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Old 09-27-2009, 09:04 PM   #1
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Newbie 12 volt fuse education.

I'd appreciate some advice.

I've got a 12 volt distribution box. Original from SMB. It is fairly well labeled. From 30 amps down to 3 amps.

The 3 amp fuse is for "stereo, gas." The "gas" does not refer to the furnace as it has a separate fuse. Any idea what this is?

Also, my understanding of 12 volt is still evolving. All the wires are the same gauge. is there any reason I can't run additional lines of that 3 amp fuse and bump it up to a 30 amp fuse? I want a couple of 12 volt outlets in the back.

I understand that the fuse if protecting the devices ("stereo, gas") from more amperage, but what does that mean in real life? Absent some short, are they ever going to trip the fuse. And if there is a short, wouldn't a 30 amp fuse work just as well?

I guess I just don't understand why they are stepped down from 30 amp to 20, 15, 10, 3. Why not put 30 amp fuses in them all?

Thanks

Tom
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Old 09-27-2009, 10:02 PM   #2
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Re: Newbie 12 volt fuse education.

Quote:
Originally Posted by bluedog225
I'd appreciate some advice.

I've got a 12 volt distribution box. Original from SMB. It is fairly well labeled. From 30 amps down to 3 amps.

The 3 amp fuse is for "stereo, gas." The "gas" does not refer to the furnace as it has a separate fuse. Any idea what this is?

Also, my understanding of 12 volt is still evolving. All the wires are the same gauge. is there any reason I can't run additional lines of that 3 amp fuse and bump it up to a 30 amp fuse? I want a couple of 12 volt outlets in the back.

I understand that the fuse if protecting the devices ("stereo, gas") from more amperage, but what does that mean in real life? Absent some short, are they ever going to trip the fuse. And if there is a short, wouldn't a 30 amp fuse work just as well?

I guess I just don't understand why they are stepped down from 30 amp to 20, 15, 10, 3. Why not put 30 amp fuses in them all?

Thanks

Tom
One reason you don't want to run a 30 amp fuse in that location is you loose the protection of the stereo. For a direct short, 30A or 3A will both provide protection. For something else (a shorted speaker) it may mean the difference between a blown fuse and a blown stereo.

If you want, you can ignore the next paragraph:

Volts and amps = power, which is measured in watts. 12V x 3A = 36Watts. That is a measure of the amount of heat that can be produced. 12V x 30A = 360Watts. A light bulb will generate light with some of the power, and heat with the rest of the power it consumes. A 36W bulb will generate enough power to burn your hand. A 360W bulb will generate enough heat to burn your entire body, or to start an electrical fire. A fuse should limit the available power to the amount that can be safely consumed by the smallest component that is protected by that fuse. That is why you don't want to overfuse a circuit. The risks may be small, but they are real

That said, the safe thing to do would be to cut the existing wire, add a 3A inline fuse and up the main fuse to 30A for the circuits you want to add. The inline fuse will protect the stereo without limiting the other circuits.

Mike
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Old 09-27-2009, 10:44 PM   #3
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Re: Newbie 12 volt fuse education.

Let's see, too much draw (amps) and the wire melts.

Whatever you put on the wire increases draw. One offroad light, 10 amps, two offroad lights 20 amps, 8 offroad lights, 80 amps. (numbers are made up).

Since you don't want the wire to melt, you add a fuse that is bigger than the draw, but smaller than the wire. So you start with 2 offroad lights (20 amps) and run a 30 amp wire to them. Cut the wire and put a 25amp fuse in it.

Then if Hillbilly Harry adds 6 more lights, the draw of 80 amps will pop the 25 amp fuse before melting the 30 amp wire.

In simple terms if Harry wants to look like a desert score truck, he needs to add 3 more 30 amp wires and 3 more 25 amp fuses ( or any combination- could be 6x 15 amp wires and 12 amp fuses, one for each ).

The problem is if Harry taps his 3 new wires into the original light power source right before the original fuse, he won't blow the fuses but all 4 wires and all 80 amps from the lights will be going through whatever wire brought the power to the original fuse.

