What gauge wire for 12v/100w car bulb?

guiri

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I'm puttin' some on the roof rack on my truck and wanted to get the wire and be ready for when my buddy comes to hook it all up. Let's say that I may upgrade to 130w just to be sure, what gauge wire do I need?
Quite a few wires going through the grommet in the body in the back so the thinner the wire I can get away with (safely) the better.

Thanks

George
 

guiri

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Ooo, someone put some time in that spreadsheet. HOWEVER, I'm too stupid to figure it out (truth) so could I tell you the distance and maybe you can tell me?

They're going in the front of the excursion but I plan on having him pulling all the wires to the back as there is an entry point/grommet in the body that I wanted to use and hopefully avoid any leaks and the excursion IS a long vehicle so I'd say maybe around 25 feet by the time he does that and goes all the way back up to the dash.

Thanks

George
 

bizzel

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I use the calculator here for most jobs, it doesn't give you a recommendation for the cable gauge straight away but you do get enough information to work out the minimum that's suitable. I plugged in your numbers with the following assumptions:

* 12.5v nominal voltage
* 8 amp load (100w/12.5v)
* 25' cable length

awg.png


You can see that the voltage drop increases rapidly once you go below 14AWG. Based on that I'd go for 12 gauge wire as a minumum, preferably 10 or 8 if that fits all your connectors okay.
 

guiri

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Ok, thanks. Not sure I can fit 12 wires thicker than 12 through that grommet...or, can I ground the lamps on the roof rack? If so, I only need the hot going to the battery, right?
 

bizzel

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Hard to say - you could always use one fat ground wire in place of several smaller ones if space is an issue.

Ok, thanks. Not sure I can fit 12 wires thicker than 12 through that grommet...or, can I ground the lamps on the roof rack? If so, I only need the hot going to the battery, right?
 

guiri

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Can they be grounded to the roof rack itself though to save on space?
 

-Virgil-

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Sheetmetal grounds are a poor idea. Use a proper wire ground to a good grounding point.
 

xul

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Using skinny wire allows you to trade off lamp intensity vs. lamp lifetime. The brightness varies by voltage^3.5 but the life varies by voltage^(-14) so a 5% drop in voltage will double your [incand.] lifetime with a 17% drop in brightness.

Current sharing with wires in parallel is pretty iffy due to varying and unpredictable contact resistance at each wire end termination.
 
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Alaric Darconville

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Using skinny wire allows you to trade off lamp intensity vs. lamp lifetime. The brightness varies by voltage^3.5 but the life varies by voltage^(-14) so a 5% drop in voltage will double your [incand.] lifetime with a 17% drop in brightness.

Why would anyone make such a tradeoff? I'll take a decrease in bulb life to get an increase in MY life by not getting killed due to choosing reduced filament luminance that comes with deliberately starving bulbs.
 

guiri

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Re: Grounding points

Ok, so would ONE ground from the battery be able to go to ALL the lamps to save on wire/thickness going through the grommet?

Also, maybe xul was just pointing stuff out. You know, FYI kinda thing

Thanks all
 

xul

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Why would anyone make such a tradeoff?
Some people have a greater utility for long bulb life over brightness. An informed consumer makes his/her own trade offs.

A 17% drop in brightness may not mean a 17% decrease in safety, but I should be able to confirm this somewhere on the Net. The response of the human eye to brightness is very nonlinear.
 
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Alaric Darconville

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Some people have a greater utility for long bulb life over brightness. An informed consumer makes his/her own trade offs.

A 17% drop in brightness may not mean a 17% decrease in safety, but I should be able to confirm this somewhere on the Net.

Confirm the "inverse square law" on the Net. A decrease in intensity at the filament is a giant decrease in intensity 150' down the road. An "informed consumer" makes their own trade-offs in such a way that it affects not only themselves, but the people they share the road with. A "well-informed" consumer knows to not make dangerous trade-offs with a large safety impact and a very slight short-term financial impact.
 

Morelite

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Why would anyone make such a tradeoff? I'll take a decrease in bulb life to get an increase in MY life by not getting killed due to choosing reduced filament luminance that comes with deliberately starving bulbs.
The OP is not reducing his safety by lowering the output since he is adding lights that where not there in the first place.
 
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Morelite

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Re: Grounding points

Ok, so would ONE ground from the battery be able to go to ALL the lamps to save on wire/thickness going through the grommet?

Also, maybe xul was just pointing stuff out. You know, FYI kinda thing

Thanks all

Your ground wiring should at least equal the positive wiring. Your current path is only as good as the weakest link.
 

Lynx_Arc

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I would rather have a bulb run at the temperature range it was designed to instead of being underdriven and shifting to a more yellower/oranger color. If he is talking about running 130 watt bulbs later then he would be silly to settle for underdriven 100 watt bulbs. A well driven and cooled 100 watt 12v bulb runs plenty long and they are pretty cheap to replace if you shop around.
 

Morelite

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I would rather have a bulb run at the temperature range it was designed to instead of being underdriven and shifting to a more yellower/oranger color. If he is talking about running 130 watt bulbs later then he would be silly to settle for underdriven 100 watt bulbs. A well driven and cooled 100 watt 12v bulb runs plenty long and they are pretty cheap to replace if you shop around.
I agree, but it was just wrong for Alaric to make it sound like he is risking his safety by running the lights underdriven. That would be true if they where the headlights or some other critical light but I don't think 6 auxillary lights on a roof rack count.
 

Lynx_Arc

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I agree, but it was just wrong for Alaric to make it sound like he is risking his safety by running the lights underdriven. That would be true if they where the headlights or some other critical light but I don't think 6 auxillary lights on a roof rack count.

I think the safety issue is possible but without more information like the speed and terrain and weather and driving conditions it may or may not be an issue. If he is using the extra light to extend his driving speed then the tendency could be to overdrive the lights, that is get brighter lights and try to drive faster than the extended range safely provides for so underdriving would definitely be ill advised. Last time I bought some 100 watt 12v halogen bulbs they were something like $5 each and lasted about 5-7 years at about 1-2 hours a night average. Unless the bulbs are $20 now and only last 1 year I would say underdriving them is unneeded.
 
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