Best way to fuse Battery - Page 2 - SailNet Community
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post #11 of 65 Old 01-28-2011
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The fuse size depends on the wire gauge used. It is there to protect the wire, not the loads. What gauge wire are you using?

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post #12 of 65 Old 01-29-2011 Thread Starter
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.....What gauge wire am I using? From the terminal fuse block to the battery switch, it's a #4 battery cable. From the battery switch, there is a #12 to the small panel (I think I remember that the inline fuse on that is a 15 amp though it might be a 20amp.... and there are a couple of other wires that go straight from the battery switch that are #16 wire But all of those have their own inline 1 amp fuses. Remember, this is a fairly minimalist electrical system.
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post #13 of 65 Old 01-29-2011
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Tree

If you are using 4awg from the battery to the main switch I would use a fuse that is greater than the draw if everything is on at once. Probably somewhere between 40 and 60 amps, which is well below the ampacity of 4awg wire.

You must have items of very low amperage to get away with 1 amp fuses on the 16awg wires from the battery switch. About the only items I can think of that draw so little would be LEDs or 10 watt lights and not much else. Unless fusing for sensitive instruments I would fuse 16awg with nothing smaller than 5 amp fuses and more likely 10 amp. Most of my wiring for interiors is 14awg and I use 15 amp fuses for that gauge. What items are they?

Hope this helps.

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post #14 of 65 Old 01-31-2011 Thread Starter
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Yes....Yes..... Yes.

One wire that is direct to the battery that I'm remembering is the "yellow" clock / memory wire to the car am/fm radio cd player. Come to think of it, I think I may have put this one on the other side of the battery switch so as to have no draw on the battery. ..... The other one is the bilge pump and ....ooops.... is way more than one amp.

I went through every thing I have on the boat that is electrical and did an evaluation of the current draw. If I had every thing on at the same time (highly unlikely) I'd need close to 30 amps. ..... So, I think the 40 amp fuse is what I'll go with.

Thank you every one for helping me think through this.
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post #15 of 65 Old 01-31-2011
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Uh... Do you have an auxiliary engine? Does it have a electric starter?

If so, you'll need more than a 30A fuse to bring it to life.

Realize that the fuse(s) sit(s) between your batteries and the main power switch. On most boats, there is no fuse or circuit breaker between the battery, and the starter solenoid. In essence, the Blue Sea fuses are protecting against a short in the battery cable between these two points. Your breaker or fuse panel then protects the individual circuits that branch off after your main power switch.

A typical starter will draw in the ballpark of 130A - 150A for the 20 seconds that it is energized. This is not a tremendous load on a battery bank's AH capacity, but it is a big load in terms of instantaneous amps. Your 30A fuse will likely fry, unless there is something that I missed.

I recently added these fuses to my house and starter batteries myself. My boat uses 2/0 battery cables. I used a 125A fuse to protect my start battery, and a 90A fuse for my house.

Last edited by eherlihy; 02-01-2011 at 12:34 PM. Reason: corrected fuse size
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post #16 of 65 Old 01-31-2011
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I would have to think that main battery fusing is more of a philosophical matter than simple engineering. Starting with the AIC rating of fuses, because I can't conceive of many fuses that wouldn't simply vaporize (as opposed to "arc shut") when subject to an overload in this kind of service.

Maybe I'm missing someting from that picture...but especially in the case of a dead short pulling 3000 amps, most fuses are going to literally explode and fail open. Maybe the AIC rated fuses are just designed to explode less vehemently, not just fail open?

A starter could easily draw 100-150 amps, but there too the numbers can deceive. A starter "should" just be running 5 seconds, maybe 10, it shouldn't be cranking and cranking. And most fuses are rated to carry several times their rating for fairly long periods, leaving the question of do you rate it for the starter load, or the crowbar protection, or your cable, whichever is the weakest link. Or maybe wire the starter on a fuse of its own. (Surely that's against ABYC code?)

I must be getting old and set in my ways, because all I see about "suitable" AIC rated main battery fuses is that they cost $10-20 EACH instead of the buck that "unsuitable" fuses cost. Breakers can arc shut, sure. But fuses? Come on, hasn't anyone ever seen or heard a fuse literally explode?

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post #17 of 65 Old 01-31-2011
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"Or maybe wire the starter on a fuse of its own. (Surely that's against ABYC code?)"

ABYC doesn't require a fuse on a wire that is dedicated to the starter. But it is a good idea. Ideally every wire on a boat should be protected from catching fire. In the case where there is a 1/2/both/off main battery switch both battery banks should be fused as they can both be used for loads other than engine starting. It is not against ABYC code.

