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Date:         Thu, 25 Oct 2012 10:11:39 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Ampacity was Re: Headlight Upgrade Double Check (fuses)
Comments: To: turbowesty <turbowesty@GMAIL.COM>
In-Reply-To:  <1CD89E84-7AEF-42FA-A295-88DA2507C9CE@gmail.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 11:04 PM 10/24/2012, turbowesty wrote: >May I ask, where does the gauge # come from and how is it defined? >The relationship between gauge and strands is quite linear >(inversely? I'm not clear on what the context for 23/25/etc is).

The American Wire Gauge is similar to the Browne & Sharpe steel plate gauge which I believe was defined in terms of thickness of a dimensioned sheet of steel having particular weights. This is also analogous to shotgun gauge, defined as diameter of a lead ball that runs so many to the pound. Because of how they're defined gauge numbers typically run contrary to intuition, i.e. big number --> small dimension.

Modern AWG is defined at two gauges and interpolated/extrapolated outside of them; see http://library.bldrdoc.gov/docs/nbshb100.pdf .

Stranding is heavily constrained by geometry because the strands have to pack solidly together in circular form. Within those constraints you get to choose the number of strands and sub-strands based on requirements. Small-diameter wire rope, for example, is commonly available in 1x19 (stiff) and 7x19 (flexible, because each of the 19 strands is itself composed of seven sub-strands twisted in opposite handedness). In general stiff wire is cheaper to make than flexible, so in electric wiring many gauges have several commercially available stranding options. Test lead wire for example is finely stranded so as to be as close to limp as possible. These stranding differences lead to minor differences in conductor area in a particular gauge, but such differences are really only of interest if you're using wire as a resistor and need to calculate precise length of a wire to obtain a desired resistance. Wire tables give resistance figures in both resistance per foot/kilofoot/kilometer and per pound/kilogram. See reference above for details of wire construction and stranding in commercial practice and in general a lot more than you want to know about how it all works.

That reference is probably authoritative in the US subject always to physics and typography. There are many much more convenient tables available on the web but beware of typographic or calculating errors.

In your van (RVC) you're interested in ampacity ratings to prevent overheating; and resistance per unit length so you can determine voltage drop in a specified supply circuit and keep it within acceptable limits. The longer your circuit the more likely that the second factor will govern. My Bosch book is as I've said mislain, but I'll go out on a limb and say that for most 12v automotive circuits a total wiring drop including return of a quarter to a half volt is probably acceptable. Starter circuits expect a somewhat higher drop.

With regard to headlights, particularly halogen ones which are universal now - halogen lamps by design burn very hot for two reasons: efficiency, because high (hundreds of psi cold) internal pressure helps keep tungsten on the filament where it belongs; and because the halogen reaction that keeps vaporized tungsten from depositing on the lamp envelope** needs high temps to run. But this means they're intolerant of over-voltages.* Lamps for automotive use are no doubt made and rated in anticipation of industry-practice wiring drops, so if you get too carried away with big wires you may find yourself with very bright lights that don't last long. This is also an issue for people who are charging their batteries with a smart regulator, or who simply have the voltage turned up high enough for a standard regulator to fully charge (and eventually boil dry) a battery.

*In standard 750-hour household tungsten lamps operating at/near rated voltage, the rule of thumb IIRC is that with increasing voltage brightness goes up with the THIRD power of the voltage but lifetime comes down with the FIFTH power. This give a very unforgiving overvoltage vs lifetime curve (rated life of a 3200K photoflood bulb is one hour, did you know?). The "run forever" tungsten lamps that used to be hawked in mail-order catalogs and such are simply 750-hour lamps with a rated voltage of 130v instead of 120v, so they give crappy light for the power consumed but last a very long time. Halogens are even less forgiving because their construction lets them run normally at temperatures that would very quickly blacken and burn out a conventional lamp.

** In ordinary lamps vaporized tungsten plates out on the cool lamp envelope, sometimes to an astonishing degree. In instrument pilot lights (#47 lamp for example) it's not uncommon to pull out a working lamp and find that you are looking at a silver-black mirror and can't see through when you hold it up to the window. One reason for the pear shape of household lamps is to give lots of area so the film will be still be pretty transparent when the filament burns out. Another reason in the case of frosted lamps is to make the lamp an area rather than pinpoint light source.

Yrs, d


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