Date: Wed, 20 Oct 1999 23:38:29 -0700
Reply-To: David Marshall <vanagon@VOLKSWAGEN.ORG>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Marshall <vanagon@VOLKSWAGEN.ORG>
Subject: Headlight wiring harnesses and voltage drops
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I am in the process of making the finishing touches to my prototype wiring
harness for installing high powered lamps in our Vanagons with out melting
our headlight switches.
This kit will run a direct, fused, line to the positive terminal of the
battery and incorportate some some relays in a waterproof enclosure that
will be mounted behind the grille. The relay will be triggered from the
input from one of the original headlight plugs.
What I am having problems with is what everyone will accept as a
satisfactory voltage loss across the harness. As we all know all
electrical devices have resistance. The more current we run through a
resister the more voltage drop we get. Lets get a little technical to
prove this point.
Lets say there is 20ft of wire between the headlights and the battery and
we have gone crazy and installed 80/100W H4s and some 130W H4s in our
Vanagon for a total of 460W of power consumption on highbeams (much like my
Double Cab only there is 2x 150W for the rallye lights!).
The electrical requirements for the lights would be:
Lights are rated at 460W at 13.5V. Meaning they require 34.074074A of
current ( I = P / V ). The resistance of these lights would be 0.396195
Ohms ( R = V / I )
The 20ft of wire has some resistance we have to calculate into the system:
- 20ft of 14ga wire at 20C is 0.05050 Ohms
- 20ft of 12ga wire at 20C is 0.03176 Ohms
- 20ft of 10ga wire at 20C is 0.019978 Ohms
- 20ft of 8ga wire at 20C is 0.012564 Ohms
- 20ft of 6ga wire at 20C is 0.007902 Ohms
Using 14 gauge wire the total resistance of the circuit would be .446695 Ohms
Using 10 gauge wire the total resistance of the circuit would be .416173 Ohms
Using 6 gauge wire the total resistance of the circuit would be .404097 Ohms
The total current consumption for the curcuit would be as follows ( I = V /
R ):
14 gauge = 30.221963 A
10 gauge = 32.438433 A
6 gauge = 33.407820 A
The voltage drop after the 20ft run of wire would be ( V = I R ):
14ga = 1.526209 V or 11.973791 V for the lights
10ga = 0.648055 V or 12.851945 V for the lights
6ga = 0.263988 V or 13.236012 V for the lights
Leaving us with less light output ( P = V I ):
6ga = 442W or 4% less
10ga = 417W or 9% less
14ga = 362W or 21% less
Now, we have a compromise between cost, ease of installation and light
output. 6ga wire is what the VW factory uses for your battery's ground
connection. It is quite expensive, especially if you get the easy to bend,
mega stranded stuff. 10 gauge is quite a bit cheaper but you are getting
up to 10% light loss especially when the tempratures go up. 14 gauge is
just too thin and would get very hot (i.e. possible fire) if used over a
long period of time. An alternative would be to run two wires to the
lights thus cutting the resistance values in half, this however would add
cost to the kit.
If you were going to install this wiring harness, what wire gauge would you
want to see used. How much would you want to be willing to pay to a "cut
no wires" "plug and play" wiring harness like this? One wire or two
running to the battery?
Thanks for the input!
-- David Marshall - - Quesnel, BC, Canada --
-- 78 VW Rabbit, 80 VW Caddy, 84 VW Westie, 85 VW Cabriolet --
-- 87 Audi 5000 Quattro, 88 2.0L VW Syncro Double Cab --
-- David's Volkswagen Home Page http://www.volkswagen.org --
-- Fast Forward Autobahn Sport Tuning http://www.fastforward.ca --
-- david@volkswagen.org (pmail) or vanagon@volkswagen.org (list) --