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Battery Fuse Sizing - How?

Feb 6, 1998
11,094
Canadian Sailcraft 36T Casco Bay, ME
Lately there have been a number of questions, on this and other forums, about battery bank fuse sizing and I wanted to give a brief explanation of how I size these fuses.

Simply put you size the fuse to the wire provided your wire is sufficiently sized.



#1 The standard.


The ABYC requirement is for a battery bank fuse to be within 7 wire inches of the battery bank. This is often hard to do, and if you can't, then as close as possible would be the next best thing.

If you're having trouble meeting the 7" rule then the Blue Sea MRBF fuses are excellent but they do add nearly 2" in height to a battery post to measure your battery compartment height carefully. The limiting factor with MRBF fuses is that they only go to 300A so on bigger engines you may need a Class T or ANL.

Blue Sea Marine Rated Battery Fuse and Holder


The Actual Fuse & Holder (this one is a double MRBF):


#2 Exceptions to the rule.


The ABYC has an "exception" to this rule for cranking motor batteries. This exception however is more broad based and written to include for large engines which have massive amounts of starting current being drawn. These engines are very expensive to fuse properly hence the "exception".. Think big sport fishing boats and our engines are about as far away from that as can be... Small diesel or gas AUX engines on sailboats would be best served fused than unprotected.

I quote our good friend Nigel Calder here:

"The net result is that nowadays, electrical shorts are probably the number-one cause of fires on boats."

There is simply no excuse for not protecting all high-current circuits , including the cranking circuit."

Nigel Calder Cruising Handbook



#3 What exactly am I protecting?


Over current protection (OCP) or over current protection devices (OCPD's) are sized to protect the wire not the devices they are powering. This is often misunderstood. You can always go smaller with OCP, than the wires ampacity rating, but ideally should not exceed the ampacity rating. The OCPD is there to prevent the wire from overheating, melting and starting a fire. In a dead short, what you are protecing agianst, the fuses will still protect wires well beyond the 100% or 150% ampacity rating, but I am not suggesting you do this.



#4 What if my engine draws more than the ampacity limit the wiring is rated for?


This is NOT uncommon. Many builders undersized starting wire for many years and got away with it due to the short duration of starting circuits. Today most have come up closer to where they should be. A good example is the Universal M-25 as shipped on Catalina's.

Catalina used to ship the M-25's with 4GA wire. They now ship that same engine with 2/0 gauge wire. That is a HUGE difference. If you have small gauge wire an upgrade to larger wire can be a very good investment and your engine will start a lot quicker and the starter will see a lot less voltage drop. Nearly every sailboat I went aboard during the last boat show was using 1GA or larger wire with 1/0 and 2/0 being the most popular in boats over 30'..




#5 Won't the starters inrush current blow my fuse?


First, what exactly is "inrush current"? Inrush current is the very brief spike in current that the starter undergoes to get the motor to begin turning over from a stopped state. The inrush duration is usually about 50mS to 250mS long and not long enough to blow a properly sized ANL, MRBF or CLass T fuse. FUSES ARE NOT SIZED FOR INRUSH but the proper type of fuse must be used. Fuse are sized to the wire they are protecting..

This video below shows the inrush as captured by a Fluke 376 meter. While this is not the fastest meter, 100mS capture rate, it gives a better guide than just about any other clamp type DC meter out there. The engine is an older 2QM20 Yanmar. The peak current draw, perhaps 2/10th of a second, is 316 amps yet this motor is protected by a fuse rated well below the inrush. It has never blown nor will at this inrush capacity. The "average starter" load during the duration is closer to 150A - 200A. This engine has a 250A ANL fuse and has started successfully 1000 times or more since this fuse was installed..




#6 If the starter can draw more than the fuse rating then why doesn't it blow?


Contrary to popular belief ANL, Class T or marine rated battery fuses (MRBF's) do not blow or trip at face value unless the duration is long enough.

As you can see below a 200A ANL can support 500% of it's rating for about .7 seconds, longer than average inrush by more than double. It can support over 300% for 1 second and 200% for as long as 5 seconds. You can push 150% through for up to 500 seconds.

Even if you figured an inrush that, by freak chance, lasted for 1 second, which ideally should not happen, a 200 amp ANL can supply 600A which is almost double what the peak inrush is for the 2QM20 in the video above so in that scenario you'd still be fine.

