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Old 10-07-2009, 14:03   #1
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Fusing the Battery Cable

My surveyor on my new boat ('79 Sabre 34) dinged me on my battery cables. They need to be fused, he said (and my insurance company sagely nodded). Sounded fairly straightforward, at first. Then I found that the simple bolt on type fuses that West sells (open 1/4" hole at either end) have a recommended fuse block. Does this mean that I have a short battery cable, to a fuse block terminal, to a fuse end, to the other fuse end, to a fuse block terminal, to my battery cable? I'm not sure I gained in safety there! My thought is that it would be much cleaner (and likely safer) if I bolted the fuse directly to the battery terminal, and then using a simple 1/4-20 bolt and nut, bolted the battery cable to the fuse -- and perhaps cover the whole mess with a battery terminal cover. Is that legal?

And, is this even an appropriate finding? I can find very little guidance on this, and none of the drawings/discussions/etc on the web address it. West Marine and BoatUS have some nice web pages on battery wiring, and none show this. I also believe that the rules (at least in the past) exempted the starter cable from fuse protection.

Finally, any recommendations on a fuse size? It's a Volvo MD11C engine. I'm thinking 150A.

Thanks,

Harry
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Old 10-07-2009, 14:41   #2
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It's ABYC now to have the large fuse in there. ABYC has no regulatory authority, but the industry follows it for lack of anything else. If you've ever seen a large battery cable melt and flame from a dead short... well you'll know it's a good idea. The fuse is usually just a copper link in a holder. It should be firmly mounted, so you need a short cable to one end of the holder and attach your existing cable to the other end. If I remember ABYC correctly the fuse needs to be pretty close to the battery, 1-2ft? I'm surprised the fuse block is only 1/4 fastners to the cable though...? The fuse should be at least as large as your largest draw. Do you have an invertor? 150 amp min I would say. I dont believe it's required on a starter cable... did he write that up? I'm real surprized those sites dont show the fuse...
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Old 10-07-2009, 15:30   #3
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A CPD (circuit protection device, i.e., fuse or breaker) is required on house battery cables running to your panel. It should be within 7" of the battery, unless the cable is in a protected channel. I believe there is still no requirement for a CPD on the starter circuit, though some believe it's not a bad idea. I have one on mine....250A.

Re: direct connection to the battery lug, I wouldn't do that. The fuses are not meant to be flexed. Buy a proper fuseholder...they're not expensive....and mount it near the battery. Make up a short cable from the fuseholder to the battery. Then, just attach your existing cable to the other end of the fuse. Simple. Troublefree if done properly.

There are also fuses which are designed to attach to battery lugs. These are probably OK, though I don't much like the idea.

Bill
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Old 10-07-2009, 15:54   #4
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Yea, I remember that now, 7" from the battery. Anyone know a boat you can do that on?!
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Old 10-07-2009, 17:28   #5
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7" is tough...

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Originally Posted by Cheechako View Post
Yea, I remember that now, 7" from the battery. Anyone know a boat you can do that on?!
It's tough.. On mine the ANL fuse is actually about 5" from the battery but for strain relief the cable itself is about 10"-12" long..
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Old 10-07-2009, 19:40   #6
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Originally Posted by Cheechako View Post
Yea, I remember that now, 7" from the battery. Anyone know a boat you can do that on?!
Blue Sea Systems makes a fuse holder designed to attach directly to the battery lug: Detailed Specifications for Terminal Fuse Block, 3/8" Mounting Hole, 2 Terminal Studs - PN 2151 - Blue Sea Systems
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Old 10-07-2009, 20:43   #7
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The only type of fuse one should use on the house batteries is a class-T fuse. The reason is that this is the only fuse with a high enough interrupt-rating to handle the short-current from the battery bank. If another type of fuse is used, it could fail to interrupt the circuit (burning up but melting and forming a circuit again). The photo is the class-T fuse on Jedi with a 400A rating on the 1200Ah house battery bank.

