# Relay & Fuse load capacity?



## NightShift (Jul 13, 2001)

I am getting a sealed beam conversion for my car which has a 4 headlamp system. Im going to have (2) 90w/100w H4 bulbs for the low/high beams and (2) 100w H1 bulbs for the high beams. Total watts for the low beams = 180 watts, high beams - 400 watts.
Now i am going to relay these directly to the battery and i was wondering if 30 amp relays and fuses would be able to handle this or would they blow right away? Would i need to find 40 amp relays and fuses...seems like the highest radio shack has are 30 amp automotive relays. I appreciate any input.


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## Brock (Jul 13, 2001)

Are you going to use the existing low beam to power the low beam relay? That should work. Then I would suggest adding 2 relays for the hi beams, one 30amp for the hi/low and one for the just the hi. That should handle it. When I redid mine I left the left side headlamps alone, so if any of my relays failed or I blew one of my fuses the left side worked like normal, then used those to power the two relays for the right side.


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## NightShift (Jul 13, 2001)

I was going to use one relay for the low beams, and one relay for the high beams. Of course the higher load would be on the high beam relay since its operating 4 lamps instead of 2. And yes, i would be using the original headlamp switch to power the relay.

Also...for wiring this stuff...does 10 guage wire sound good for about 25-30 amps?


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## Brock (Jul 14, 2001)

Yes 10 should be fine, I sounds like your on the right track just make sure you have it fused at the battery side.


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## Go Go Gadget Flashlight (Jul 16, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Brock:
*Are you going to use the existing low beam to power the low beam relay? That should work. Then I would suggest adding 2 relays for the hi beams, one 30amp for the hi/low and one for the just the hi. That should handle it. When I redid mine I left the left side headlamps alone, so if any of my relays failed or I blew one of my fuses the left side worked like normal, then used those to power the two relays for the right side.*<HR></BLOCKQUOTE>

I'm with Brock on this one for sure.





I did a car of mine with a single relay for 4 high beams and fried the 30 amp Bosch relay I used. You really ought to use 3 relays. One for the Low's of the dual beam, one for the High's of the dual beam, and one for the High bulb. 

I ran a fresh supply wire from the Positive battery terminal to the supply side of all 3 relays and used parallel fuses (two 25A's) to get 50 Amp supply capacity.

The trigger for each relay is the OEM wire from the headlight harness(es).

That's how I did mine after melting the relay and it was fine after that.

Let me know if you want a schematic, this is hard to describe.


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## NightShift (Jul 16, 2001)

Before I checked back and read that last post, today I hooked the high beams up using a 30 amp relay and fuse. I was thinking of changing the wire from 16GA to about 10 or 12GA from the battery to the relay and from the relay to the light, but I guess it wouldnt do any good since the rest of the lights are wired with like 16GA.
(Note I only had to hook up one of the high beams and it activates all 3 other lights.)

But everything worked fine...I drove it around tonight with the high beams on for like 10 minutes and it all worked fine and they are much brighter. I felt the wires after that and they felt slightly warm, but so was the engine which they were near...so couldnt really tell. 
Im doing the low beams tommorow.


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## Go Go Gadget Flashlight (Jul 18, 2001)

Try this....





With engine running...
Check the voltage at the back of the bulb (High beams ON). 
Compare this to the battery voltage (also with High beams ON).
If there's a big difference (greater than about 0.5 to 1.0 volts), you are wasting power due to wire resistance. 
Ex: If you have a 1 volt drop from battery to High beam bulb, at about 33 amps, you have 33 Watts of waste.

If that's the case, you might want to step up to the 10 awg wire.

On the flipside, the wire resistance (16 awg) may help to make your bulbs last longer.


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## NightShift (Jul 18, 2001)

After i did all the wiring, i did check the voltage at the bulb and battery and was dissappointed to see at the battery it was 12.17V and the lights were at 11.70V. But it was better than before when the lights were at 9.30V. Next time im at radioshack i'll get some 10 guage wire and connectors...then all i have to replace is two out of the four wires on the relay cuz the switch activating the relay is low current. I'll check the voltage again today.

You dont need a big fat ground wire, right? Is it just the positive that needs to be big?


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## Go Go Gadget Flashlight (Jul 18, 2001)

The ground carries the same current, but since you are dropping less than 0.5v, you prolly shouldn't worry about it.





Sounds good the way it is, and the 10 awg can only help.


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## NightShift (Jul 18, 2001)

Ok , well i have to re-do everything anyway...the right side of the car's lights are only getting 10.95V...so im gonna pick up some 10 guage wire and fuse holders and do it over tommorow. Thanks for the help





Gonna need a bigger alternator after all this lol...


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## Go Go Gadget Flashlight (Jul 19, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by NightShift:
*Ok , well i have to re-do everything anyway...the right side of the car's lights are only getting 10.95V...so im gonna pick up some 10 guage wire and fuse holders and do it over tommorow. Thanks for the help





Gonna need a bigger alternator after all this lol...*<HR></BLOCKQUOTE>

Don't laugh, you may very well burn out this alternator if the new bulbs load it past it's rating. Good luck.


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## Brock (Jul 19, 2001)

It is true, most alternators will supply about 1/2 the rated capacity on a continuos basis. I burned mine out, but I was running over 1o0 amps on a 80 amp unit. I replaced my alternator with a 150 amp marine unit. If you have a standard 60 amp unit it should run about 30 amps. Also the 60 amp rating is at peak output or hi-way speeds, in city driving it will probably only put out 20 amps.

Overall you will probably be fine, but just keep this in mind if you are running the brights for a while and other large 12v loads (AC or trailer lights).


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## NightShift (Jul 19, 2001)

I was planning on upgrading to a 130 amp (or higher) alternator too...


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