5ktq Headlight Wiring

[Ameer Antar]

At 10:48 PM 11/8/2000 , you wrote:
>Temperature does in fact (non-linearly) track power loss. At
>steady state, the wire's waste heat is dependent on a temperature
>differential to drive the heat out of the wire into the
>surrounding (air, body, other wires in the harness). If you're
>wasting, say, 10 watts per foot, then the wire has to be maybe 1X
>degrees hotter than the surroundings. If you're wasting, say, 40
>watts per foot, the wire has to be maybe 3X degrees hotter than
>the surroundings.
>Unfortunately, our wires are coated with an (electrically _and_
>thermally) insulating compound.

No one can really say how temp and wattage are related in this case, b/c it 
depends on so many variables like insulation, amount of air moving over it, 
etc. The only way to relate heat and wattage is thru a thermal 
characteristic used for heatsinks [deg C/W]. Ie. a heatsink w/ .25 deg C/W 
will rise 25 deg C under 100W. But as long as yer not dissipating more than 
a few watts total, this won't even be a concern.

>On this I, and science, agree. However, most wiring runs are a
>large fraction of the original length (maybe 3/4), so I can't see
>the logic of downsizing the gauge. All you do is reduce the
>available voltage at the component, which in this case has light
>output proportional to the square of that voltage.

Yer relating voltage drop to wire gage, which is not true. Voltage drop is 
equal to the current * resistance...Ohm's Law. Nowhere did he mention 
anything about gage. We've already discussed that a short wire will have 
less resistance [R=L/A*1.7e-8 for Cu]. So a short [2"] skinny wire may have 
similar resistance to a long 16 ga wire. In other words, there is just as 
much of a drop, since the resistance is about the same and the current is 
as well.

>According to the National (US) Electric Code, there is no mention
>of length in the current/gauge tables. Sorry. The variation is
>for dielectric makeup: a Teflon-PFA jacketed wire can stand up to
>1XX degrees C, so it's rated for over 100 amps in open air, but
>the gyppo PVC stuff we use around the house is only good for 20
>amps.

yes, this is for homes, or a set max. length of wire. You can't possibly 
convince me that a long wire in a 3000' building carrying 20A will have the 
same gage as a 20A circuit in your kitchen. The NEC is a guideline I 
believe for standard homes w/ a limited max. length. It would be ridiculous 
to use it on a commercial business. Go here and you'll find a chart 
relating length and gage: 
http://www.off-road.com/jeep/swb/articles/electrical2.htm

>Respectfully, methinks you need to study up a bit on ohm's and
>kirchov's laws and electrical terminology.

Me thinks not. All of the concepts I talked about are backed up w/ real 
formulas. My point is, just b/c a wire is thin, doesn't mean it can't carry 
a high load or that it will cause a large drop. In fact that's what a lot 
of countries use to their advantage. They use 220V so they can transmit 
power at a higher voltage and less current. Transformers are then used 
convert that high voltage into lower voltage, but higher current. Remember 
voltage drop in a wire is I*R. So by transmitting less current, they can 
use much thinner wire [prolly half the size of 120V lines]. Even in the US, 
those high-tension lines can be 10000V or more. The voltage is so high that 
they need to be spaced apart and away from the ground. The high voltage is 
to decrease the current and thereby reducing voltage drop in these wires 
which travel for 100's of miles. The current can all be regained by using a 
transformer to drop the voltage at the other end of the line. You're right 
about the 12ga. rating, but I have seen [when working on a 1940's home] 
that used 12ga. for a 30A circuit. It also was all cloth-covered wire. One 
thing to keep in mind is that home gage ratings won't be the same in the 
car since we're dealing w/ 12vdc not 120vac, and also the distances are 
much shorter. But again, I would recommend using 14 ga. wire. On mine I 
used 12ga. wire throughout but I'm still using the OE bulb connectors which 
are about 2" long. I don't think I'll see any difference if I replace the 
connector w/ a 14ga type. Nor do I believe it's any source of fire or heat...

-ameer