Blue Seas ANL fuses (6000 AIC rating) aren't that expensive - $6 to $9 as I recall.

As for sizing the fuse here are some segments from Blue Seas resources section.
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post #18 of 65 Old 01-31-2011
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Quote:
Originally Posted by mitiempo View Post
Blue Seas ANL fuses (6000 AIC rating) aren't that expensive - $6 to $9 as I recall.
I wish! The Blue Seas Terminal fuses (what we're discussing here) are more like $11-16 each.


See Genuinedealz > Marine rated battery terminal from Blue Sea Systems

Last edited by eherlihy; 01-31-2011 at 09:37 PM.
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post #19 of 65 Old 01-31-2011
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I was referring to the fuses shown below. I may be a bit low but you don't actually expect to fuse a battery bank with a $1 fuse do you?
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post #20 of 65 Old 02-01-2011
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Sorry guys. I'm going to need to back up here a minute.

HS - I'm not sure if you are agreeing or disagreeing with me.

I agree with you in that over current will vaporize the fuse. I have seen, and heard, fuses pop from an over current draw. I also agree that a fuse can sustain an over current draw for short time periods. However, if you're saying that the over current situation can exist for 5 or 10 seconds, I have to disagree. "Short time periods," in this context, means milliseconds.

A starter can, and should be capable of turning for up to 30 second bursts, with a 60 second rest (33%, or less, duty cycle).

Mitempo - the OP started this thread asking about the Blue Sea MRBF Terminal fuse. Sorry, but I got confused when you injected the ANL fuse into the discussion.

Tree - HS is right in that the fuse for the starter circuit is not required (or that is what I think that he means by stating "main battery fusing is more of a philosophical matter..." You'll read below that it is not required.

However, I'll tell you why I agree that this is a good idea. My boat has a 1986 Universal M-25 engine. The original alternator bracket on these engines were infamous for breaking when you least expected a problem.
You can read about that here. When this happens, the still spinning alternator would either slam into the timing case cover and break it, or touch on the positive post of the alternator. In the later situation, an uninterrupted circuit would be created from the selected battery bank through the switch, to ground. I believe that it could also have run through the ammeter wiring, thus frying the entire electrical instrumentation system. You can read about that here.

Mitempo is right in pointing to Blue Sea as a resource for the discussion. Here is what Blue Sea says (at Part 2: Select a Fuse and Fuse Holder For Your DC Product Installation - Resources - Blue Sea Systems) about selecting the appropriate fuse for any given application;
Quote:
U.S. Coast Guard regulations, and ABYC E-11.10.1.1.1, require every ungrounded wire except those in the engine starting circuit to have circuit protection. Many DC installations employ fuses rather than circuit breakers. Choosing correct fuse amperage and the best fuse holder or fuse block will help protect your boat and your safety.

Always select a fuse size to protect the wire according to its rating. In some cases, a product manufacturer will specify a fuse value and it will usually be lower than that required for protecting the wire. If this fuse value is too high to protect the wire, use a bigger wire in the circuit. Review Part 1 for more information on choosing wire size for a DC circuit.

Follow the steps to choose fuse amperage and a fuse holder or fuse block.

Choosing the Fuse Amperage

In this step, find appropriate fuse amperage within a range of minimum and maximum

A Find the MAXIMUM FUSE AMPERAGE by following your AWG wire size (from Part 1 of this series) across the chart. Maximum fuse amperage reduces nuisance blows but offers less protection for the wire. Select:

Single Wire or Bundled Wire column, and
Outside Engine Room or Inside Engine Room grey bar
Example: For a 4 AWG single wire outside an engine room, maximum fuse amperage is 150A.

B Calculate MINIMUM FUSE AMPERAGE by multiplying product amperage rating by 125%. Minimum fuse amperage provides more protection for the wire but may result in nuisance blows.

Example: 80A x 125% = 100A.

C Choose FUSE AMPERAGE near the middle of this range. Mid-range values are usually acceptable. Consider the requirements for each individual circuit.

Example: 125A is between 100A minimum and 150A maximum.

D Find AVAILABLE FUSES by using the chart.

Example: Fuses available at or near 125A include MIDI®/AMI®, MRBF, MEGA®/AMG®, and ANL fuses.
- Used without advance permission, but I am clearly referring to, and crediting Blue Sea's website, so I hope that they will agree that I am retaining their copyright.

I hope this helps you select the right fuse for your application.


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USCG Licensed OUPV Captain, ASA 101/103/104/105/106/118 Certified Instructor - Also certified in Recreational Marine Electrical Systems
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