Fusing the Battery Cable

[Ziggy]

Originally Posted by sailingharry

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.

The Blue Sea battery terminal fuses are listed here: Terminal Fuses - Blue Sea Systems. It goes in the Terminal Fuse Block (Terminal Fuse Block - Blue Sea Systems). The rated ampacity of #2 wire in engine space is 178A, so I suggest 175A fuse. You should have a separate, smaller fuse protecting the panel, say 50A or 60A.

[sailingharry]

Originally Posted by Ziggy

The Blue Sea battery terminal fuses are listed here: Terminal Fuses - Blue Sea Systems. It goes in the Terminal Fuse Block (Terminal Fuse Block - Blue Sea Systems). The rated ampacity of #2 wire in engine space is 178A, so I suggest 175A fuse. You should have a separate, smaller fuse protecting the panel, say 50A or 60A.

Actually, you've hit on two of the problems with fusing the starter wire. The fuse should be rated for the wire. But starter wire is allowed to have MUCH higher current than the same wire for other purposes because of the short duty cycle. So, to meet the surveyor's "ding," I'd need a 175A fuse as you suggested. But, someone else here said that a ~30HP diesel can take upwards of 300A to start. The cable can take it, the fuse can't.

With regard to the Blue Sea battery terminal fuses. Nearly every picture I've seen, including the West and Blue Sea advertising pictures, show the unit with the fuse installed. The fuse is a funky 1" square, 3/4" thick block with a bolt hole in it. Pretty neat, once you figure it out.

Harry

[Ziggy]

Originally Posted by sailingharry

Actually, you've hit on two of the problems with fusing the starter wire. The fuse should be rated for the wire. But starter wire is allowed to have MUCH higher current than the same wire for other purposes because of the short duty cycle. So, to meet the surveyor's "ding," I'd need a 175A fuse as you suggested. But, someone else here said that a ~30HP diesel can take upwards of 300A to start. The cable can take it, the fuse can't.
Harry

You should check that 300A figure. Just for comparison, the max surge current on a Yanmar 3YM30 (30 HP) starter is 200A. While the Volvo may be more, you should take the 300A with a grain of salt until you can verify the starter spec. If the max surge current is really higher than 175A and you're concerned about the surveyor's dings, you should upgrade the starter cable to #1 or 1/0 and fuse it accordingly.

[Cheechako]

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...

[chala]

Originally Posted by s/v Jedi

Harry: 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.
cheers,
Nick.

AWG 4/0 is approximatively 107 mm2 this seems to be a little over the top.
If I assume that your starter motor is a 1.2 kW which normally equip Volvo2030 and Perkins Perama M30, both engine are rated at 29 bhp, then the workshop manual indicate that the total battery cable resistance shall not exceed 0.0018 ohm.
The resistivity of copper at 20 deg C is 0.0175 assuming a 5 meter circuit, the section of the cable required will be 48.61mm2. (0.0175 * 5)/ 48.61 = 0.0018 ohm.
50 mm2 according to some tables should be protected by a 155A fuse but can be increased up to 4 time for equipment subject to short-time overload current.
The workshop manual also indicate for the above starter a no load current of 60A or less or a maximum current of 540A or less. On a M30 I have recorded a maximum current of 330A. The maximum cranking time vary between 15 sec to a maximum of 30 sec with a rest period of 30 sec. The manual does not show a fuse on the starter line and for this reason does not indicate the value of a fuse.
It as to be realised that a fuse do wear of and can blow at a much lower value and surely at the least desirable time it is why some boaties prefer a non fused starter line and a mechanical engine stop. This is maybe a point to be brought to the attention of your surveyor.
Over current protection and fault current limiter.
In the simplest case, a 1mm2 wire should be protected be a 6A fuse, a 1,5A by a 10A, a 2.5mm2 by a 15A, a 6mm2 by a 25A and so on. When the manufacturer of a GPS indicate that the GPS should be protected by a 1A fuse, then if the supply line is 1.5mm2 then the 10A should be replaced by a 1A fuse. The lower the rating the least the damage.
Fault current limiter. When a fuse blow, depending of it’s construction the fuse may not be able to stop the flow of current throught the fuse, the current will just arc across the fuse. It is the same for a circuit breaker. So a 6A rated CB or fuse will be also given a fault current rating in kA. My experience with battery is what ever is the fault current rating of the protecting device the battery tend to split in half well before the protective fault current rating is reached. At this time someone may require some rubber gloves, sponges and backing soda to mop and neutralize the acid.
Installing in close proximity of a battery an over current protection device is a risky business unless this over current protection is of an ignition proof design. After all even a sealed battery will bear the warning of no sparks.

