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Thread: need advice on wire size choice for power and ground

  1. #1

    Default need advice on wire size choice for power and ground

    I am running two 450w GE landing lamps on my car, they have 6AWG wire which i THINK may be overkill, i want to redo my wiring to clean things up a bit. With that said, they are designed for 16.5v, and i realize a car is 12v.....so for one thing, how would you end up figuring out how many amps you now would accommodate for? I hope that makes sense, i know they will not be as bright if they arent powered by a true 16.5v, but the car when running is typically 14.3v.
    Ok so after figuring out the new wattage, divided by voltage (no idea if you would use 12v or the 14.3), you get the amperage, i understand that....but so many wiring charts show different size requirements, some say 8AWG, 10AWG, etc....so i have no clue if i can get away with 10AWG wire.

    Thanks for any advice!

  2. #2
    Flashaholic Lightdoctor's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Would not use anything smaller that 8AWG...to much risk of a melt down. When you build this harness, make sure the connections are tight.

    (I sure hope that you're using these lights offroad only.)
    Last edited by Lightdoctor; 07-21-2011 at 06:16 PM.

  3. #3

    Default Re: need advice on wire size choice for power and ground

    i hardly drive the car, i live in the middle of farm land and there are coyotes and other animals that you cannot see until its too late....long 5 mile stretches with vineyards hiding potential animals.
    Would multi-strand wire be of any advantage heat-dissipation-wise aside from being more flexible to route? i have a soldering iron that i am learning how to use instead of crimping them on.

  4. #4

    Default Re: need advice on wire size choice for power and ground

    Multi-strand wire is the only acceptable kind in automotive service. Solid wire can't withstand the vibration.

    Be advised that if you are running lamps with a rated voltage of 16.5v on 14v (I suspect they are probably actually seeing less than 13v with what is probably massive voltage drop in their circuit, but we'll go with your 14v figure for calculation) then they are producing only 57% of their rated light flux, and luminance will be very poor. You can do a lot better than your present setup with more thoughtful selection of lamps.

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    Administrator Norm's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Mr Bright please resize the pic in your sigline. Sigline images cannot exceed 500 pixels wide, 100 pixels high and 20k.

    If you do happen to post in the wrong forum do not open another thread in the correct forum, click the triangle symbol directly under your post, this is the report button, explain your problem and send a report this will go to all Admins and Mods, someone will move your thread to the correct area. - Norm

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    Flashaholic ryguy24000's Avatar
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    Default Re: need advice on wire size choice for power and ground

    I am not a auto wireman, but 900/16.5=54.6 amps In the construction world that would require at least a 6awg.

  7. #7

    Default Re: need advice on wire size choice for power and ground

    forgot to mention that each light has its own circuit off of the battery with its own fuse and 50A relay.

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    Flashaholic* Alaric Darconville's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by Mr Bright View Post
    forgot to mention that each light has its own circuit off of the battery with its own fuse and 50A relay.
    That's all fine and dandy, but mathematics are against you. The bulb is rated for intensity at 16.5V. You're feeding it, at the most, 14.0V.
    (14.0/16.5)^3.4 = .57

    Give it 0-gauge wire if you want, you're not going to conjure up more brightness until you conjure up more voltage.

  9. #9

    Default Re: need advice on wire size choice for power and ground

    I would consider the 6 AWG the minimum size. I use 2 AWG (not 2/0 AWG) for grounds and power. Be sure to use a proper hex crimp tool and use heat shrink tubing with adhesive at the lugs/connections. I would suggest you read some 4X4 forums and see what others are using for driving lights. Your fuses and relays are a must.

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    Flashaholic* Alaric Darconville's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by HotWire View Post
    I would consider the 6 AWG the minimum size. I use 2 AWG (not 2/0 AWG) for grounds and power. Be sure to use a proper hex crimp tool and use heat shrink tubing with adhesive at the lugs/connections.
    It's a non-starter. The filament luminance will be awful. The color will be nearly brown.
    Current draw will be ginormous because the filament's resistance won't be maximal until the filament is hot enough, but it won't get hot enough at 14-15V.

