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Old 07-23-2010, 03:32 PM
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CardinalFJ60 CardinalFJ60 is offline
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Default aux. fuse block...what gauge wire to power?

I have a 6 blade fuse panel that'll handle 30amp per fuse with a total of 32v. I'm going to clean up the wiring under the hood of the 40 and wondering what gauge wire I should use to bring power to it? I'll probably run driving, fog and rear aux lights off the rack, as well as radio and CB and in the future Ham. Lights aren't anything crazy, just 55w hellas that I have already.
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Old 07-23-2010, 04:00 PM
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Shawn,

Do a search here as I know this has been discussed before and there were some good links to websites posted up about gauge sizing etc.

A quick Google search netted this: http://www.the12volt.com/info/recwirsz.asp

The answer depends on how many total amps you plan on running off the block (looks like the max you could do is 180) plus the distance from the battery. You want to make sure you are getting the full power needed to the block and not losing a bunch in heat/resistance in the cable do it being too small. I ran 2ga power and ground wire from my battery in the LX to the fuse block under my PS seat. For you that might be overkill and either 4ga or 8ga would be plenty robust.

I wanted to exceed the requirement and only do things once so I went with the big and forget about it plan - So, if you figured your max is 180 amps and the block will be 8' or less from the battery I'd go with 4ga cable and forget about it. 8ga would limit you to about 1/2 of the max the block will pull at that distance, theoretically. Of course you'll probably never have all of the light and the CB and the HAM etc. all running at the same time so...
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Old 07-23-2010, 04:10 PM
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CardinalFJ60 CardinalFJ60 is offline
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Default Thanks!

That's the just nudge in the right direction I needed. I'll searchy-search and post my progress when I get to this project.
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Old 07-23-2010, 04:18 PM
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I just did this in my 80, I used 6ga I believe.
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Old 07-23-2010, 04:19 PM
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If you think you'll use 30A on each of the 6 fuses, then you need about 4AWG, maybe 2AWG, for 180 amps. But that's unrealistic, you should add up what the maximum sustained power you'll need and size the feeding fuse and wiring for that. Then you size the individual fuses based on the size of the branch wiring required for the load.

So say you have 55W fog lights, a 120W compressor, 65W radio and 35W aux lights. That's a total of 275W and at 12V that means you need to feed the block with 23 amps. Give yourself some head room and feed it for 40A, which is about 10AWG and matching 40A fuse or breaker. Then size each load for 15A, which is about 14AWG and a 15A fuse or breaker.

You match the fuse size in distribution to the wire size, not necessarily the load. Each load (save for lights probably) will have a fuse or breaker to protect it. If the load does not have a self protecting fuse, then you can fuse it appropriately. But even lights have a fuse, the filament itself will melt open. Ultimately the distribution fuse is designed primarily to protect the wire insulation from melting and causing a fire, not protection of the load.
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Old 07-23-2010, 05:14 PM
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Dave explains it so much better than I

Yeah, don't forget to put a fuse inline, or breaker, on the power feed to the aux fuse block like Dave mentions. Like I mentioned, my setup is overkill but I'll never have to redo it

Question I guess for Dave...If a person only needs 10ga wire but chooses to use say 8ga, or 4ga, or even 2ga, there isn't any "risk" in doing so right? Other than wasting the $$ on the more expensive cable/wire I guess .

In my case I planned to power my CB and HAM, plus some DC power outlets (for the fridge etc.) off the aux fuse block and I wanted the cleanest power I could get without taking stuff all the way up to the battery. So I figured go big on the power cable to fuse block would help insure clean power...It will be REAL overkill when I get my second battery installed and the power run to the fuse block gets even shorter
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"...anything else i can do for you guys, how about i wash your car or mow your lawn while you figure out your firewall system? I am now boarderline insane/unibomber." Kipper

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Last edited by corsair23; 07-23-2010 at 05:38 PM.
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Old 07-25-2010, 09:49 AM
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The fuse block probably also has an overall rating (80A or 100A) is likely. It is unlikely that it is rated to handle 180A.

On my 80, I wired & fused the block at its rated max (100A, in my case).

In sizing my wiring, I target <3% voltage drop. Typical automotive is 10% or more.

To do this, you need to full circuit length (2x the distance from the battery to the fuse block, "as the wire runs"). You'll also need the max draw of the fuse block. You can find charts on the internet that give you the resistance per foot for various wire sizes. From there, a bit of E=IR (Voltage = Current * Resistance) arithmetic tells you the voltage drop to expect. You then divide this into 12V (I use 12.8v) to get the % drop.

The results typically result in wire a size or two larger than typical charts tell you.

If your fuse block supports it, you should also consider running both a + and - cable from the battery to the fuse block. (Only the + needs to be fused.) This gives you a "clean", "known-good" ground, and ensures that your sizing calculations are correct (body grounds can have surprisingly high resistance).

You'll find that many Ham & CB radios will recommend direct-battery connections (no body grounds).

If you have marine-grade hardware & charts, they are typically rated very conservatively. You can safely use their numbers/sizes directly.

I find that most automotive/generic stuff is unrated, or aggressively rated. I use my own calculations for these.
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Old 07-25-2010, 10:30 PM
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4 gauge and forget it. It is cheap insurance and you can load it up in the future as plans change.
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Old 07-26-2010, 01:34 PM
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I used 6 gauge and it is fine for the light loads I have on it. I guess I'll recalc since I'm ready to add seat heaters!
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Old 07-26-2010, 02:52 PM
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[quote=corsair23;156369]Dave explains it so much better than I

If a person only needs 10ga wire but chooses to use say 8ga, or 4ga, or even 2ga, there isn't any "risk" in doing so right? Other than wasting the $$ on the more expensive cable/wire I guess .
QUOTE]

There is no risk with using a wire gauge larger than your minimum requirements as long as the lugs in your fuse block can accomodate it. The risk is when people start using larger fuses than what the wire size is rated for. Then the wire becomes the fusibile link and the risk of fire increases.
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