Date: Mon, 18 May 1998 10:05:00 -0700
Reply-To: Mark McCulley <raven@HALCYON.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Mark McCulley <raven@HALCYON.COM>
Subject: Re: wire size for headlight circuit
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Marcus and Drew,
For a given power output, you must use 10 times the current at 12V than at
120V. A 100W light bulb uses less than 1 amp (100/120=0.83) in your home. In
your vehicle, a 100W headlight bulb consumes over 8 amps (100/12=8.33). As
far as the capacity of the wire is concerned, only current matters. Thus,
you need a much bigger wire to handle 100 watts in a 12V system than in a
120V system. For a graphic example, look at the size of the wire between the
starter and your battery. It's about as big as the wires that bring power
into your house!
My actual requirements are to handle 210 watts of lighting. Worst case is 2
55W low beams plus 2 50W fogs. Thats 210 watts total. I'm discounting the
current spike that occurs when you flash your brights and both the low and
high beams are on at the same time. That would add another 120 watts but I
figure I'll have enough headroom designed in to handle this. OK, that's
210W. I want an extra 200W for future expansion (2 100W driving lamps or
whatever). Now I'm up to 410W. I probably wouldn't be using the fogs and the
driving lamps at the same time, but who knows. I want to be able to do so
safely if the occasion arises.
410 watts at 12V uses 34 amps (410/12=34.16). According to the charts I've
seen, 10 gauge wire is rated at 30 amps. Thus I need to use 8 gauge wire,
which is rated at 40 amps.
Is this really overkill? Where does my thinking go wrong on this? It seems
sort of fruitless to me to do this with 12 gauge as some of original
lighting circuit is 10 gauge (all wires labled 2,5 in Bentley are
essentially 10 gauge). I'd like to keep the voltage drop on this circuit to
less than 100 mV if possible.
-Mark McCulley
At 11:12 AM 5/18/98 -0700, marcus wrote:
>Wire size is 8?? Must be a typo. My 4ton home AC uses 8 gauge. 12 gauge is fine
>for your project
>
>Mark McCulley wrote:
>
>> OK, I've decided to run a dedicated, relay-switched +12V line to power my
>> headlights and fog lights. My questions is what gauge wire should I use?
>> I've got some 8 gauge--that should handle it right? I believe it's rated at
>> 40 amps--that would handle up to 480 watts of lighting. Comments?
>>
>> Mark McCulley
>
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