Starter batteries are not required to have a fuse and opinions on what's better differ 50/50. We opted not to have a fuse there.

cheers,
Nick.
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Old 11-07-2009, 06:00   #8
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Quote:
Originally Posted by s/v Jedi View Post
The only type of fuse one should use on the house batteries is a class-T fuse. The reason is that this is the only fuse with a high enough interrupt-rating to handle the short-current from the battery bank...
A T-fuse may be a better solution, but it's not the only solution.

“Class T” Fuses are rated to 20,000 Amp Interrupting Capacity* (AIC).
“ANL” Fuses are rated to 6,000AIC.

ABYC requires that large 12 & 24V battery banks, up to 1,100 Cold Cranking Amps (CCA), be rated (Fuse, Cct. Breaker, or Both) to 5,000 AIC.
Smaller battery banks require 3,000 AIC or 1,500 AIC (banks under 650 CCA)

* Interrupting capacity is the total (fault) current the fuse can interrupt without damage. Higher AIC’s are better; but ABYC only requires up to 5,000 AIC in it’s most stringent case.
Accurately calculating the available fault current (dead short) is not a simple task. It’s this available fault current that determines the required interrupting capacity.

Terminal Fuses, as offered by Blue Sea Systems are rated 10,000 AIC (@ 14V), which will be more than adequate for large battery banks.

Terminal Fuses - Blue Sea Systems

Terminal Fuse Block - Blue Sea Systems

See also:
DC Overcurrent Protection (Fuses or Breakers)

PS: I presume the Fuse Cover was removed from your picture only in the interests of visual clarity.
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Old 11-07-2009, 15:48   #9
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So should I have each individual battery within my house bank (4 @ 215 Amp. hour) fused or one fuse for the whole bank?

Thanks,
Extemp.
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Old 11-07-2009, 15:57   #10
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Yes, several of the issues have been addressed here. But I'm still confused.

First, I'm not running a "house" and "starter" bank (at least not yet). So, the batteries I have are deep cycle, and they also feed the starter to my little 27hp Volvo. And it all runs through an old fashioned 1-both-2 switch. Basic, archaic, but still functional after all these years. I do intend to make changes, starting with the factory original 35 Amp internally regulated alternator. But, for now, I've got two "starter" batteries.

Now, with regard to the 7" requirement, it seems that no one can figure that out. And those that have come close have a LOT of extra joints in the high current circuit, that in my case is a starter circuit. I'm not sure I like pushing 125 or more amps through 4 extra terminals. But, my preference aside, I'm wrestling with the "how" and to make matters worse, I have to do the "how" times TWO, because I have two batteries. The Blue Sea battery terminal fuse blocks may well be the answer -- the simplicity I was looking for in a battery mounted fuse, with the durability issue raised above addressed. At $20 each plus the fuse, they aren't cheap, but nothing on a boat is. Anyone know what kind of fuse, and where it actually goes? Blue Sea's site is pretty much mum on the issue.

Now, on the fuse size, how do I decide how large? House loads are trivial (perhaps 50 amps on a really bad moment. The LectraSan has 1/2 of that), but the starter is the issue. The Volvo manual is pretty mum. It's an even bigger issue than the fuse, because I also have welding cable that I need to replace, and I'm trying to size it. I'm figuring #4 is adequate, but I'm probably going with #2 to be safe and ready for the future. It's only a 34' locally sailed sailboat, after all.

Thanks for all the input!

Harry
1979 Sabre 34-I #063
Annapolis
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Old 11-07-2009, 16:43   #11
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Wiring?

Fuses are, of course, essential to the safe operation of a small sailboat.

However perhaps more important is to ensure that all wiring is properly sized, in good condition and mounted securely so as to avoid damage. Good insulation and grommets (to avoid chafe) should also form part of the electrical system. Crimps should be insulated where necessary and all connections should be snug and firmly bolted (where a nut is necessary) using quality hardware.

After all, the fuses are only there in case the wiring fails catastrophically. Fires can start with only a spark.
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Old 11-07-2009, 22:14   #12
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Gord: large battery banks up to 1,100 CCA are in fact very small banks for most cruisers. A simple 12V, 80 Ah Trojan AGM battery is 440 CCA already.

It is indeed hard to find data but using Google books I found that a 800Ah battery bank can deliver a short circuit current of up to 8000 amps. The typical maximum value is reached 5-25 milliseconds after completing the short circuit.

