Over Current Protection On Pos Battery Connection - Page 2 - SailNet Community
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post #11 of 38 Old 02-24-2012 Thread Starter
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I'm getting ready to add a fuse to my setup. Not sure your exact application but MRBF fuses from Blue Sea look pretty easy to add to an existing setup. Their site also says 7" is AYBC requirement. Terminal Fuse Block, 1 Terminal Stud - PN 5191 - Blue Sea Systems
Thanks for everyones input. I was considering this Terminal Fuse Block at the Battery Terminal, but I also have about 85 watts of solar panels that tie directly to the battery. Seems I may not be able to attach both the boat loads and the solar panel to the same terminal fuse as the terminal fuse would be rated higher than the wire size of the solar panel- thus the wire would not be protected. I do have a blue sea safety hub:
SafetyHub 150 Fuse Block*—*Blue Sea Systems
But still need to run a wire from the battery to it. Maybe use the Terminal fuse block at the battery post and run a large cable to the safety hub. At the safety hub the solar panel and panel board would be attached (and have their own fuse protection).
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post #12 of 38 Old 02-24-2012
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I would fuse the solar panel separately, and it could still be connected directly to the batter terminal or the terminal fuse.
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post #13 of 38 Old 02-24-2012 Thread Starter
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I would fuse the solar panel separately, and it could still be connected directly to the batter terminal or the terminal fuse.
I was thinking using the high amp connection (which is fused) on the safety hub to connect the solar panal output wire. This solar output wire would be fused at at about 20 amps with the battery post fused sized at say 100 amp (with appropriate wire connecting the battery to the safety hub).
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post #14 of 38 Old 02-24-2012
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Fuse size should be based on the maximum amp capacity of the equipment on the load side; usually based on wire size. The main terminal fuses are very good and have come down in price. I used ANL fuse blocks instead when I rewired the batteries a couple of years ago- three batteries: one start battery, fused alone at 300a, and two house batteries in parallel, ANL fused at 100 amps leading to a sub panel (rated at 100a)for house circuits. This little fuse block is located within the 7" limit and serves as the solar panel input/fuse point as well, wired to a bulkhead plug that is easy to access to plug/unplug panels as needed. I also installed a crossover charging relay. The individual house circuits for lighting, etc, are fused at wire ratings, usually 15 or 20a for #14 or #12 wire. (note:I understand that some bilge pumps have started fires before reaching designated fuse rating).All large 2/0 wires off the batteries meet the 7" spec. that the CG uses. Have not had any trouble blowing the 300a starter fuse. The ANL fuses have good "slo-blo" characteristics if you look at their graphs. It is amazing that more of the old boats did not go up in flames with the long, unfused main wires they used.
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post #15 of 38 Old 02-27-2012 Thread Starter
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Fuse size should be based on the maximum amp capacity of the equipment on the load side; usually based on wire size. The main terminal fuses are very good and have come down in price. I used ANL fuse blocks instead when I rewired the batteries a couple of years ago- three batteries: one start battery, fused alone at 300a, and two house batteries in parallel, ANL fused at 100 amps leading to a sub panel (rated at 100a)for house circuits. This little fuse block is located within the 7" limit and serves as the solar panel input/fuse point as well, wired to a bulkhead plug that is easy to access to plug/unplug panels as needed. I also installed a crossover charging relay. The individual house circuits for lighting, etc, are fused at wire ratings, usually 15 or 20a for #14 or #12 wire. (note:I understand that some bilge pumps have started fires before reaching designated fuse rating).All large 2/0 wires off the batteries meet the 7" spec. that the CG uses. Have not had any trouble blowing the 300a starter fuse. The ANL fuses have good "slo-blo" characteristics if you look at their graphs. It is amazing that more of the old boats did not go up in flames with the long, unfused main wires they used.
Good advice. Thanks. I will run calcs on the wire size and loads and size the fuse protection accordingly.

I am surprised my boat has not caught fire. Half the wire does not have proper over current protection and the 225 amp hour battery is held down by steel angle iron. A slight shift of the battery will put that angle iron across the + and - battery terminals and then puff.... That item is on my list.
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post #16 of 38 Old 02-27-2012
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I ran across this document from Blue Sea while researching the fuse size for my battery. http://bluesea.com/files/resources/r...010_Rev002.pdf. It has fuse sizes for each wire size.
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post #17 of 38 Old 02-27-2012
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Good advice. Thanks. I will run calcs on the wire size and loads and size the fuse protection accordingly.

