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Old 05-13-2012, 03:40 PM   #1 (permalink)
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Default Fuses and relays for lighting ??

I must preface this post by acknowledging electricity is not my strongest suit, I know just enough to run wiring, make connections and follow basic schematics..... and start fires..... and letting the smoke out of delicate electronics.

88 Samurai. I just replaced the factory headlamps with DOT approved clear lens composite lamp housings and replaceable H4 Osram Night Breaker Plus 60/55w bulbs. I may go up a step or two in wattage depending on how these perform. I will not go to HID w/o the correct projectors, I want bright but I don't want to be that guy that feels the need to blind other motorists needlessly by putting HID bulbs in lamps designed for halogens.

I am also adding 130w 6.25" lights. Depending on how they perform they may get HID converted later on as it wont effect other road users, unless I choose for them to on special occasions.

Also adding an air horn, Nautilus by Stebel. Its an integral air horn/compressor, compressor is about half the size of a can of pop. I have no idea how much amperage it draws, no wattage rating or anything on it. I have two of these and will add the 2nd one if one fails to penetrate a thick skull with a cell phone grafted to it. I drive in Rapid City where the pinnacle of douchedom and negligence is realized daily by far to many. I hate using the horn/s but when I do it must WORK!

I plan to run power straight from the battery to a 3 slot fuse box, one fuse and relay per each new circuit.

Will the 30amp relays I already have work safely for the above mentioned circuits under circumstances where extended use is expected, if no, what do I need?

What fuse amperage value should be placed in each of these circuits?

Open to all suggestions including wire gauge and schematic recommendations. I saw the headlight wiring upgrade schematic somewhere and I planned something similar.

Thanks for any input.

Tony

Last edited by caseyjones955; 05-13-2012 at 03:43 PM.
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Old 05-13-2012, 10:57 PM   #2 (permalink)
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Headlights:
DOT approved marking on the headlight may not be definitive. There's apparently little or no enforcement. The brighter 80/100W headlamps may not be highway-legal. You may want to verify.

Wire size is dictated by the current (Amperes) it carries.
Amperes = Watts/Volts. So your 65W headlights would draw around 65W/12V= 5.5 Amperes each. Your 130W lights would therefore draw just over 10 Amperes each.

Wire size is chosen by either of two limiting factors:
1) Temperature rise, determined by the current (squared) flowing in the wire
2) Allowable voltage drop/loss, which is determined by the wire length and the current.

#14 AWG (American Wire Gauge) wire is generally rated for 15A based on temperature rise. Short, single, un-bundled lengths of wire might be OK for 20A. Similarly,
#12 AWG wire is generally rated for 20A, but short, single un-bundled lengths might be OK for a bit more, say 25A.

In a 12V application, any voltage loss is 10 times larger (in percent) than with domestic 120V at home. For headlights, I would urge that you be very conservative and select slightly heavier wire (lower gauge numbers) than minimally required in order to minimize voltage drop and thus maintain brightness. Use your relays to keep the high-current runs as short and direct as possible. The relay coil-control wires can be any convenient length of skinny wire, as they carry only about 1/8 Ampere, a trivial amount.

Fuse selection:
Tungsten filament lamps draw a huge cold, inrush surge, possible as much as 10 times the running (hot) current. Select your fuses to protect the wiring, not the load. What you're trying to do is protect against a short circuit/fire, not a simple overload. The surge is no problem as long as your circuit can handle it. So, run your relays around half their rated load current current, say 15A on a 30A relay, and they'll last as long as you need them to. Same with the motor for your air horns, although here you can be less conservative, since the duty cycle is so low (hopefully!). Standard automotive ATO/ATC fuses could be a good choice, as long as you observe a generous de-rating margin. Use a 20A fuse in a circuit that draws no more than 15A. Fuses can and do fatigue over time, causing unexpected failures, so it's best not to stress them unnecessarily. Carry spares.

Last edited by Merlin93; 05-14-2012 at 01:11 AM.
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Old 05-14-2012, 01:28 PM   #3 (permalink)
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Excellent response! Appears I have everything I need to get this going.

Thank you!

Tony
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Old 05-23-2012, 01:59 PM   #4 (permalink)
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I have it pretty well sorted, just mounted all the lamps and ready to begin wiring.

I looked at some schematics for the headlamp improvement and they show one relay per headlamp. According to calculations of amperage draw wont a single 30a relay be perfectly adequate for both headlamps? Is there some other reason I would need one per lamp?

Thanks
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Old 05-24-2012, 05:15 AM   #5 (permalink)
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There's a number of headlight rewiring schematics floating around, and each has some good points. Suzuki uses an always-hot, switched ground approach for headlights, which is different from the way others may choose to wire headlights. Relays, like any electro-mechanical device (switches, connectors, fuses, etc.), are relatively high failure-rate devices. Consider the consequences of a component failure. Suzuki fuses each circuit separately, so you'd at least have high-beam if the low-beam circuit fails, and vice-versa. In my line-of-work, we call this a single-point failure and we try very hard to arrange our circuits so that we cannot lose the mission if any single component fails. Maybe you'd don't need spacecraft-level reliability, but at least _consider_ the result of the more likely component failures -- at night, in the rain and miles from civilization.
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