Just as a point of reference a 1987 Universal M-25, with a 600Ah bank of fully charged AGM batteries, wired with 2/0 wire, draws roughly 250A +/- in-rush on an 80F day. This is the in-rush load though. All engines are slightly different as are batteries and wiring.. Some draw more and some draw less but the in-rush is still a very short duration on all of them.

None of this changes the fact that you are still protecting the battery cable with the fuse and not the battery or other systems. The fuse is still sized to protect the battery cable and this in-rush data is simply a reference point..

200 AMP ANL Seconds vs. Amps


.7 Seconds =1000A
1 Second = 600A
5 Seconds = 400A
500 Seconds =300A

Image courtesy Blue Sea Systems






#7 What type of over current protection for my bank/banks?

Generally speaking larger banks should be protected with ANL, CLASS T or MRBF fuses which are available from Blue Sea and others. They should however be "Ignition Protected" fuses and Blue Sea is the only one I know of offering ANL fuses with ignition or spark protection for ANL's. No Class T fuses are ignition protected but they do have an AIC rating of 20,000 amps for DC, so the possibility of blowing one open, to create a spark, would be extremely rare even with a Lithium bank.

These fuses have AIC (amperage interrupt current) ratings that will work with decent sized banks. If you have a large bank of Odyssey or LiIon batteries then a Class T would be best as the AIC rating of Class T fuses is nearly 20,000 amps.

The concern with AIC is that some breakers can literally weld shut before tripping, if the bank has enough amps behind it. For decent sized battery banks you ideally want an AIC rated fuse or breaker of 5000A AIC or greater. AIC is a greater concern for breakers but fuses are also AIC rated and can fail dangerously when subjected to shorting amperage greater than their AIC rating. The actual ABYC requirement for batteries is that any bank over 1100 CCA needs 5000 AIC rated protection or greater. Even two parallel group 27 batteries can supply more than 1100 cold cranking amps...

Be aware that not all fuses are created equal. Quality fuses must be chosen and Blue Sea is




#8 How do I determine my wires ampacity rating?


Below is Table VI from the ABYC E-11 Electrical Standard. It is for single conductor wires not bundled together and organized by jacket temperature rating. UL1426 Marine Wire is all 105C rated. Manufacturers like Pacer, Ancor, Berkshire, Cobra and others all build marine battery cable to UL1426 standards and most any chandlery will have it, as does Sailboatowners.com your gracious hosts!

The ABYC does allow you to go to 150% of the Table VI ampacity rating, if necessary, but if you do this always round down to the next size fuse rather than up. My personal preference is to size the wire correctly so that use of the "150% rule" is not necessary.

Data courtesy ABYC:




So, if you were using 2/0 wire and it was outside the engine space then you could use a fuse up to 330A @ 100% of the ampacity rating.

If you needed to go bigger with your fusing you could use the 150% rule and use 330A X 150% = 495A fuse, or rounded down to the next commercially available size.



#9 What do manufacturers suggest?

Here are some minimum manufacturer suggestions for battery/starter cable from Westerbeke & Universal.

Universal / Westerbeke BATTERY CABLE REQUIREMENTS (wire length round trip)

2GA = 8'
1GA = 10'
1/0 = 14'
2/0 = 18'
3/0 = 22'
4/0 = 28'


It does not take much to have 10' of wire length even with batteries just a few feet from the engine. remember these numbers are wire lengths along the conductor not the "as the crow flies" distance


This Vessel is Protected With A 300A ANL Fuse:

This is another prime example of "inrush" vs. average starting current and what will blow the fuse and what will not. In this screen capture we can see that the starter drew slightly over 640A peak inrush:


In this screen we can see the "average" starting load was 286A. It should be noted this was on a day when the air temp was 20F and the batteries in the bilge at 32F so these currents were slightly higher than they normally would be on a sailboat. This engine also starts in 0.76 seconds as evidenced by the "TIME 765mS". If you look back at the trip delay curves you'll see why the 300A ANL fuse has never blown despite well over 750+ starts on this engine since the installation..