I am surprised my boat has not caught fire. Half the wire does not have proper over current protection and the 225 amp hour battery is held down by steel angle iron. A slight shift of the battery will put that angle iron across the + and - battery terminals and then puff.... That item is on my list.
You're welcome. My old boat had no main fuse protection at all and a 2/0 starter wire that snaked through the bilge, through the bulkhead, and under the engine for about 8'! It was an electrical fire waiting to happen. I built in a new battery box, low and up against the bulkhead in the starboard cockpit seat hatch that fits 3 big batteries. Used a piece of 5/4 X 3 cedar with a 1" X 1/8" strap on top of it to hold down the batteries. The wood is less likely to create any sort of circuit and is less abrasive to battery plastic. It keeps the strap well away from terminals and also served to mount the two ANL fuse holders within the 7" limit. It also makes it pretty easy to disassemble the whole thing to yank the batteries in the winter.

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post #18 of 38 Old 02-28-2012
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Fuse size should be based on the maximum amp capacity of the equipment on the load side; usually based on wire size.
Well, yes and no. The fuse protects the wire, so it is ALWAYS based on wire size. If you have 80 amps worth of equipment on the load side, but the wire is only rated for 50 amps then you had darned well better not have anything bigger than a 50 amp fuse on it!

So, first you figure out the max current the wire might have to carry. Then you pick a wire that is capable of carrying AT LEAST that much, and preferably a bit more. And finally you fuse the wire so that it cannot exceed its rated amperage.

Of course, if your wire is grossly over-sized for the load, you can use a smaller than necessary fuse if you wish. This would be the ONLY case where you might pick a fuse based on the capacity of the equipment instead of on the capacity of the wire. But, even in this case, really, you are still fusing the wire so that it cannot exceed its rated amperage.

That's the bottom line that a lot of people lose sight of. The fuse is NOT there to protect your equipment, or to control the amount of electricity that can get to your equipment. It is there to protect the wire, and thereby prevent fires.
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post #19 of 38 Old 02-28-2012 Thread Starter
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I ran across this document from Blue Sea while researching the fuse size for my battery. http://bluesea.com/files/resources/r...010_Rev002.pdf. It has fuse sizes for each wire size.
That is a helpful document. Did not find it a few days ago when I serched the Blue Sea web site for such an item.
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post #20 of 38 Old 02-28-2012
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Well, yes and no. The fuse protects the wire, so it is ALWAYS based on wire size. If you have 80 amps worth of equipment on the load side, but the wire is only rated for 50 amps then you had darned well better not have anything bigger than a 50 amp fuse on it!

So, first you figure out the max current the wire might have to carry. Then you pick a wire that is capable of carrying AT LEAST that much, and preferably a bit more. And finally you fuse the wire so that it cannot exceed its rated amperage.

Of course, if your wire is grossly over-sized for the load, you can use a smaller than necessary fuse if you wish. This would be the ONLY case where you might pick a fuse based on the capacity of the equipment instead of on the capacity of the wire. But, even in this case, really, you are still fusing the wire so that it cannot exceed its rated amperage.

That's the bottom line that a lot of people lose sight of. The fuse is NOT there to protect your equipment, or to control the amount of electricity that can get to your equipment. It is there to protect the wire, and thereby prevent fires.
In house wiring, of course wire size and breaker ampacity always go together. This is true from your service entrance to the 15a lighting circuits. The only reason I mentioned the word "usually" is because of the bilge pump issue that someone else pointed out to me a few months back. Apparently some of the well known brands of pumps have been melting down and actually burning (as in flames) well below recommended fuse size. I have lowered the pump fuse sizes in my boat to a minimum to try to minimize this possibility. It's also common practice to fuse sensitive electronics well below wire size. I guess the difference on a boat is that you are often fusing one particular device and not a string of generic outlets. I would not want to fuse my solar panels at 20a although I use #12 wire to connect them. They never produce> 100w so a smaller fuse just raises the safety level.

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