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

  1. #1

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

    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

  2. #2

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

    How long is the run? You just need to run the numbers through a voltage drop calculator, thinner wire will just be less efficient, you would have to go quite thin before it became dangerous. below is a link to a voltage drop calculator.


    http://www.wind-sun.com/ForumVB/showthread.php?t=29

  3. #3

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

    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

  4. #4

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

    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



    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.

  5. #5

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

    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?

  6. #6

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

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

    Quote Originally Posted by guiri View Post
    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?

  7. #7

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

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

  8. #8

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

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

  9. #9
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    Default Re: What gauge wire for 12v/100w car bulb?

    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.
    Last edited by xul; 08-29-2011 at 10:51 AM.

  10. #10

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

    Quote Originally Posted by Scheinwerfermann View Post
    Sheetmetal grounds are a poor idea. Use a proper wire ground to a good grounding point.
    Thick, solid steel roof rack if I'm not mistaken..

  11. #11
    Flashaholic* Alaric Darconville's Avatar
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    Default Re: What gauge wire for 12v/100w car bulb?

    Quote Originally Posted by xul View Post
    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.

  12. #12
    Flashaholic* Alaric Darconville's Avatar
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    Default Grounding points

    Quote Originally Posted by guiri View Post
    Thick, solid steel roof rack if I'm not mistaken..
    That does not guarantee a good ground. Good ground points are typically where other factory grounds are connected, or directly to the negative battery terminal.

  13. #13

    Default 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

  14. #14
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    Default Re: What gauge wire for 12v/100w car bulb?

    Quote Originally Posted by Alaric Darconville View Post
    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.
    Last edited by xul; 08-29-2011 at 05:02 PM.

  15. #15
    Flashaholic* Alaric Darconville's Avatar
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    Default Re: What gauge wire for 12v/100w car bulb?

    Quote Originally Posted by xul View Post
    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.

  16. #16
    Flashaholic* Morelite's Avatar
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    Default Re: What gauge wire for 12v/100w car bulb?

    Quote Originally Posted by Alaric Darconville View Post
    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.
    Last edited by Morelite; 08-29-2011 at 05:27 PM.

  17. #17
    Flashaholic* Morelite's Avatar
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    Default Re: Grounding points

    Quote Originally Posted by guiri View Post
    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.

  18. #18

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

    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.
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  19. #19
    Flashaholic* Morelite's Avatar
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    Default Re: What gauge wire for 12v/100w car bulb?

    Quote Originally Posted by Lynx_Arc View Post
    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.

  20. #20

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

    Quote Originally Posted by Morelite View Post
    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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  21. #21

    Default Re: Grounding points

    Quote Originally Posted by Morelite View Post
    Your ground wiring should at least equal the positive wiring. Your current path is only as good as the weakest link.
    Sorry, but I'm not too bright. Can you say this again in idiot terms?

    Thanks

  22. #22

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

    Well, I'll address the speed issue. I will not be driving faster due to more light, I just like to see where I'm going, ESPECIALLY in the country side

    Here's a pic of the business end by the way http://www.georgepics.com/p30027000/h352b19f9#h352b19f9

  23. #23
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    Default Re: Grounding points

    Couldn't get any real numbers but it seems substandard lighting is the norm.
    "According to the AAA Foundation for Traffic Safety, upwards of 90% of the vehicles on the road have some degree of lighting or illumination blockage due to these compromised cloudy headlights."

    and

    "Vehicles equipped with auxiliary lamps, a spotting light or any other lights used for lighting or clearance should not emit a beam with an intensity more than 300 candle power. Such vehicles will be restricted to four such lighting devices, in addition to the regular lighting equipment.

    Federal Lighting Specifications
    Upper beams shall range from 20,000 to 75,000 candela in intensity for each lamp. The lower beam will range from 15,000 to 20,000 candela in intensity for each lamp. These restrictions apply to Type 2 or 2A light classifications.

    The Type 1 or 1A limitations for lighting stipulates an upper beam that will range from 18,000 to 60,000 candela intensity for each lamp.
    "
    Last edited by xul; 08-29-2011 at 07:23 PM.

  24. #24

    Default Re: Grounding points

    Well, I didn't like the output on my regular Excursion lights. Was told to upgrade to Sylvania Silverstars and I'm not impressed by them either which is why I got the 4 extra lights. Not all that impressed AND, I like the look of the roof rack so I'm adding more

    My Volvo when I lived in Sweden had a total of 700 watts on brights. It was beautiful and god help anyone that didn't dim their headlights

  25. #25
    Flashaholic* Morelite's Avatar
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    Default Re: Grounding points

    Quote Originally Posted by guiri View Post
    Sorry, but I'm not too bright. Can you say this again in idiot terms?

    Thanks
    All that means is if you need 12ga wiring then you need it for both the positive and negative. It doesn't matter if you use one wire or six as long as it adds up the the needed current capacity.

  26. #26
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    Default Re: Grounding points

    And the vehicle body doesn't conduct as well as copper for the same cross section, but the vehicle body has a much larger cross section.
    http://resources.schoolscience.co.uk...elech2pg1.html

  27. #27

    Default Re: Grounding points

    So, I can use one stout wire for the ground AND six thinner wires (if I had to and as long as they hold up to the power going through them) if needed?

  28. #28
    Flashaholic* Morelite's Avatar
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    Default Re: Grounding points

    Quote Originally Posted by guiri View Post
    So, I can use one stout wire for the ground AND six thinner wires (if I had to and as long as they hold up to the power going through them) if needed?
    You could but keep in mind that you will need a wire rated for a minimum of 50 amps (100w/12v = 8.33A x 6 = 50A)

  29. #29

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

    Quote Originally Posted by guiri View Post
    Here's a pic of the business end by the way http://www.georgepics.com/p30027000/h352b19f9#h352b19f9
    And you're adding more lights on a roof rack? If the lights on the grill are the only lights you can just run thin gauge wire through the grommet in the firewall to a large relay and the run from the battery to the relay to the lights will only be ~5 feet and you can run heavy gauge wire no problem.

  30. #30

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

    I hope you have a larger than stock alternator in mind running these.
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