5ktq Headlight Wiring

[James Marriott]

> what's wrong? I don't get it. I did say that there can be
voltage drop in
> wires, esp. if they are long and skinny. You used a formula
(I^2*R losses)
> which is the power in watts which is wasted as heat in the
wire. Wattage
> does not equal temperature.

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.

> Depending on how much heat dissipates from the
> material and how much surface area there is, more watts do NOT
necessarily
> mean higher temp, since some material, such as copper conduct
heat away
> much quicker than others.

Unfortunately, our wires are coated with an (electrically _and_
thermally) insulating compound.

> Here are the formulas needed: R of a wire is equal to
length/area times the
> material resistivity coefficient [which is 1.7e-8 ohm-meters
for Cu]. so
> the shorter the length, the lower the resistance. The lower the
R, the less
> power wasted [I^2*R]. In other words, a short thin wire can
replace a long
> fat one.

> So a 3" length of the original wiring [18-20 ga.] will have
much
> less resistance than the original ckt, and prolly even a 10'
length of 14
> ga. wire.

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.

> I would encourage the use of 16-14 ga. wire [as in my
conversion] for the
> rest of the system. It is commonly thought that current rating
is based
> only on wire gage. It also is heavily dependent on length, as
shown by the
> formula for resistance.

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.

> That's why extra long extension cords can be 12ga.
> instead of 14ga.

Extra-long cords have larger conductors so there's still enough
power left to run the drill.

> The 120A rating of the 12 ga wire doesn't mean anything
> w/o distance.

The dielectric of the wire is rated at 120 VOLTS, the standard
current rating of 12 gauge is 20 AMPS.

> In households 12ga. wire is used in 30-40A circuits,

Not on anything that meets the NEC.

> but nothing like 120A.

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

> That's b/c the rating is from a certain distance from
> the main line. Even those huge 000 gage wires in power lines
can contribute
> to quite a voltage loss. They're so thick, but the distance is
enormous.
> This is why there are many transformers on the line which
increase line
> voltage and decrease current, cutting power losses. This is all
basic power
> technology. Point is a thick wire doesn't necessarily mean more
current,
> unless we talk about length.
>
> No fire to worry about...

At least not on MY car . . .

best wishes to all and cu, James Marriott
'87 4kq (alias "late-B2 90q"), 180k, 12-ga H4s
'89 200q (MC1, Procon10/no bag), 126k, 12-ga H4s
Boise, ID, USA     http://www.webpak.net/~marriott/

> -ameer
>
> >Sorry folks, this is just flat wrong--don't burn your car up.
> >_Voltage drop_ depends on length, current capacity depends on
> >YOUR allowable wire temperature (I^2*R losses). If you can
stand
> >really hot wire, then higher current is OK (at work we use
some
> >mil-spec TFE-jacketed wire that is rated at something like 120
> >amps in a 12-gauge section--wow! We don't use it for the high
> >capacity (it only carries 8 amps) but for the high temp (the
> >heater it powers runs at 190 dC and is infrared)). If you
don't
> >want a fire, use big wire. If you want reasonable voltage at
the
> >output, use big wire.