Date: Sat, 28 Apr 2007 16:10:20 -0500
Reply-To: Matt Roberds <mattroberds@COX.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Matt Roberds <mattroberds@COX.NET>
Subject: Re: wiring and fuses for 80/100w bulbs
In-Reply-To: <20070428180220.WMGV17357.fed1rmmtai104.cox.net@fed1rmimpi03.cox.net>
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> From: Florian Speier <groups.florian@GMAIL.COM>
> Date: Sat, 28 Apr 2007 08:46:22 -0700
>
> i am going to run 80/100w H4s for now, so i suppose i need a toatal of
> 160w in low beam mode and 2x100 + 2x55 = 310 watt in high beam mode
> (am I correct in assuming that in high beam inners and outers run
> simultaneously?)
This I do not know.
> my new wire from the battery is 12awg (dont have any ten), fused with
> 30A at the battery, giving allowing 360W.
My gut feeling is "for 310 W, get some 10 gauge". In your house, 12 AWG
is only good for 20 A; you need 10 AWG for 30 A. OK, in your house, it's
usually in a bundle with two other wires and the voltage is different,
but it's still some kind of reference point.
To get scientific: 10 AWG wire is about two-thirds of the resistance
of 12 AWG wire - about 0.9989 ohms/1000 feet vs. 1.588 ohms/1000 feet.
I agree with the 160 W in low-beam mode. I don't know if you'll have
200 W or 310 W in high-beam mode. Worst case, you'll have 310 W at
12.0 V, which is 25.8 A. If you end up with ten feet of wire, that
will be 0.009989 ohms of 10 AWG or 0.01588 ohms of 12 AWG. With 25.8 A
flowing, the voltage drop will be 0.26 V in the 10 AWG wire and 0.41 V
in the 12 AWG wire. Over the entire length, the 10 AWG wire will be
dissipating 6.7 W and the 12 AWG wire will be dissipating 11 W.
If you end up with much less than 10 feet of heavy wire, or if the high
beam load is only 200 W, then the 12 AWG is probably fine. If you do
buy wire, get stranded wire, and finely stranded at that - the stuff
they sell at the hardware store for house wiring (usually "THHN") isn't
very finely stranded - it's hard to route and may not like living in
the vibration of a car. Fortunately, the popularity of car stereo
systems that take as much power as a small aluminum smelter means that
it's relatively easy to get finely stranded wire in various gauges.
> I was considering running two 12awg wires through their a) separate
> fuses to gain more power
You'd gain a little power; the fuse holders add resistance but the two
wires in parallel lower it. It also adds a little redundancy - if one
of the two wires shorts to ground, probably only one of the fuses will
blow, leaving you with one fuse and wire for the lights.
> b) through the same fuse just to beef up the wiring.
This would also about halve the voltage drop. You do need to take care
to make both wires the same length and to have good connections on both
of them, so the current divides evenly between the wires. If either wire
shorts to ground, the fuse will blow, which is OK; if one wire opens
the other wire is probably enough to carry the load, so that's probably
OK as well.
> Option C would be to run one wire and fuse to high beam and one to
> low beam, which gives the system some redundancy.
This does gain redundancy at the expense of not reducing the voltage
drop for the high beams. You could step down to a 25 A or 20 A fuse
on the low-beam wire for some additional protection.
Matt Roberds
