Fuses for the Battery cables

[Shoalcove]

Now that it's too cold to actually do anything I've decided it's time to plan some spring projects. One thing I know I need is to put some big fuses on the battery cables. The only trouble is I can't really figure out what size I need. I've checked Calder, Wing, and Casey and while they all suggest I should have a fuse near the battery they don't suggest the size. Is it as simple as checking the wire sizing charts (wire size x length) to determine the fuse or is there other factors? What about on the alternator output? I've got a 100 amp Alt. Do I size for that or the wire capacity? Fudge factor?
I've got 2 Grp 27 (230 amp/hr) for the house bank but plan to upgrade to 4 6volt Golf Carts (400 amp/hr). I'm open to ideas. Thanks, David

[Chris Campbell]

This isn't very scientific, but I would size it for a little higher than what I felt was an overload for the circuit. Meaning whatever I figured the circuit would not normally ever go above unless something had gone wrong. And since my battery cables are always sized much too large to keep resistance down, that would mean the fuses would be well under what the cables were able to carry, but maybe 25-50% above what I thought they might carry as a maximum.

But as I mentioned that's not very scientific!

[Tim]

The fuse should be higher than the maximum load you ever anticipate running through those cables (i.e. if you turn every device on the boat on at once--highly unlikely in and of itself), but smaller than the ultimate amperage capacity of the cable itself. You don't want the fuse to be too close to the max draw you anticipate, though; these fuses are for worst-case scenarios, like a short circuit.

The large fuses designed for battery cables tend to be 200A, which is typical and will work just fine for something like 4AWG battery cable or larger with the loads we're typically talking about on a smallish sailboat.

[Ryan]

As said above, the fuse is to protect the cable, not the load. Choose the fuse size that protects the wire itself.

Alternator wire should be sized for the output of the alternator, and the fuse (placed at the battery) is sized for the wire.

[Shoalcove]

Thanks to all! I appreciate the help.
David

[JonnyBoats]

In addition to specifying the amperage of the fuse, you need to be concerned with the Ampere Interrupt Capacity (AIC) as well.

Cutting to the chase, a good, cheap fuse that will meet your needs can be found at:

http://bluesea.com/category/5/22/produc ... erview/379

To learn more about AIC take a look at:

http://bluesea.com/viewresource/98


These fuses are realy made by Buss, so you could source them from Buss directly if you prefer.[/url]

[Popeye1865]

i would suggest a breaker not a fuse as you can reset a breaker if you inadvertently drop a wrench on your starter or other self inflicted short, rather than having to disassemble wires, just reset the breaker when the problem is solved. With that being said very often i use no fuse at the battery and just place a battery switch as close as possible. after the batt. switch i use a distribution block with fuses for each major component: DC panel, Alt, charger. this ensures that if your Alt. cooks it dosn't take down your whole elect system but only blows its associated fuse (better yet breaker) and the likely hood of the two batt. cables (neatly run out of the way and chafe free) to the batt. switch being a problem are very small. it also allows you the ability to shut down the whole system easily while working on wiring or when you leave the boat for a long while.

[Tim]

A large, high-amperage fuse (or circuit breaker, if available) in the battery supply cable is not mutually exclusive with battery switches and individual circuit breakers and device fuses. A well-designed electrical system typically incorporates all these components, as each performs a specific function and/or protection for various and specific portions of the system. One thing does not replace the other in a good system.

High-amperage circuit breakers of the 200-250 amp ratings we're talking about here are either unavailable, or expensive if available. With so little (read: no) benefit for the high price and/or unavailability, why look anywhere but a Mega Fuse block or similar, available complete for under $20 for what is likely a lifetime installation? Changing these fuses takes little more time than flipping a breaker, but the point of a big fuse like this is that if it blows, the user ought to be looking for a serious problem, not tempted to just flip a breaker.

[Ryan]

I am a fan of breakers over fuses, but at the individual load level (ie the wire run to the GPS, autopilot, etc). Large fuses are still my choice for the battery cables and main distribution runs. They are cheaper and do the job at least as well as breakers.

the likely hood of the two batt. cables (neatly run out of the way and chafe free) to the batt. switch being a problem are very small.

For this very reason, fuses are a great choice. You pay a premium for a breaker due to its ability to be quickly reset, and in this application, that isn't an issue

[Quetzalsailor]

Bearing in mind that I have not done any of this but...