Originally Posted by s/v Jedi

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.

I have read many books on this topics, they tend to disagree with each other, good old Nigel seems not to realize that contrary to other source of supply, battery supply has a limited amount of energy.
quote:
"The prospective fault current of larger batteries, such as deep-cycle batteries used in stand-alone power systems, is often given by the manufacturer.
In Australia, when this information is not given, the prospective fault current in amps "should be considered to be 6 times the nominal battery capacity at the C120 A.h rate," according to AS 4086 part 2 (Appendix H)."

Gord, derating, what are the chance on a small pleasure craft that all these circuits be all fully loaded to their maximum current carrying capacity all the time? and also a starter motor will have limited time of operation. Inverter and capstan motor are the one who will need to be seriously considered.

[s/v Jedi]

Gord: I knew you would chime in with the details ;-) When you try to bring the main fuse as close to the batteries as possible, with a single positive cable, I think there would be no other cables bundled with it. I would change that if there were.

Chala: you are right, we don't have 4/0 for the starting service. I was describing our house battery bank which is also what the photo with the class-T fuse is about.

The biggest conductors we have aboard are between the busbars in the salon and those in the engine room. They are separated by 22 feet and have dual 4/0 cables (2 x red plus 2 x black) That's a lot of weight... The reason for those is the 12 V, 420 A charge current from the main engine alternators to the house batteries.

The risk of blowing the fuse in the starter-circuit is the reason we don't fuse that circuit and this is indeed allowed. If the surveyer says otherwise, he is wrong and should be replaced by an upgraded (de-fused?) model.

Chala: that fault current sheet you attached is for AC. AC is much easier to handle because the voltage is zero 100 or 120 times a second (zero-crossing of the sinewave at 50 or 60 Hz). An arc is likely (or easier) to distinguish during a zero crossing. An arc that is powered by DC does not get that "break".

I don't understand your remark on Calder but the quote appears logical to me. Calder was just an example of a book that many cruisers have and which explains correctly how to terminate and secure wiring. I think all the books agree on those issues.

cheers,
Nick.

[CharlieJ]

s/y Jedi-I disagree with your recommendation to utilize a Class T fuse to feed anything other than an inverter as they are very fast acting. In the B+ to an entire DC system, their inadvertent blowing, caused by a heavy start up load for a motor, or an accidental B+ to B- spark during maintenance or installation would knock out your entire electrical system, and at about $40 a piece, relatively expensive to replace. On the other hand, an ANL fuse does have sufficient AIC and a much more forgiving trip curve and are therefore more appropriate in the application described by the OP if
the downstream wiring is correctly sized. And they cost about $15.

One practical approach is to bring the battery B+ leads to a bus bar, placed as closely as possible to the battery box, and then distribute to your loads via appropriate fuses and circuit breakers. The B+ to the starter is not required to be protected so with this scheme, it is easy to accomplish this.

A true belt and suspenders approach, and a refinement of this approach, with a house bank performing both house and start duties, would be to use 300 amp MRBF fuses on the B+ posts of each battery, then run their respective leads, now protected, and of any length, to the bus bar and proceed as discussed above. Their 10,000 amp AIC at 14 VDC would satisfy even the most conservative among us!

[s/v Jedi]

Charlie,

Originally Posted by CharlieJ

s/y Jedi-I disagree with your recommendation to utilize a Class T fuse to feed anything other than an inverter as they are very fast acting. In the B+ to an entire DC system, their inadvertent blowing, caused by a heavy start up load for a motor, or an accidental B+ to B- spark during maintenance or installation would knock out your entire electrical system, and at about $40 a piece, relatively expensive to replace.

I think you misread the specs. They are very fast acting for a short circuit. Pls. explain me why you don't like that. The $40 400A fuse can save a much more expensive battery bank that way. Also, the ANL fuse will fail to interrupt a short circuit current of 5,000 A or more. Logic dictates that if your battery bank can supply that 5,000 amps you must not use an ANL fuse. As demonstrated earlier in this thread, a single 100Ah Odyssey battery can supply 5,000A.

Also, a 400A class T fuse does not inadvertenty blow with a start-up load of a motor, but it will if you create a short with a wrench or similar. It should.

On the other hand, an ANL fuse does have sufficient AIC and a much more forgiving trip curve and are therefore more appropriate in the application described by the OP if the downstream wiring is correctly sized. And they cost about $15.