    I would suggest you read some 4X4 forums and see what others are using for driving lights.
    Better yet, go to danielsternlighting.com and ask for recommendations there, or search this forum more carefully.

    Your fuses and relays are a must.
    As are bulbs designed for automotive voltages.

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    Flashaholic* VegasF6's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by Alaric Darconville View Post
    That's all fine and dandy, but mathematics are against you. The bulb is rated for intensity at 16.5V. You're feeding it, at the most, 14.0V.
    (14.0/16.5)^3.4 = .57

    Give it 0-gauge wire if you want, you're not going to conjure up more brightness until you conjure up more voltage.
    Alaric, can you please explain how you arrived at that formula? Is there a name for it?

  12. #12
    Flashaholic* Alaric Darconville's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by VegasF6 View Post
    Alaric, can you please explain how you arrived at that formula? Is there a name for it?
    http://www.danielsternlighting.com/t...ys/relays.html

    I don't know if there's a name for it, though. Scheinwerfermann may know the name if there is one, and will probably have all kinds of snazzy documentation for it, or you can ask Daniel Stern at danielsternlighting.com.

  13. #13

    Default Re: need advice on wire size choice for power and ground

    It's just standard filament lamp math. The usual exponent is 3.4 for filament output with voltage change, 1.6 for filament power consumption with voltage change, and -13 (negative 13) for filament lifespan with voltage change. These aren't carved in stone; each individual lamp (bulb) design will have slightly different characteristics and so there's no one exponent that is exactly, precisely correct for all bulbs. But the exponents here are close to the middle of the range of variance, and will get you close to the middle of the ballpark for calculations like this on whatever kind of filament lamp you have in mind.

    The generic form is m=(v2÷v1)^x where:

    m = multiplier
    v1 = original or rated operating voltage
    v2 = changed or actual operating voltage
    x = exponent as given above.

    Running the equation gives you a multiplier which you then apply to whatever rating you're looking at. If you are using the lifespan exponent of -13 to figure the change in lifespan with changed voltage, you multiply the lamp's rated lifespan by the multiplier to get the expected lifespan at the new voltage. If you're using the 3.4 exponent to figure the change in light flux with changed voltage, you multiply the lamp's rated output by the multiplier. If you're using the 1.6 exponent to figure the change in power consumption with changed voltage, you multiply the lamp's rated actual wattage (watch out for inaccurate nominal wattages) by the multiplier.
    Last edited by Scheinwerfermann; 07-26-2011 at 07:08 PM.

  14. #14

    Default Re: need advice on wire size choice for power and ground

    Get a pair of cheap HID offroad lights. Only 35 watts and standard car voltage.
    Stinkloads of light.

  15. #15
    Flashaholic* VegasF6's Avatar
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    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by Alaric Darconville View Post
    That's all fine and dandy, but mathematics are against you. The bulb is rated for intensity at 16.5V. You're feeding it, at the most, 14.0V.
    (14.0/16.5)^3.4 = .57

    Give it 0-gauge wire if you want, you're not going to conjure up more brightness until you conjure up more voltage.
    Quote Originally Posted by Scheinwerfermann View Post
    If you're using the 3.4 exponent to figure the change in light flux with changed voltage, you multiply the lamp's rated output by the multiplier. If you're using the 1.6 exponent to figure the change in power consumption with changed voltage, you multiply the lamp's rated actual wattage (watch out for inaccurate nominal wattages) by the multiplier.
    Well that sure is handy to know. If I am understanding you then, is the formula being used correctly above? When you say the lamps rated output you are talking lumens, or luminous flux?
    The only data given is watts at a specified voltage, so the only thing we could extrapolate would be the change in watts consumption with the 1.6 exponent?

    *edit* after a re-read I guess Alaric was using the formula right, that resulted in his multiplier to figure approximate lumens. So, after you multiply the rated output by the multiplier, you then add the result to the original rating?
    Last edited by VegasF6; 07-26-2011 at 03:57 PM.

  16. #16

    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by VegasF6 View Post
    Well that sure is handy to know. If I am understanding you then, is the formula being used correctly above?
    Yes.