Don't you fuse both hot and ground near the battery bank (I've seen arguements about this but the logic is perhaps shaky)? And are these not the last items in failsafing the power system, that is, failures down the line somewhere should be isolated by fuses or breakers on the hot side? These fuses in discussion are to protect the batteries, and the boat, from failure in the wire between batteries and distribution or batteries and ground. As such we are looking for a no-brainer - no maintenance - non-deteriorating setup for fuses close to the battery. Kinda wishful thinking since ordinary lead-acid batteries are tough on electrical stuff near them. The saving grace is that, normally, distances between the posts and conductors and grounds are large at the batteries, but it's not many inches down the wires, away from the batteries, before these things get closer to potential evils.

I think that a substantial something just outside the battery box could be mounted carrying the fuses. Relatively short conductors tag to the batteries, run carefully to avoid potential damage. I wonder whether it's necessary to isolate the batteries in a bank with fuses? I suppose it would be appropriate to protect all the other wires that are tagged to the battery posts - 110v charger wires for one example. My Balmar alternator has an awesome collection of heavy wire which includes fuses.

Dad had all of the engine control wire in the Morgan 27 burn one cheerful afternoon. Those circuits were not protected and the wire could not have carried enough power to blow a battery fuse. I have also enjoyed an instantaneous and total electrical failure on an expressway when the battery-to-starter-solenoid wire shorted to the engine.

Dad's anecdote is telling. As noted in other posts, fuses are sized to protect the wire or the object, never larger.

[Ryan]

It is entirely appropriate to fuse any wire (charger, alternator, bilge pump, etc) connected directly to the battery positive post, and to do it as close to the battery as possible. The battery bank(s) can provide the largest amount of fault current, and as such anything attached to them should be protected from that current.

I have yet to see (and don't anticipate seeing) a solid logical argument for fusing negative battery cables within a normal electrical system.

Dad had all of the engine control wire in the Morgan 27 burn one cheerful afternoon. Those circuits were not protected and the wire could not have carried enough power to blow a battery fuse. .

Yikes!

[David]

It is entirely appropriate to fuse any wire (charger, alternator, bilge pump, etc) connected directly to the battery positive post, and to do it as close to the battery as possible. The battery bank(s) can provide the largest amount of fault current, and as such anything attached to them should be protected from that current.

I have yet to see (and don't anticipate seeing) a solid logical argument for fusing negative battery cables within a normal electrical system.

Dad had all of the engine control wire in the Morgan 27 burn one cheerful afternoon. Those circuits were not protected and the wire could not have carried enough power to blow a battery fuse. .

Yikes!

Rather than attaching individual circuits to the positive battery post (or the negative post), a cleaner way for several reasons is to run one cable sized to the amperage of the battery to a positive (or negative) distribution block. That way the positive feed cable can be fused with the correct fuse for the wire size, and the individual positive leads off the distribution block can be individually protected. Care should be taken with voltage sensing leads since a breaker or fuse can alter the wire's resistance.

[Shoalcove]

Thanks for all the input! I printed off the Blueseas article (and a few others) to add to my ideas/methods binder. I think that I will gather up the stuff to get this project done this spring. I'd like a new panel and a battery monitor someday too but this looks like a priority.
Thanks again, David

[Ryan]

It is entirely appropriate to fuse any wire (charger, alternator, bilge pump, etc) connected directly to the battery positive post, and to do it as close to the battery as possible. The battery bank(s) can provide the largest amount of fault current, and as such anything attached to them should be protected from that current.

I have yet to see (and don't anticipate seeing) a solid logical argument for fusing negative battery cables within a normal electrical system.

Dad had all of the engine control wire in the Morgan 27 burn one cheerful afternoon. Those circuits were not protected and the wire could not have carried enough power to blow a battery fuse. .

Yikes!

Rather than attaching individual circuits to the positive battery post (or the negative post), a cleaner way for several reasons is to run one cable sized to the amperage of the battery to a positive (or negative) distribution block. That way the positive feed cable can be fused with the correct fuse for the wire size, and the individual positive leads off the distribution block can be individually protected. Care should be taken with voltage sensing leads since a breaker or fuse can alter the wire's resistance.

I wasn't advocating running a separate wire to the posts for each load, and I agree 100% that this (distribution block, or a fused distribution block) method would be the preferred method, if for no other reason than it makes the install look better and it is a lot easier to remove the batteries when there is only one wire per post. Regardless of how you distribute the loads, the point is to protect the wire(s).

Good Luck!

[Shoalcove]

Thanks for all the pointers! Stand by for more questions!
David

[Maine Sail]

Generally your biggest load will be either a windlass or your starter. A 200 amp ANL type fuse will normally suffice with all but the worst offending starters..