For most small sailboat aux engines a 250A to 300A fuse should be used, if the battery bank could ever be called upon to start the motor. This size fuse will avoid nuisance trips and start just about any Universal, Westerbeke, Yanmar, Beta and other sailboat AUX engines with ease. If your battery cabling is small this may mean using the 150% rule. As a bare minimum for a small diesel I would advise a 200A fuse. Below 200A and you begin to get into nuisance-trip range during starting episodes...
 
Dec 2, 2003
1,637
Hunter 376 Warsash, England --
I Worry.

Nice post, thanks,
But,
I worry about this maximum of 7" distance from battery to fuse because it often means the fuse will be in the battery compartment.
A wet cell house bank just off charge is likely to have plenty of H2 and O in a perfect explosive mixture. ANL fuses I have handled look as if they are not fully enclosed thus could emit a spark when fusing. Likewise fuseholders are 'open'.
So I always mount my main battery fuse in an adjacent compartment notwithstanding the "Standard".


#1 The standard.

The ABYC requirement is for a battery bank fuse to be within 7 wire inches of the battery bank. This is often hard to do, and if you can't, then as close as possible would be the next best thing.

 
Feb 8, 2009
117
Sabre 34 MK-1 Annapolis, MD
Mainsail,

Outstanding and very well presented post, as usual! I have a bit of an issue with your item #7, however, where you discuss ABYC AIC (Amp Interrupt Capacity) requirements. I think I understand what issue ABYC is addressing, and it is the welding shut of contacts in the breaker before it has a chance to open, just as you have stated.

My issue is with regard to fuses, and it is two-fold.

* First, my reading of ABYC 11.10.1.5 (which is where AIC is discussed) is titled BREAKERS and, equally important, by using the term breaker and not "over current protection device" as is used throughout the rest of the text seems to clearly exempt fuses. 11.10.1.6, which is titled FUSES, does not discuss AIC. Table IV-A, where the AIC ratings are listed, lists BREAKERS in the title. This is a good thing, because it can be very difficult to find AIC ratings on fuses, and even harder to find high AIC ratings.

* Second, my opinion is that this wording is not an accident, but rather it is intentional. A breaker can arc-weld closed, and then will not open until something vaporizes -- the guts of the breaker, the #10 wire to your bilge pump that is being protected, etc. In the case of a fuse, I can't see the "failure mode." As an exercise in imagination, take a little 1A glass automotive fuse, and connect it by 2/0 cables as a dead short to a battery -- a power source capable of many thousands of amps. In a micro-instant, the fuse melts away. At that point, what is the current path for further problems? Electricity will conduct in dry air, at 10,000V per inch -- but we only have 12V so that won't work. I guess that you could get conduction through vaporized metal inside the glass, but I tend to doubt it at 12V -- mercury vapor lights are much higher voltage than that. This explains why vendors like Blue Sea or Zantrax can tell you to connect their critical devices direct to the battery, through 16 ga wire, with no more protection than a glass automotive fuse.

I'm not quarreling -- I'm looking for input. I gave up worrying about it once I realized that ABYC clearly was exempting fuses. I was pulling my hair out when this first came to my mind, because on any but the smallest of runabouts, branch circuit breakers require at least 1500 A AIC, and no affordable, small fuses can meet that. If this requirement applied to fuses, you could not put a "blade" or "glass" type fuse any place on a typical sailboat -- even the 1A fuse on the backup power to your radio!

That's my take, but I'm open to other views!

Harry
Sabre 34
 
Feb 6, 1998
11,094
Canadian Sailcraft 36T Casco Bay, ME
Mainsail,

Outstanding and very well presented post, as usual! I have a bit of an issue with your item #7, however, where you discuss ABYC AIC (Amp Interrupt Capacity) requirements. I think I understand what issue ABYC is addressing, and it is the welding shut of contacts in the breaker before it has a chance to open, just as you have stated.

My issue is with regard to fuses, and it is two-fold.

* First, my reading of ABYC 11.10.1.5 (which is where AIC is discussed) is titled BREAKERS and, equally important, by using the term breaker and not "over current protection device" as is used throughout the rest of the text seems to clearly exempt fuses. 11.10.1.6, which is titled FUSES, does not discuss AIC. Table IV-A, where the AIC ratings are listed, lists BREAKERS in the title. This is a good thing, because it can be very difficult to find AIC ratings on fuses, and even harder to find high AIC ratings.

