Headlight wiring gauge

what gauge wire for headlight harness

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discussion starter · #1 · sep 24, 2015

i was very surprised to find that wiring to the headlight plug in the kit was a very sad 18 gauge. as i want neither “warm” wires nor dim lighting, i replaced the wiring with 12 gauge, to match the lines from the relay. looking forward to bright lights ahead. others may want to confirm the gauge on their set.

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well, ok? an 18 ga wire is more than enough to handle all the dash gauges. factory headlight wire on fox body mustangs were 14-16ga.

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the following article gives all the details. i went with the relays and 10 ga. feed wire from battery to relays and relays to lights, with the rf leads triggering the relays. says 14 min., 12 preferable, and 10 is max.

daniel stern lighting consultancy and supply

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well, ok? an 18 ga wire is more than enough to handle all the dash gauges. factory headlight wire on fox body mustangs were 14-16ga.

he’s talking headlight wire gauge, rich, not gauge lights.

i can’t imagine using 18 ga wire for headlights..who the hell supplies the harness with that size wiring?

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you are talking about the plug, correct? 18 gage in the plug should be sufficient for over 175 watts of bulb. or am i missing something?

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discussion starter · #8 · sep 24, 2015

yes, it could carry 175w, but at what heat rise, and corresponding voltage drop. halogen lights, when operating at 100% voltage, produce 100% of the designed light output. at 95% of the voltage, the light drops by more than 15%. how much light do you want? i want as much as i can get. also realize the capacity is based on a mean 30c degree air temperature. heat also causes resistance to increase. the change, near room temperature, is about 1/2% per degree c. hotter wires have higher resistance. i don’t know what the temperatures are like in your engine bay area, but mine are nowhere near that temp when running.

for the effort involved, no reason the shortcut with 18 gauge wire. let there be light.:white:

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i remember they looked pretty light duty to me too but isn’t the fact that they are such s short run also a factor in terms of load capacity? i looked on the shelves at the local auto parts store and saw many headlight plug/pigtails that used the same size wire. i know the kit supplied (halogen) sealed beams are only 35/55 watts though!

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yes, it could carry 175w, but at what heat rise, and corresponding voltage drop. halogen lights, when operating at 100% voltage, produce 100% of the designed light output. at 95% of the voltage, the light drops by more than 15%. how much light do you want? i want as much as i can get. also realize the capacity is based on a mean 30c degree air temperature. heat also causes resistance to increase. the change, near room temperature, is about 1/2% per degree c. hotter wires have higher resistance. i don’t know what the temperatures are like in your engine bay area, but mine are nowhere near that temp when running.

for the effort involved, no reason the shortcut with 18 gauge wire. let there be light.:white:

considering that most bulbs run less than 100w, even at 18 ga there is considerable headroom. i’d bet that at 100w any difference would be immeasurable without some pretty sophisticated equipment.

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a typical h4 55/60 watt bulb draws approx. 4.58 amps. (some builders use higher amp bulbs though).

with 18 ga wire, the safe length to be running say 5 amps, is pretty short, according to this chart. 18 ga would be fine for triggering the relay, but personally, i wouldn’t use it from relay to headlights, or for 12v+ power to the relay.

headlight wiring gauge

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thanks bill.

so if i’m reading your chart right (wire length in feet) then 18 gauge could carry 5 amps up to around 7 feet of wire length?? i think the plug/pigtails are less than 6 inches (mine are only 3-4″). do you think it’s a problem to use what ffr supplied? what are you using?

sean

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canuck1, an 18 gauge wire is plenty safe to run 5 amps for the headlights on high beam (60 watts). the safe limit of an 18 gauge wire is about 15 amps. the problem is voltage drop over the length of a given wire. the chart above says 18 gauge will lose about 2% voltage at 7 feet, which reduces 12v down to about 11.75v… hence, dimmer lights. not much, but it’s there.

with a 14ga, you’d have to run out almost 20 feet before you get that 2% voltage drop.

the 18ga is safe and fine to use, just maybe not the best for the brightest lights.

