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

1. ## 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. ## 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.

3. ## 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. ## 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
* 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. ## 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. ## 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.

Originally Posted by guiri
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. ## Re: What gauge wire for 12v/100w car bulb?

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

8. ## 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. ## 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.

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

Originally Posted by Scheinwerfermann
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. ## Re: What gauge wire for 12v/100w car bulb?

Originally Posted by xul
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. ## Grounding points

Originally Posted by guiri
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. ## 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. ## Re: What gauge wire for 12v/100w car bulb?

Originally Posted by Alaric Darconville
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.

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

Originally Posted by xul
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. ## Re: What gauge wire for 12v/100w car bulb?

Originally Posted by Alaric Darconville
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.

17. ## Re: Grounding points

Originally Posted by guiri
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. ## 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.

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

Originally Posted by Lynx_Arc
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. ## Re: What gauge wire for 12v/100w car bulb?

Originally Posted by Morelite
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.

21. ## Re: Grounding points

Originally Posted by Morelite
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. ## 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. ## 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.
"

24. ## 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. ## Re: Grounding points

Originally Posted by guiri
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. ## 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. ## 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. ## Re: Grounding points

Originally Posted by guiri
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. ## Re: What gauge wire for 12v/100w car bulb?

Originally Posted by guiri
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. ## 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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