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Old 12-15-2005, 12:35 PM   #1
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Battery Fuse

I have just finished my electric install... but have a couple safety questions.


I have 2 12V 100AH batteries in parallel. How many amps should the fuse be on my positive line?

Do I need a large grounding strap directly from the negative terminal to the shell or is the smaller (10 gauge) grounding wire from my distribution box to ground sufficient?

I have a 65 amp wfco charger/converter. Should I have an additional fuse between this converter and my dc distribution box? If so, should it be 65 amp?

Thanks!

Carlos Ferguson
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Old 12-15-2005, 01:25 PM   #2
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Carlos,

The size of the fuse has nothing to do with the size or number of batteries. It's all about the size of the wire connecting to the distribution panel.

The size of the wire or strap connecting the ground should be the same or larger than the positive cable. You want to put a fuse on this line also. The only exception would be if you have a large load, like an inverter, with fully wired positive and negative cables.
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Old 12-15-2005, 01:31 PM   #3
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Quote:
Originally Posted by markdoane
Carlos,

The size of the fuse has nothing to do with the size or number of batteries. It's all about the size of the wire connecting to the distribution panel.

The size of the wire or strap connecting the ground should be the same or larger than the positive cable. You want to put a fuse on this line also.
Hi Mark, thanks,

I have a 4 gauge wire connecting my batteries to the converter/charger, then a 10 gauge wire running from the converter/charger to the dc distribution panel. Should the 10 gauge be larger?

With this in mind, how would you fuse the wires?

I should use three fuses - one for the ground line, one for the battery to converter, one for the converter to distribution box?

Thanks for your help,

Carlos
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Old 12-15-2005, 02:06 PM   #4
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Carlos,

It depends a lot on what the total loads are that you intend to power from the distribution panel. If it's only a few lights and other circuits then the 10ga would be fine, with a 30a fuse. You can fuse it larger, depending on the length, location, and temp rating of the wire you used. 30a is conservative, you can go as high as 60a with 105F rated wire in non-engine locations.

I put in a 60a intellipower, and I ran #6 wire to the battery and to the distribution panel. I have 15 separate DC circuits. I ran both positive and ground lines directly from the battery to the converter to the panel. I have separate ground connections to the chassis at the battery, at the converter, at the distribution panel, and at selected locations.

In each case, I put in a fuse or circuit breaker that corresponds to the size of the wire.
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Old 12-15-2005, 02:32 PM   #5
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Thanks for the info Mark -- That sounds good - basically I have the same setup. Do you have a reference for how much current a wire can carry (and thus what fuse to use) or do you just have a sense of it from having wired a lot? I am still wondering about the fuse for the battery - I assume larger than 65 amps since the converter could potentially put out that much.


Thanks again,

Carlos
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Old 12-15-2005, 02:40 PM   #6
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Carlos,

Here's a chart I use. Note the ampacity is different for DC and AC circuits.

I don't trust my memory for anything anymore.

http://www.cmsquick.com/Tech.html
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Old 12-15-2005, 02:45 PM   #7
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Great!

Thanks,

Carlos
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Old 12-16-2005, 09:07 AM   #8
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Mark - I wonder if you could look this over for my fusing solution:

1 - the charger line (10 gauge wire) to the umbilical will have a 40 amp fuse

2 - the main line from the battery to the converter (4 gauge wire) will have a 100 amp fuse

3 - the line from the converter to the dc distribution box (10 gauge wire - though I may increase this size) will have a 40 amp fuse

4 - all branch circuits will have a breaker at the dc distribution box, sized according to wire size.



Now, if this is correct - I wonder if you could describe what these fuses look like and where I can buy them?
I have a 40 amp circuit breaker that I bought at an auto supply store for about 3 dollars - I am wondering if this type will work - I will attach a picture of of what it looks like.


Thanks, your help is great.

Carlos
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Old 12-16-2005, 09:16 AM   #9
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This is another model I found that might work?


60 AMP AGU FUSES

60 Amp. 1.5" long x 0.41" diameter. Gold-plated fuses for high current applications. Sold in packages of 5 pieces. 32 Volt max.
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Old 12-16-2005, 09:40 AM   #10
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Quote:
Originally Posted by jcferguson
1 - the charger line (10 gauge wire) to the umbilical will have a 40 amp fuse

2 - the main line from the battery to the converter (4 gauge wire) will have a 100 amp fuse

3 - the line from the converter to the dc distribution box (10 gauge wire - though I may increase this size) will have a 40 amp fuse

4 - all branch circuits will have a breaker at the dc distribution box, sized according to wire size.
This is how my 75 Trade Wind is fused. All of the fuses you listed are on the 12 Volt fuse Panel.

1. 40 amp
2. 50 amp
3. Not fused
4. 20 amp (4 circuts)

My setup is pretty much original. I have the old style glass fuses and the univolt that hums along nicely. Most of my camping with out hook ups, so this is not an issue. When I upgrade (most likely inteli-power) I will replace the fuse setup with one with the current automotive style fuses. The reason I believe #3 is not fused is because everything else is. Adding an extra fuse would not hurt.
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Old 12-16-2005, 10:21 AM   #11
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Carlos,

I think I would use a 30a fuse on the charger line. It is a long run and you can't be certain of the temperature rating on the wire throughout the run. Also, I don't think you will ever get more than 15 to 20 amps through the charger & umbilical anyway. If a 30a works, you will have a little more safety. If a 30a blows continually you should look for a leak somewhere.

The 'legal' fuse sizes according to NEC are:

18 awg = 6a.
16 awg = 8a.
14 awg = 15a.
12 awg = 20a.
10 awg = 30a.

I know these are more conservative than the chart I gave you yesterday, but this chart is for lower temp wire.

Also, the minimum wire size for the battery ground is 8awg, unless the power lead from the battery is greater, in which case the wires shall be equal size.

For the main battery fuse, I would use a type T fuse rather than a glass fuse like the AGU you have shown. I don't know if the AGU is even available in 100a? I would use a 110a type T on a #4 power lead.

The circuit breaker you show is very common. You can buy premade connecting straps to gang together a row of circuit breakers. Let me know if you want a vendor. You can also make connecting straps out of copper.

I think a 40a fuse on the converter-distribution box is good if the run is short and you use good wire. This line might already be fused in the converter.
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Old 12-16-2005, 12:42 PM   #12
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Mark,

A vendor would be great.

How do you feel about automotive atc fuses - or fuseable element cartridges? I could get them at any auto parts store if they blew... and I found some with a nice cover that I could install inline between the converter and dc box - and on the charge line.

A 110 amp fuse on both the positive battery lead and ground line?

Carlos
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Old 12-16-2005, 07:38 PM   #13
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Carlos,

Sorry for the delay, I was Christmas shopping.

Here is the vendor for ganging together automotive breakers, although I think I would just drill some holes in a strip of copper. This vendor has everything you will ever need.

http://waytekwire.com
(go to the Products page, then circuit breaker/automotive-accessories category)

As far as a fuse on the ground side of the battery, i don't think it's makes much difference. If you put a 110a fuse on the + side, using the same size on the neg would reduce the number of spares you need to carry.

I really like the ATC fuses. The only good thing about the glass tube fuses was you could see if they were blown, and since my eyes are as good as they once were that isn't an advantage anymore.
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