Overcurrent Protection at Batteries

[Kettlewell]

This question has cropped up before in other discussions, but I have never seen what I consider an "answer" to the question. What is appropriate overcurrent protection at the batteries, or is it even worth it? Many here seem to think so, yet ABYC says it is not necessary on wire intended to carry starting current between the batteries, the main battery switch, or the starter. I currently have both my house and starter bank fused, but I wonder if it is really worth it. I can see why ABYC says this. Let's say you want to protect the wire. You'll need at least 00 wire to get up to the ability to handle 200 amps. Yet, some folks find that a 200-amp fuse just doesn't cut it and will blow too often on the starting circuit. So they up the fuses to 300 or 350, which would require 000 wire or above to handle that level. I suppose you will get some protection using these fuses even if the wire coating lights on fire, because eventually the fuse will blow during a dead short even if the wire is still intact and red hot. The problem I see is the short that only lets 300 amps of current go through, still evaporating or lighting on fire your wire coating, but not having enough oomph to break your fuse. So, these fuses appear to only be useful for the very rare catastrophic dead short situation, while not doing anything for the more likely scenarios. And of course putting any fuse, connections, etc. in the line adds more potential places for a wire to come loose, or resistance to build up, possibly causing the sort of catastrophe you're trying to avoid by installing the devices.

By the way, what do folks think of Ample Power's suggestion to put the main battery fuses in the negative side of things? Their argument makes a lot of sense: no need to shield the fuse as it is ground, making it safer to work in the battery box with tools that might be conductive, and making it easier to change the fuse or disconnect it too, because you don't have to worry as much about grounding them. I know they aren't ABYC approved this way, but does it make sense?

[djmarchand]

I agree with ABYC's position on not requiring (or even recommending) fusing the starting circuit. A starter can pull much more than the wires ampacity but since it is such a short duration the insulation doesn't have time to heat up and cause a problem. Also as you noted starting circuits need to be short and direct and fuse adds connections that can get loose and cause a voltage drop.

I hope that you are misreading Ample Power's negative fusing recommendation. What happens when you get a fault somewhere between the battery, the load and the negative fuse. That wire will get fried. If the negative wire fuse is the ONLY one and it is installed near the battery before any grounds then it might work. But that goes against the first paragraph and normal boat wiring practise. The negative wire from the battery terminal usually goes to the engine block as it is the return path for the starter.

David

[smurphny]

Last year, I installed main fuses on both starter and house circuits. My thoughts were that the long wire leading to the starter is just too exposed to any number of accidents to leave unfused and the consequences of dead short here would likely cause a major meltdown/fire. I calculated the max. amp load of the starter and installed fuses the next size above the potential load. Have had no problems or blown fuses. I keep a spare fuse handy.

[CarlF]

Electrical fires from shorts are among the most frequent cause of fires on boats. There's no excuse as fuses can reduce the risk to almost nothing.

A 300 amp fuse is going to blow long before insulation melts on a battery cable much smaller than 00. My bow thruster comes with a 500 amp fuse and I believe uses a short segment of 00 wire. Modern batteries (especially AGM) in the large banks that are common on bigger cruising boats can deliver much higher amperage to a short. You are right that no single fuse is adequate. Smaller wires need smaller fuses. When in doubt, add another fuse or breaker.

While I fuse the battery positive side, I can understand the argument for fusing the negative side (no need to insulate the fuse holder and possibly fewer wires at the battery post). In any case, a fuse on the negative post is much better than no fuse at all.

The ABYC guidelines are full of outdated standards left over from when boats had much simpler electrical systems. How about the stupid wire color coding scheme (Brown with yelllow stripe for the blower wire!). Or the extremely dangerous use - until recently - of black for DC negative when it's also the color used for AC hot.

Two things everyone should check on their boats:

-- Inspect all larger red cables for chafe. Look especially carefully for anyplace the cable crosses a black cable or comes near a piece of metal. Install a piece of split reinforced water hose at chafe points

-- Tighten all postitive cable nuts (especially at the starter). Install a 2nd nut at the starter to make sure the cable can't loosen with vibration and drop onto a piece of the engine causing a dead short. While you're at it, clean and re-tighten the engine ground as that's so often the cause of so many mysterious electrical problems.

Carl

[GordMay]

I believe that Ample Power’s negative fusing advice is just plain wrong!
Fusing must be placed in the ungrounded conductor (or both); which, on a standard negative ground boat, will be the positive wire.

ABYC 11.10.1.1.1 Overcurrent Protection Device Location
Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point at which the conductor is connected to the source of power measured along the conductor.

➥ ABYC Section E-11 ~ Table 15 - Location of Overcurrent Devices in DC Circuits, per New Electrical Section E-11 (effective 2004) Cruisers & Sailing Photo Gallery

[Maine Sail]

If not fusing, it is a very good idea to "protect" or additionally sheath the wire. A starter fuse can also be sized up to 150% of the Table VI ampacity rating if not bundled with other wires.

This would make a 2GA wire safe to fuse up to 315A outside an engine space. Considering that heat is really bad for batteries most of this wiring is likely to be outside an engine space except for a few inches of starter wire. If inside an engine space you could still fuse up to 268 amps with only 2GA rated at 105C.