* Second, my opinion is that this wording is not an accident, but rather it is intentional. A breaker can arc-weld closed, and then will not open until something vaporizes -- the guts of the breaker, the #10 wire to your bilge pump that is being protected, etc. In the case of a fuse, I can't see the "failure mode." As an exercise in imagination, take a little 1A glass automotive fuse, and connect it by 2/0 cables as a dead short to a battery -- a power source capable of many thousands of amps. In a micro-instant, the fuse melts away. At that point, what is the current path for further problems? Electricity will conduct in dry air, at 10,000V per inch -- but we only have 12V so that won't work. I guess that you could get conduction through vaporized metal inside the glass, but I tend to doubt it at 12V -- mercury vapor lights are much higher voltage than that. This explains why vendors like Blue Sea or Zantrax can tell you to connect their critical devices direct to the battery, through 16 ga wire, with no more protection than a glass automotive fuse.

I'm not quarreling -- I'm looking for input. I gave up worrying about it once I realized that ABYC clearly was exempting fuses. I was pulling my hair out when this first came to my mind, because on any but the smallest of runabouts, branch circuit breakers require at least 1500 A AIC, and no affordable, small fuses can meet that. If this requirement applied to fuses, you could not put a "blade" or "glass" type fuse any place on a typical sailboat -- even the 1A fuse on the backup power to your radio!

That's my take, but I'm open to other views!

Harry
Sabre 34
Harry,

The problem with simply reading E-11 is that you miss all the nuances that come out in training for certification in E-11. While the standard makes it look as if only circuit breakers are required to meet AIC the training manuals make it quite clear that it is suggested for both fuses and circuit breakers. I have also discussed this at length with John A. and their are going to be quite a few changes made in the next revision around battery over current protection including AIC changes due to new battery technologies.

AIC is not just for arc welding contacts shut it is also inclusive of "damage" to the fuse that could be considered dangerous. AIC ratings for fuses are very easy to come by unless you are trying to find them for glass fuses, which are not even rated. I have seen my fair share of shattered or cracked glass fuses after a trip and perhaps this is why they don't get an AIC rating. Blue Seas list the AIC ratings of all their fuses and breakers except for glass/Bosch style.

This is the Blue Seas definition of Interrupt Rating/AIC

"Interrupt Rating (AIC)
The fault current that a device, normally a fuse or circuit breaker is capable of breaking without damage."

Keep in mind that the minute you install a fuse on the battery bank meeting the AIC any other fuses down stream are not required to meet this same level of protection.

Blue Seas AIC ratings are definitely inclusive of fuses..

Glass Fuses = Not Rated
ATO/ATC = 1000A AIC
ATM = 1000A AIC
MAXI Fuses = 1000A AIC
MEGA/AMG - 2000A AIC
MIDI/AMI = 1000A AIC (Blue Seas has a 5000A AIC version but only rated at 12V)
MRBF = 10,000A AIC @ 14V
ANL = 6000A AIC
Class T = 20,000 AIC
Blue Sea Panel Breakers "A Type" = 3000A AIC
Blue Sea Panel Breakers "C Type" = 5000A AIC


Here's an excerpt from the ABYC Electrical Certification manual on AIC.

"All fuses and circuit breakers shall have an AIC rating. The concern with the AIC rating is what it defines. It is basically a specification that defines how much amperage a fuse or breaker can be subjected to without locking or welding itself into the closed position or failing in a unsafe condition."

And this one from Calder:

"Even so, quite small batteries can deliver several thousand amps of current for a brief period of time (more than enough to melt that wrench). For this reason, the American Boat and Yacht Council (ABYC) and the ISO in Europe have always recommended that the primary OCP device on a boat have an AIC rating from 3,000 to 5,000 amps, depending on the size of the battery bank."


Unfortunately the current standard of 3000-5000 AIC is well undersized in many applications and is why it is being re-tooled. LiIon and TPPL (Odyssey) batteries can far exceed the AIC of just about anything but a Class T in a dead short situation so even a 10,000 AIC MRBF may fail to protect adequately in a dead short with a LiIon or large TPPL bank...

