recommendation for wire gauge in 100/90 bulbs

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Jan 22, 2009
Manila, Philippines

i am about to install an osram all season 100/90 h4 bulbs in my car and i am also 100% sure that the housing can handle the heat. i have all the necessary materials ready such as ceramic terminals, relays, connectors, except the wires.

do you have any recommendation on the wire gauge that i need to use for this kind of bulb? i am choosing between 10 and 12.
lets do the math:

V=volts=12 (we'll chose the low side)

so putting it together:

200/12= 16 amps
you could technically use as small as 16awg wire but for we all like overkill so 12awg is more than adequate with less than a 1% voltage drop.
I'm sure you can find one on the net. charts give you sort of a rule of thumb.

I calculated the circular mils area (CMA) of the wire needed for copper wire, 30' long, 16 amps with a 1% voltage drop then looked on a CMA chart

Originally Posted By: spyghost
is there a generic table that says what stranded automotive wire gauge to use for a particular current drawn?

What Tom said. If you do find a good chart it will be for general use. For a head light circuit you want low voltage drop because a small drop in voltage = a large drop in light output. 1% is a good number and the general charts allow a bigger voltage drop. They are more concerned with not setting your car on fire.
is it right to say at this time that going further to gauge 10 won't be much of a benefit at all?

also, given that i will be using a gauge 12, assuming that there occurs a short, will a 20a fuse break first before the gauge 12 wires burn the insulator?
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sorry for being too much of an "asking person." but just to clarify again, i found this



Use only stranded wire, never solid (household type) wire, in automotive applications.

Wire gauge selection is crucial to the success of a circuit upgrade. Wire that is too small will create the voltage drop we are trying to avoid. On the other hand, wire that is of too large a gauge can cause mechanical difficulties due to its stiffness, particularly in pop-up ("hidden") headlamp systems. The headlamp power circuit ought to use no less than 14-gauge (2.5 mm2) wire, with 12-gauge (4.0 mm2) being preferable. 10-gauge (5.2 mm2) can be used if bulbs of extremely high wattage are to be used, but it's usually overkill. Be sure to pick a kind that flexes easily if yours is a hidden-headlamp system. Do not fail to use the large wire size on both sides of the headlamp circuit! Voltage drop occurs due to inadequate grounding, too! you will only sabotage your efforts if you run nice, big wires to the feed side of each headlamp, and leave the weepy little factory ground wires in place. Most factory headlamp circuits run the too-thin ground wires to the car body. This is an acceptable ground--barely--on a new car. As a car ages, corrosion and dirt build up and dramatically increase resistance between the car body and the ground side of the vehicle's electrical system. It takes little extra effort to run the new, large ground wires directly to the battery Negative (-) terminal or to the metal housing of the alternator, and this assures proper ground.

are my bulbs (osram all season 100/90) 100watts hi and 90 watts low classified as one of those "extremely high wattage"? if not, what are those "extremely high wattage" bulbs?
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Email Mr. Stern and get his opinion directly, he loves chatting about lighting and seems to be a really nice guy.

IMHO 100w is not extremely high wattage, If you used 150w bulbs you'd need to step up to 10awg. The numbers I've provided are calculated for your need, not a generic chart. 16 amps is not a huge amount of power.
spyghost, I believe I read somewhere that the life expectancy of these high wattage bulbs was less than most people might desire, especially in a daily driven car/truck. But, if one was using these bulbs in a racing, that is a different story as this is the price to pay for optimum lighting. Can anyone chime in on this to verify or contradict ?? Bill
make sure to get the right type of wire too, there are specific insulation types for automotive/underhood applications.
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