Going to 2/0 gets you to 495A and 421A with UL 1426 marine wire rated at 105C.

I have been fusing both start and house banks on sailboat aux engines for the better part or 15 years and never once had a nuisance blow. I even had my big old Cummins fused with a 350A fuse and that too never blew once even in winter.

Keep in mind too what the ABYC trains and teaches not just what is in the "standards"..

Here's a direct quote from the Electrical Certification Manual:

"Starter, or cranking motor, circuits are not required to have overcurrent protection, although in recent years some builders have begun to install appropriately rated over-current protection in starter circuits. It is a good idea if it can be done practically. Starter current draw has gone down over the years due to design characteristics of the starter motors, making the use of over-current protection for the circuits more affordable."

"For wires connected directly to a battery terminal, the rule says that the over-current device can be placed up to 72" down the length of the wire run. The caveat to this is that the cable run must be sheathed or enclosed to minimize the risk of chafe on the conductor."

I run a 44HP Westerbeke currently, with a 200A ANL. The motor has started hundreds and hundreds of times even in quite cold temps well below 30F.. These fuses can go well beyond the face value rating to handle the in-rush for short durations.

Having experienced a direct short to the engine WITH a fuse on the cranking battery I am darn glad I had it. The builder did not ship the boat with two fuses and I luckily added a second one before taking delivery. Still took a good chunk out of the engine mount and cable.

Our Yacht Club now has our entire fleet protected with fuses after we lost this chase boat last summer.


If you want to wire to accepted US standards then fusing of the Neg is not acceptable. Not that it can't work but the standards require fusing of the "ungrounded" conductors not the negative or grounded conductors.

Our boat has three group 31 Deep Cycle batteries in parallel. They can supply over 2300 CCA as measured with both the Midtronics and Argus analyzers. I am certainly not going to mess around with 2300+ amps potentially finding a dead short to ground. For the $30.00 it cost to fuse each bank I find it good insurance.

P.S. This came off a boat who only had the house bank fused. Fortunately he starts and runs house loads of the fused house bank and the fuse blew when it grounded. It grounded out on a pulley for refrigeration after a plastic wire tie failed. He now has religion and his start bank is also fused.

[btrayfors]

I agree with Maine and Carl...fusing of the starting circuit makes a great deal of good sense. I've done mine...several years ago on a 4-108...and I recommend to clients that they do theirs.

The danger is not just shorting the (unfused) battery cable. It's also the risk of a dead short in, e.g., the starter or solenoid. With an unfused lead to the solenoid from the starting battery, that's a fire waiting to happen. And, it DID happen to one of my clients, while they were 100 miles offshore. Only fast action on their part prevented a huge, life-threatening disaster.

BTW, if fuses are chosen properly and are properly installed, there's very little chance of them coming loose or developing unwanted resistance. With proper fuseholders (e.g., the Blue Sea series) for ANL or Class-T or terminal MRBF fuses, and with proper tensioning and connection of ring terminals (using a high-quality crimper and adhesive heat-shrink tubing), these connections will be good for many years.

Bill

[hellosailor]

"Our boat has three group 31 Deep Cycle batteries in parallel. They can supply over 2300 CCA as"
Maine, if three of those are only supplying 2300A something is limiting the current, or they're half dead. IIRC even a single battery of that size can exceed the usual 3000A arc limit of conventional breakers, if it is fully charged and then someone drops a crowbar across the posts or heavy cables. I suspect your measurements are low because the equipment is averaging things out, and the initial surge from the batteries is way higher than what they're showing.

[foggysail]

Those who support fuses, even as high as 300+ amperes are in the right camp. I cannot imagine wiring something only to have the wire explode in my hand because the circuit is not protected. Even small wires can carry very large currents in fault conditions.

Fuse locations---- this is the first time that I read about fuses being placed in the negative wire. I first thought this could be a good idea IF DONE PROPERLY. Sure, 12,24 or for that matter 36vdc is well within safe limits for personnel protection. But again, there are 'buts'. Low voltage it may be, it will still cause great damage if one gets a ring or a wrist watch across the circuit that somehow allows current to flow.

Foggy

[Maine Sail]

Originally Posted by hellosailor

"Our boat has three group 31 Deep Cycle batteries in parallel. They can supply over 2300 CCA as"
Maine, if three of those are only supplying 2300A something is limiting the current, or they're half dead. IIRC even a single battery of that size can exceed the usual 3000A arc limit of conventional breakers, if it is fully charged and then someone drops a crowbar across the posts or heavy cables. I suspect your measurements are low because the equipment is averaging things out, and the initial surge from the batteries is way higher than what they're showing.

I referenced CCA not short circuit amps. Each battery is rated at 675 CCA however they have always performed higher than rated since new, which many battery brands do and why I always print out a baseline from which to work off in the future.

3 X 780 CCA = 2340 CCA. The MCA is rated at 875 @32F but at 68 degrees they can supply about 1040 CA so 1040 X 3 = 3120 for the combined bank CA @68 degrees. I have not yet seen a single group 31 supply 3000 cold cranking amps. Even the Odyssey PC2150 has 1150 CCA "rated" but my meter measures new ones at 1314 CCA. Short circuit amps are not the same as CCA and it is often tough to get a SSA rating from a manufacturer and why the ABYC defaults to CCA.

