Not sure I am asking this right, but > it's the best I can do with the electrical knowledge I have.. > thanks, Don Hanson >


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