Ah, I immediately agree that the OP poster doesn't need a class-T fuse. Once again we demonstrate how important context is ;-)

A true belt and suspenders approach, and a refinement of this approach, with a house bank performing both house and start duties, would be to use 300 amp MRBF fuses on the B+ posts of each battery, then run their respective leads, now protected, and of any length, to the bus bar and proceed as discussed above. Their 10,000 amp AIC at 14 VDC would satisfy even the most conservative among us!

Indeed, that is the best solution for the OP.

cheers,
Nick.

[sailingharry]

I'm that "OP" that has been so discussed the last 24 threads. I'm still here, and still listening, and still learning a lot from the thread. I'm starting to firm up a position as well.

First, as has been occasionally forgotten and then remembered, I'm not very high power. Currently, two "dual purpose" 12 Volt Gels (that's what came with the boat. I'm not impressed, but it's what I have now). Also, these both can server the starter. And, beyond the starter (and my lectrasan with a 25A breaker close to the switch), I have a #10 wire leaving the battery switch, so you know what my loads look like!

I really don't like (and don't believe I need to) the idea of a fuse on my starter. I don't know what it really draws, and it could be 200Amps on a bad day. But I also don't like the fact that the #10 leaving the battery switch is not protected at all.

Here's what I think I'm going to do, and it's copied straight off Blue Sea's web site. I'm going to put that neat terminal based fuse on the output of my battery switch to feed my panel. I'll probably leave the LectraSan hanging off the battery switch as well. The breaker isn't but 2' from the switch, and cutting the wire length is a good idea (the unit is 20 feet away!). I don't want to add all that load to the fuse, because then the fuse gets bigger, and then the cables need to be replaced. The big cables from the battery to the switch, and to the starter, will be unprotected.

Now, the #10 needs to be replaced too. But it's worked for 30 years, and has plenty of ampacity (51 amps in the engine room), even though there is a voltage drop issue. I'd like to see it be #6 from the switch to the main panel, and also to the alternator, since then I'd have the ampacity to support 100A (big alternator?) and have trivial voltage drop issues.

Further down the road, I intend to go to a dedicated starting battery with an echo charger off the main battery, and a pair of Trojan T-105's for the house bank. When that happens, I'll not have a fuse between the starting battery and the starter, and I'll move my little fuse holder to the Trojans.

Does my plan make sense?

Oh, and ALL of this is "down the road." This season, but I've got bigger fish to fry, like an exhaust pipe rotted in two, and a shaft seal that leaked like a sieve (that's past tense. Had to move the bloody engine 4" forward to get clearance to replace that lip seal!).

Thanks for all the input!

Harry

[chala]

Nick you are right, the fault current sheet is for AC. Fault current sheets for DC are not so readily available. The sheet was provided to illustrate fault current. Originally the thread started with a question about over current protection then shifted logically to fault current. I can understand some reader being confused by the two different matters and the way they were explained. This is why I joined in, hoping to be able to provide more understanding of the subject.
AC circuit breakers of the type represented are adequate for DC operation up to 80A above this rating, and, to help extinguishing the arc, a magnet is normally fitted in the arc chamber of a DC circuit breaker.
Fuses come in 3 categories:
Normal
Fast blow: mostly to protect electronic equipment (obviously a T fuse in America)
Slow blow: to withstand the starting current of a motor (also a T fuse in other countries, confusing)
Any fuse should be installed in such way that it cannot start a fire.

Originally Posted by s/v Jedi

As demonstrated earlier in this thread, a single 100Ah Odyssey battery can supply 5,000A.
cheers,
Nick.

For how long and what will be the voltage?

[hellosailor]

Harry, don't be so sure you aren't "high power". Even a "car battery" can throw 3500A into a dead short, so a pair of car batteries could put you in the 6000A range. One slip with a wrench or screwdriver, and you'll see what 6000A at a measly twelve volts can do.

One quickly learns to remove any rings or watches before working on batteries, too. The USN has some photos on one of their web sites showing what happened to one seaman who left his wedding ring on. "Traumatic amputation" is the technical term.

[FrankZ]

Blue Sea has a nifty little weblet that helps in picking out cabling.. and fuses.

[Ziggy]

By the way, you can find the Volvo shop manual here: http://www.bluemoment.com/manuals/Vo...1C_D_17C_D.pdf. While it does not give the max starter surge current, it does specify #1 AWG cable.

[Cacique]

A freind recently chartered a new (Less than 1 year old) French boat, after starting the engine the starter jammed and started working as a generator and overheated the wiring starting a serious fire, fortunately they stopped the engine and extinguished the fire with no injurys.
If the starter cable had a fuse it would have probably saved the day.

[CharlieJ]

...and that is exactly why there is an ABYC requirement for a battery switch in the B+ conductor to the starter.