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5,825 posts

canuck1, an 18 gauge wire is plenty safe to run 5 amps for the headlights on high beam (60 watts). the safe limit of an 18 gauge wire is about 15 amps. the problem is voltage drop over the length of a given wire. the chart above says 18 gauge will lose about 2% voltage at 7 feet, which reduces 12v down to about 11.75v… hence, dimmer lights. not much, but it’s there.

with a 14ga, you’d have to run out almost 20 feet before you get that 2% voltage drop.

the 18ga is safe and fine to use, just maybe not the best for the brightest lights.

thanks,

i’m electrically challenged. so is the 4″ run of 18 gauge wire (just the headlight plug/pigtail) represent a potential reduction of light? i have a short length of 12 gauge wire feeding my headlights after a relay wired directly to battery but i just used the kit-supplied plugs/pigtails. the standard headlights suck but my new h4 55-65 bulbs are much better. i just used the short, 18 gauge (i’m assuming that’s what they are) pigtails that come off the three prong headlight plugs. i did not upgrade them. is there any need to in terms of light reduction/loss?

sean

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some here are confusing running 18 gage for the entire harness rather than the actual situation which is 18 gage on the bulb plug pigtail. 18 gage on the pigtail is absolutely fine and will not result in warm wires, measurable voltage drop, dim lights, or any other adverse effect.

it doesn’t matter to me if someone wants to run 00 with a high power contactor for their headlights. i’m only chiming in so that others don’t mistakenly believe that their 18 gage plugs are not sufficient for the task at hand.

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lots of good info on this thread….voltage drop is the real issue. if you want the most from your headlights do yourself a favor now and either: 1. change to 16 or 14 gage or 2. add relays. if you ever plan to add better lights (i.e. cibie”s) …you need to do this anyway. daniel stern is a great lighting reference imho

daniel stern lighting consultancy and supply

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124 posts

thanks,

i’m electrically challenged. so is the 4″ run of 18 gauge wire (just the headlight plug/pigtail) represent a potential reduction of light? i have a short length of 12 gauge wire feeding my headlights after a relay wired directly to battery but i just used the kit-supplied plugs/pigtails. the standard headlights suck but my new h4 55-65 bulbs are much better. i just used the short, 18 gauge (i’m assuming that’s what they are) pigtails that come off the three prong headlight plugs. i did not upgrade them. is there any need to in terms of light reduction/loss?

sean

you’re welcome. no, a 4″ section of 18ga will not make a difference with your lamps. you’re fine.

any and all wire will have a voltage drop of some amount. your 4″section of 18ga wire with 12v applied at 5.5 amps will have a voltage drop of about .02 volts. so yeah… you’re fine. :wave:

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discussion starter · #18 · sep 26, 2015

didn’t mean to create a major debate. i just surprised and the small gauge. the engineer/purist in me wants to do it right. the effort to correct was minimum, and i get to smile at the 0.02v i gained.

the build is all about being able to look back how you made it better, and made it your own.

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didn’t mean to create a major debate. i just surprised and the small gauge. the engineer/purist in me wants to do it right. the effort to correct was minimum, and i get to smile at the 0.02v i gained.

the build is all about being able to look back how you made it better, and made it your own.

:thumbup:that is the way i’m doing it. i went with 10ga from relays triggered with the 18ga wire. i am going to play around with the off road halogen h-4 bulbs that you can easily buy for about $7.00.:evil:

i am using 12ga for extra driving and fog lamps, with relays, which also use the h-4 style base bulbs.

george

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10,377 posts

4″ of 18 ga wouldn’t be an issue, sean. i believe mine is 14 ga. easy test would be to feel the wire with the headlight on high beam. heat is resistance.

depending on how a builder wires up his headlights, and/or the harness used, some may use just one wire leading from the relay, then branching off to both headlights.this would mean 10 amps passing through that wire.

according to one automotive electrical pundit, this (wire length/ga) aspect, should also be kept in mind..

“the wire footage is double the distance you measure, if you are running a separate ground wire, and not using a direct chassis ground, at, or close to, the point your feeding power to. most people do not think about this factor. it is a common mistake. so, if you are using the correct gauge wire – based on the length of the wire, and the wire is getting hot, chances are you did not calculate the return trip to the battery.”

perhaps that’s why ground wires on most vehicles wire harness’s are usually a larger gauge. i know never really thought about that aspect, when calculating feed wire length/gauge.

Source: http://tonupboys.com
Category: Headlights