Many manufacturers do not tell you what the short circuit amperage is but it is usually significantly higher than the CCA and in many cases as much as 3-4 times CCA. I don't know of any tester/analyzer that will test for a short circuit rating, though I'd like to have one.

The ABYC goes by "Total Connected Battery (Cold Cranking Amperes)" which is why I use CCA as a measurement.

For a bank larger than 1100 CCA, mine is 2300 +/- CCA, you'd need a minimum AIC of 5000A. For batteries like Odyssey you'd likely want a Class-T on any sufficiently sized bank. This is why many breakers are often not suitable for large banks as they do not have an AIC rating high enough.

[hellosailor]

"I don't know of any tester/analyzer that will test for a short circuit rating,"
Just put a 5000A fuse across the battery terminals, see if it blows. Then repeat with a fuse 1/2 or 2x the rating, until you find one that can carry the load, right?

If you can't find a 5000A fuse, a socket wrench ought to do. Funny thing, those manufacturers won't tell you how many amps those are good for, either.<G>

[Maine Sail]

[QUOTE=hellosailor;685297]"I don't know of any tester/analyzer that will test for a short circuit rating,"
Just put a 5000A fuse across the battery terminals, see if it blows. Then repeat with a fuse 1/2 or 2x the rating, until you find one that can carry the load, right?

Makes for expensive and fun experiments!

Originally Posted by hellosailor

If you can't find a 5000A fuse, a socket wrench ought to do. Funny thing, those manufacturers won't tell you how many amps those are good for, either.<G>

I have a few 5000A rated wrenches with the burn marks to prove it... The chrome plating is not 5000A rated however...

[btrayfors]

I hope nobody takes you guys seriously!

DO NOT TRY THE WRENCH TRICK AT HOME :-)

Seriously, I've taken to using a stubby 1/4" Snap-On socket wrench, with 1/2" and 9/16" sockets for the battery nuts. Works like a charm, easy to get the right torque and, I believe, lessens the possibility of a short between terminals on the battery.

Not sure of the Snap-On's ampacity, though :-)

Bill

[hellosailor]

Gee, Bill...No one's gonna try the wrench trick at home. They're gonna do it in the engine spaces of their boat!

Making your caution really unnecessary.<VBG>

[Mark Johnson]

Originally Posted by Kettlewell

This question has cropped up before in other discussions, but I have never seen what I consider an "answer" to the question. What is appropriate overcurrent protection at the batteries, or is it even worth it? Many here seem to think so, yet ABYC says it is not necessary on wire intended to carry starting current between the batteries, the main battery switch, or the starter. I currently have both my house and starter bank fused, but I wonder if it is really worth it. I can see why ABYC says this. Let's say you want to protect the wire. You'll need at least 00 wire to get up to the ability to handle 200 amps. Yet, some folks find that a 200-amp fuse just doesn't cut it and will blow too often on the starting circuit. So they up the fuses to 300 or 350, which would require 000 wire or above to handle that level. I suppose you will get some protection using these fuses even if the wire coating lights on fire, because eventually the fuse will blow during a dead short even if the wire is still intact and red hot. The problem I see is the short that only lets 300 amps of current go through, still evaporating or lighting on fire your wire coating, but not having enough oomph to break your fuse. So, these fuses appear to only be useful for the very rare catastrophic dead short situation, while not doing anything for the more likely scenarios. And of course putting any fuse, connections, etc. in the line adds more potential places for a wire to come loose, or resistance to build up, possibly causing the sort of catastrophe you're trying to avoid by installing the devices.

By the way, what do folks think of Ample Power's suggestion to put the main battery fuses in the negative side of things? Their argument makes a lot of sense: no need to shield the fuse as it is ground, making it safer to work in the battery box with tools that might be conductive, and making it easier to change the fuse or disconnect it too, because you don't have to worry as much about grounding them. I know they aren't ABYC approved this way, but does it make sense?

It is surprisingly easy to be working on the engine, and the wrench hit the starter's + bolt, or doing the same on the back of a three way selector switch. When this sort of thing happens, IF you are fast, you just can wiggle the now "welded" wrench back and forth, and with considerable effort, break it free! Since you created a 380A. direct short, the wrench glowed red, and you've now BRANDED your palm! It could have been MUCH worse.

Don't ask how I know this.

My practice, and what I recommended to my clients, is to mount a Blue Sea fixed fuse, (like 200A), within 1' of the battery, on the + side. If you size it large enough, it can handle the current of engine cranking or the windlass, just fine. In 15 years I only blew one from normal use. I then moved up another 50A, and it never happened again.

I have an identical fuse base next to the engine battery as well.

With these in place, if I do something stupid, I can LET GO OF THE DAMNED WRENCH. The fuse will blow, and I won't loose countless hours of work spent wiring the boat, or the boat itself!

It is a really quick and inexpensive safety feature, that is well worth the effort. IMO.

Mark