But wait... I just bought a couple of Odyssey PC2150S batteries for starting service. Each one is 1150 CCA and "more than" 5000A short circuit current. But it is just a 100Ah battery.... see Odyssey PC2150 series battery

So, my observation is that 1150CCA equals 5000A short circuit current and for deep cycle batteries it's about 10 times the Ah rating (instead of 50 times like with the Odyssey).

Boats with house battery banks of 500 Ah and up should really use a class-T fuse.

Extemp: The price of the fuse and the holder will make you use 1 fuse for the whole bank if your bank is substantial.

Harry: you should be very glad to spend that $20.- per holder plus the $15 per fuse. You get away with this very very cheap. My class-T fuse holder costs $50 and the fuses $40 each. Next are the AWG 4/0 cable and terminals for connecting it, the busbars where the parallel strings of batteries terminate with all their cables, terminals etc.
Your 2-battery setup is screaming for that terminal-fuse solution. You're ready to go at $85.- total cost (2 holders + 3 fuses). Installation time will be minutes.

Boracay: what these main battery fuses and switch-panels breakers do is protect the cabling itself, not the devices powered by them (they should have their own fuses for that). So you must know the maximum current that is allowed through the size wire used and fuse at that value or a lower value. If that's not enough amperage, both the wire and fuse should be replaced, not just the fuse.
Damaged wiring from chafing is common, especially when connecting a device that vibrates like a diesel engine. I also see a lot of damage that occured during installation of wires, like when pulling through a bulkhead. I find it amazing that so many cruisers without the knowledge install electrical systems themselves without reading and following the many books on this topic. Most of them have at least Nigel Calder's book on the shelf.

cheers,
Nick.
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Old 11-07-2009, 22:16   #13
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For completeness:

ABYC standard E11.12.1.1.1 states that each ungrounded conductor connected to a battery charger, alternator, or other charging source, shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point of connection to the DC electrical system or to the battery.
The exceptions to this are:
  • When the conductor is enclosed in a sheath or enclosure such as conduit in addition to its insulation the 7" dimensions can be increased to 40".
  • If the conductor is connected directly to the battery terminal and is enclosed in a sheath or enclosure such as conduit in addition to its insulation the 7" dimension can be increased to 72".
  • The conductor connects to the starter for the engine.
cheers,
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Old 12-07-2009, 05:52   #14
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But wait... I just bought a couple of Odyssey PC2150S batteries for starting service. Each one is 1150 CCA and "more than" 5000A short circuit current. But it is just a 100Ah battery.... see Odyssey PC2150 series battery ...
Higher interrupting capacities are certainly better, but 5,000 - 6,000 AIC will be sufficient for most applications.

If you goto Odyssey’s “PC 2150" ‘More Information’ pages, you’ll see that they specify the “Short circuit current” (“Isc”, available fault current) at 5,000 Amps (not “more than”).
Ie: ➥ PC2150 MS Odyssey Battery

The PC 2150 Battery is listed as having an internal resistance of 2.2 milli-Ohms* (0.0022 Ohms), which (by Ohm’s Law, Isc = Voc ÷ R) would suggest an Isc of between 5,000A (@ 11.0 Voc) through 5,454A (@ 12 Voc) to 5,909A (@ 13 Voc). Internal resistance varies with a number of normal factors, all of which increase it’s actual Ri, in use, thus reducing the actual short circuit current. None of the above accounts for External Impedance, which further reduces the fault current available at the fuse.

See ➥ Odyssey high quality drycell batteries - It can deep cycle as well as provide enormous cranking power

* This is an uncommonly low (excellent) internal resistance.
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Old 12-07-2009, 11:26   #15
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The pragmatic approach is that installing a fuse is a good thing, regardless of whether you can do it in 7" or not. You should have one for each bank of batteries, but not necessary for each battery in a bank. There are portions of the ABYC that a lot of people dont agree with, or that are impractical as heck. It's like any type of regulation, sspecification or code, it's a generalization that is trying to fit all circumstances... but the real world has a lot of variables...
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