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Discussion Starter #1
i bought a small fuse box to hook up the two wires from the alternator to the relays. Im also going to hook up the switching wire to this fuse box also.

I need to know what size fuse i should run between the alt. and the relay. These are the diamond type h4 lights for sale on ebay that im using, Also which sized fuse for the switching power wire? The one connected to the headlight switch that tells it when to turn them on. Thanks!
 

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When I did my conversion, I used 10 amp fuses for the headlights. Considering that would be good for at least 120 Watts (more like 140 Watts with typical charging systems), that would probably work for you. I left the old fuses in place for the relays.
 

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The fuse is supposed to be your failsafe in the circuit. Too high an amperage fuse and you defeat its intended purpose. A balance is necessary.
Questions:
What are the wattages of the bulbs? (high / low)
What is the gauge of the wires you are using?
What are your relays rated to? (amps or watts)
How many relays?
Do you have any idea what the circuit will draw? (peak amperage at initiation)
 

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Discussion Starter #4
relays rating=30 amp relays
wires=12 guage wiring
bulb wattage=not sure, i know they are h4 and very bright!
number of relays=2 relays
circuit amp drawing power=not sure about the amps at initiation.
 

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The wattage of the bulbs is on the base of the bulbs. Without those numbers everything is pure speculation. This is why you are installing relays; to feed enough current to the bulbs to light the filaments correctly and not cause everything to melt down, fuse or combust.
A 100/90 watt bulb requires more than say a 60/55 watt. You can go with the stock headlight fuse amperage and adjust up if they keep blowing.
It is a good practice to replace all fuses annually because as they heat and cool over time they do fail. Just remember that whatever is not in balance will be the first to go. Usually it's the relay because a high amperage fuse is used and the relay points fuse internally. This is seen frequently in cheaper relays, they just don't take the current or live up to their rating.
Rally / Off Road folks may carry short, dual spaded, jumper wires to bypass the relay if the visibility becomes paramount over safety of circuitry. Try to keep the power feed fuse to the relays at or below the rating of the relay, if 30 amps then say to 25 amps. You may want to use resettable circuit breakers on the feed side, budget permitting.
If the bulbs are very high wattage you may need seperate relays for each lit filament. Two relays may not be enough to carry the demand over time, especially if you are lighting all filaments simultaneously, a popular thing to do with E code headlights.
BTW: The wire gauge may be too light if the bulbs are of a very high wattage. Also high wattage bulbs have a shortened life and can be made even shorter if fed the wrong voltage, ('halogen cycle' requirements), placed in high vibration environments, and lit up in very cold temps.
That's why I always recommend that you use the best components you can afford. There really is a difference and it's no fun losing your headlights on a dark windy road at speed!
 
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