In a traditional auto fuse box this "wire" feeds the one side of all the fuses and can handle their combined amperage, and usually sufficiently more.

SMBs added fuse box could probably support 30 amp x slots. Mine has 6, so I could probably pull 180 amps, although by the time I put all those fuses in I would probably start a new box. The gist is you can probably use the positive side of the fuse box for a few more things safely. At autoparts stores you can find a fuse sized splitter that allows 2 fuses in the place of one- one for the original and one for an additional item.

The reason it's OK to step down is a 30 amp wire won't melt before anything smaller in a fuse- 25, 20,... 3, they all pop before the wire starts to melt. But you can't step up because a 15 amp fuse might represent a 20 amp wire and replacing the 15 amp fuse with a 30 amp fuse to power 25 amps of accessories might melt the wire without popping the fuse.

The quickest way to overamp your + wire is to simply touch it to ground... or cut across a hot and ground together metal wire snips (D'oh!). If the fuse weren't there to pop the wire would get hot, if you're lucky at the tip (which is how we have welding) and if not somewhere inaccessible.

I'm not sure about the labeling but my 3 amp SMB installed fuses go to the CO detector and the Propane detector. I don't think 3 amps is enough to power a stereo, so it might be to power the relay of your stereo switch in the house, or that little red light that tells you when the stereo switch is on. The best way* to find out is to turn on the accessories in question and remove the fuse. If they go off, bingo!

*best way: as in how I do's it. Remember I'm the same guy who cuts across hot and ground at the same time...
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Old 09-28-2009, 12:29 AM   #4
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Re: Newbie 12 volt fuse education.

This is getting old because this is all a quick search found but still beware of cheap China fuses. If you have an old multipack from Harbor Freight then smoke may be in your future.
http://www.thumpertalk.com/forum/archiv ... 68007.html
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Old 09-28-2009, 05:17 AM   #5
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Re: Newbie 12 volt fuse education.

Thanks all. That helps.

Tom
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Old 09-28-2009, 05:42 AM   #6
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Re: Newbie 12 volt fuse education.

Quote:
Originally Posted by Stumpalump
This is getting old because this is all a quick search found but still beware of cheap China fuses. If you have an old multipack from Harbor Freight then smoke may be in your future.
http://www.thumpertalk.com/forum/archiv ... 68007.html

Sorry to bore you Stumpalump. What would you quicksearch to get this answer. I have not had much luck with our search engine.

Good tip on Harbor Freight. I've got a package of their fuses sitting in my van right now.

Thanks

Tom
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Old 09-28-2009, 07:45 AM   #7
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Re: Newbie 12 volt fuse education.

Quote:
Originally Posted by bluedog225
Quote:
Originally Posted by Stumpalump
This is getting old because this is all a quick search found but still beware of cheap China fuses. If you have an old multipack from Harbor Freight then smoke may be in your future.
http://www.thumpertalk.com/forum/archiv ... 68007.html

Sorry to bore you Stumpalump. What would you quicksearch to get this answer. I have not had much luck with our search engine.

Good tip on Harbor Freight. I've got a package of their fuses sitting in my van right now.

Thanks

Tom
I think a googled "fuses burning" but this came up under "fuse recall": http://money.cnn.com/2007/09/05/autos/f ... /index.htm
As far as search engines go then I allways use : http://www.google.com/
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Old 09-28-2009, 10:30 AM   #8
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Re: Newbie 12 volt fuse education.

Quote:
Originally Posted by Stumpalump
This is getting old because this is all a quick search found
Are you bored, or are you saying the issue of Harbor Freight fuses is getting dated and turning into ancient history which is difficult, then, to turn up in Google among the gabillion other fuse results?
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Old 09-28-2009, 08:07 PM   #9
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Re: Newbie 12 volt fuse education.

Chuckle.. I was saying that it was too old a story to find some good old threads on the topic.
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Old 09-28-2009, 10:48 PM   #10
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Re: Newbie 12 volt fuse education.

Stumpalump

My apologies. I completely misunderstood your post.

Tom
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