yea, I can read wiring diagrams and reproduce harnesses......but the math side of amps and volts and watts is kind of over my head....so here's my question I just installed a set of combo fog/driving lights. they come with 2 pairs of 55w H3 bulbs, and a switch designed to only have one pair on at a time. I've replaced the switch with 2 switches, to allow the 2 pairs completely independent control (including both on at the same time). and replaced the driving bulbs with 100w bulbs. I'm wondering if it's safe to run them all on at the same time on the included 12 or 14 gauge power wire, and what fuse I should run (I blew the included 15a one after running them both for only a few minutes). short version: what amperage would 2 100w bulbs and 2 55w bulbs draw? and would that be safe to run through a 12-14 gauge wire?
I know that: Amps = Watts / Volts and standard amperage ratings are: 12 ga = 9.3 amps 14 ga = 5.9 amps
call it 26 amps (total watts divided by voltage error on the high side), and no (unless you are only talking 2-3 inches). That kinda of amp draw, you best bet would be to build up a relay bank and feed it with some 8ga wire. From each relay out to the bulbs you could get away with the 12-14ga wire. If you use a relay bank, you are keeping your total length if wire shorter on the power side, and you can do whatever you want on the switch side. So I will give you a short setup that I have on my truck. 96 Gm pu, seperate high, low, and rally lights. The stock harness sucked. Had about 12 different connections and about 8-10 miles of wire. At the back of the bulb with the truck running, i wouldonly get 10-11 volts. I set up a relay bank that is fed with 4ga wire. the stock harness controls each relay. So now I have 14.5 at the back of each bulb. When i turn on the high beams, it will also leave the low beams on. I have a 3 way switch for my rally lights that will energize the circuit 2 different ways. either though the switch, or with the high beam switch. So I can actually have all 6 lights on and still have 14.5 volts at the back of each bulb and no large current draw through any of the harness wires. Russ
only if they chatch me, but where I have the relay bank and such located, no normal cop will ever know. I can turn off the rally lights 2 different ways, so the only thing they could really get me for would be the high and low beams being on at the same time, but then that falls into the 4 forward facing beams thing and I should be ok there. Russ
off the top of my head and with no math (just audio experience) i'd run 12 gauge to each light with a 25-30 amp fuse getting it exactly right really isn't that big of a deal- just don't put a 100 amp fuse on 12 gauge (because it could catch fire before the fuse would blow) just my opinion -drew
if you run it and it blows fuses, then upgrade. but I'd just go 8-10 right away anyway, that way your new fun bulbs will get what they need to shine as bright as they can.
Those values aren't applicable in this case. This is "Chassis Wiring" and hence 14ga can carry up to 32A and 12ga up to 41A. However, depending on the length of the wires, voltage drop may be a big factor. I would rewire each pair of lights through a relay and use a 12ga power wire to feed each set. The ground wire should be the same gauge too, the wires controlling the relay can be pretty much anything because the current draw of the relays is so small; 18ga would be fine.
If your runs are long (and if you use relays, they shouldn't be), then you might want to step up from the 12ga. For instance at a 17A load a 10' run of 12ga would have a voltage loss of about 5.7%. Going to 10ga would drop that to 3.5% and 8ga to 2.2%
they are a bit, with the included harness, there's probably ~6ft of wire between the battery/lights and the relays. this way I can get the relays up in the cabin instead of in the engine compartment (always my preference, I don't like leaving any more than just wiring in the compartment). I should see what voltage I'm getting at the lights. the included 15a fuse worked for quite awhile. so the draw can't be too much more than that. I replaced it with another 15a last night, and it's held pretty well, primarily with just one set of lights on at a time, but ran both at once more than occasionally. I'm thinking the easiest way to upgrade, would be just to add a second wire from the battery to one of the relays, and use the existing wire for the other one. and maybe use a slightly thicker wire, and run it to the relay for the driving lights (100w bulbs).
So wait, you had 15a fuse on a wire feeding all of the 310w worth of bulbs and it worked for a while? Get different fuses first.
If he had enough voltage drop across the length of the wire the amperage would have dropped to low enough for a 15A fuse to almost make it.
yea....why I'm not really buying the 26a number. I ran these same lights (old pair, all 55w bulbs), another pair of lights on the roof, my radiator fan AND an amp on a 25a fuse (4 gauge wire to the relays) and didn't blow it. maybe I'll check the voltage at the lights tomorrow.....but it's really freakin cold.....maybe I'll wait. btw, used both pairs of lights for awhile again tonight, 15a fuse holding just fine.
Ther are a couple of things going on also. Most cheapy fuse's have a 25% tolerance, so that 15 amp fuse COULD be as high as 19 amps. Also, the 26a figure I gave ya is 310 watts divided by 12 volts. If you did 310 divided by 14.5 volts that goes down to 21a. Now, what wattage are the bulbs?? Really, what wattage are the bulbs. You can't honestly tell me that a 2.99 bulb is going to hold a untra tight tollerance. Basically you are skirting the edge of the fuse. Maybe the 100w bulbs are really on 80watt bulbs, that would drop your load down to 18a at a 14.5 volts. Again, I would just run a large main power wire (8ga or larger) to your relay bank and from there I would run 12ga out to each light. You are ALWAYS better off running larger wire, and the more stands in the wire, the better. Russ