    When you say the lamps rated output you are talking lumens, or luminous flux?
    Er...say what? Luminous flux is stated in lumens. Sometimes, usually from American vendors, you will see it stated in MSCP or MSCD (mean spherical candlepower, sometimes labelled just "candlepower") instead; to get lumens multiply MSCP by 4π (12.57). Example: a 32cp 1157 bulb produces 32 ×4π = 402 lumens.

    The only data given is watts at a specified voltage, so the only thing we could extrapolate would be the change in watts consumption with the 1.6 exponent?
    The only specific value we can come up with given what we know is the power consumption. But we can come up with relative (percentage) values for luminous flux and for lifespan. Alaric's statement of ~57% of rated output is correct.

    So, after you multiply the rated output by the multiplier, you then add the result to the original rating?
    No. Why would you do that...? I don't understand your thinking here. Let's run through an example. Take a 9006 bulb rated 55 watts, 1000 lumens and 1000 hours at 12.8v. Let's run it at 11.9v and see what happens to…

    …Luminous flux:

    m = (11.9 ÷ 12.8)^3.4

    m = (0.9296875)^3.4

    m = 0.7805

    0.7805 × 1000 lumens = 780.5 lumens at new operating voltage.

    …Power consumption:

    m = (11.9 ÷ 12.8)^1.6

    m = (0.9296875)^1.6

    m = 0.89

    0.89 × 55w = 49w at new operating voltage

    …Lifespan:

    m = (11.9 ÷*12.8)^-13

    m = (0.9296875)^-13

    m = 2.58

    2.58 × 1000 hours = 2580 hours at new operating voltage.

    -----
    It's worth keeping in mind that the decrease in luminous flux is not the only vector of reduced performance. Filament luminance nosedives, which sharply reduces the "punch" of the beam.

  17. #17
    Flashaholic* VegasF6's Avatar
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    Default Re: need advice on wire size choice for power and ground

    I was trying to compute for over voltage, and I reversed the 2 voltages in the formula. So when I arrived at a # lower in flux I figured you had to add the two together. I see my mistake now. I will be sure to reference this page in the future.

  18. #18

    Default Re: need advice on wire size choice for power and ground

    Looks like the error is actually mine -- I've corrected my post above to
    read (V2 ÷ V1) rather than the erroneous (V1 ÷ V2),
    which would have had you dividing rated voltage by operating voltage. In fact, you need to divide operating voltage by rated voltage. I do these calculations so often it just goes through the spinal cord and I seldom ever have occasion to write them out in general form like that. Sorry!

  19. #19
    Flashaholic*
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    Default Re: need advice on wire size choice for power and ground

    With vehicle running, measure voltage at light. Get an ammeter and measure current. Then, you should be able to figure out the gauge that you need, and the actual wattage that you're running those 450w bulbs at.

    An option is to convert to a 16v battery/alternator for your lighting(either independent 2nd alternator/battery or vehicle conversion). Conversion will use several step down resistors to power the each of the car circuits at ~13v. Check with a competent car stereo shop or race shop. Higher voltage systems are common enough for amplifier setups or for more ignition power.

  20. #20

    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by deadrx7conv View Post
    An option is to convert to a 16v battery/alternator for your lighting(either independent 2nd alternator/battery or vehicle conversion). Conversion will use several step down resistors to power the each of the car circuits at ~13v.
    And while you're at it, you could travel from Los Angeles to San Diego by way of Calcutta, too -- which would be comparably cost-effective.

    Just put in more thoughtfully-selected lights and be done with it!

  21. #21

    Default Re: need advice on wire size choice for power and ground

    Quote Originally Posted by Scheinwerfermann View Post
    Just put in more thoughtfully-selected lights and be done with it!


    That's what I'm sayin'.

    If you want cheap-n-stinky light, Godlights, Thor's thunderbolt, just mount a set of 8" HIDs up there. Offroad use only, don't vaporize your neighbor's retinas.

    If you want road-legal driving lights, there are good halogen options and I'm sure Scheinwerfermann has a list of what works best.
    I've always been fond of the Hella Rallye 4000s since I'm from a Jeep background. They're big and ugly (like me) but have a nice even wash of useful light.
    And no obscene power demand...

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