Don't beat yourself up over this. You, as a boat owner, are not "required" to meet ABYC standards, though it is never a bad idea to meet or go beyond.
You should however consider safety and what your insurance company would think if you grossly ignored current accepted best practices and standards and a fire resulted. The vast majority of vessels out there have zero OCP at the batteries so you're already doing ten times better even if your fuses don't meet the AIC rating.. I woudl MUCH rather see a fused battery bank with out of spec AIC than no fuse at all.

AIC is not just for marine electrical, shore rated fuses and breakers also need to meet AIC standards too and both fuses and breakers are AIC rated shore-side as well..



Nice post, thanks,
But,
I worry about this maximum of 7" distance from battery to fuse because it often means the fuse will be in the battery compartment.
A wet cell house bank just off charge is likely to have plenty of H2 and O in a perfect explosive mixture. ANL fuses I have handled look as if they are not fully enclosed thus could emit a spark when fusing. Likewise fuseholders are 'open'.
So I always mount my main battery fuse in an adjacent compartment notwithstanding the "Standard".






Don,

That's why I suggested only using the Blue Seas ANL fuses as they are spark or ignition protected as are their MRBF's and Class T fuses. Most ANL's are not spark protected and you're right about their location if not using Blue Sea ANL's.
 
Feb 8, 2009
117
Sabre 34 MK-1 Annapolis, MD
Main Sail,

I'll go with your belief (which I've seen many other places, and you've quoted ABYC training to support it as well) that ABYC intends the AIC rules to apply to fuses as well as breakers, I have a question about fuses. I emailed Blue Seas about this as well, and their answer was pretty unhelpful.

The AIC tables list values for both the main breakers and for branch circuit breakers. Their definition of "branch circuit" is any breaker downstream of the main breaker. For any battery bank with a CCA over 650A (which is pretty much everyone), that requires an AIC of 1500A for branch breakers. That means that glass fuses, blade fuses (ATC), ATM fuses, Maxi fuses, and MIDI fuses are all innappropriate for use as branch fuses anywhere on a boat. Yet, Blue Seas and others sell many of these fuse panels and fuse blocks. Balmar and Zantrax inlclude inline glass fuses in their equipment.

Since ABYC is silent on specifically addressing fuses, and it's up to those who have heard it elsewhere to know what ABYC "really" meant, this gets really confusing. Since there are no suitable high AIC/low value fuses on the market, is it really ABYC's intent to prohibit fuses anywhere? A classic installation is a switchboard breaker feeding a small 6-fuse panel for local distribution at, say, the Nav Station, which can't be done.

What is ABYC take on blade fuses, and why do Balmar, Zantrax, Raritan, and others (who sit on ABYC) still sell equipment with fuses -- and tell you to connect direct to the batteries? For example, Balma's regulator installation instructions suggest the sense wire, with a 1A ATC fuse, connects to the battery terminal post -- a location requiring 3000 to 5000 AIC!

Harry
 
Feb 8, 2009
117
Sabre 34 MK-1 Annapolis, MD
Keep in mind that the minute you install a fuse on the battery bank meeting the AIC any other fuses down stream are not required to meet this same level of protection.
I missed this line the first time I read your post. Are you saying that fuses downstream of the battery fuse don't have to meet ANY AIC, or just a lower level (the same rating as downstream breakers, which as I pointed out in my other post is still unachievable)?

My boat has nice ANL battery fuses, which provide 200A of protection. The rest of the boat is still nearly stock, (meaning those fuses are too big!) but I'm working my improvements slowly downstream. As you said, a battery fuse alone is a HUGE improvement over most stock systems. Note that, as you pointed out in your original post, a 200A fuse is amply large even for a starter. Discounting inrush (which you clearly demonstrated is NOT an issue), my ancient Volvo MD11C cranking current is right around 100A -- and your curves tell me I can crank at 300A for almost 10 min without blowing. A 200A fuse is plenty large -- but will blow in a heartbeat on a serious short.

Harry
 
Feb 6, 1998
11,094
Canadian Sailcraft 36T Casco Bay, ME
I missed this line the first time I read your post. Are you saying that fuses downstream of the battery fuse don't have to meet ANY AIC, or just a lower level (the same rating as downstream breakers, which as I pointed out in my other post is still unachievable)?

My boat has nice ANL battery fuses, which provide 200A of protection. The rest of the boat is still nearly stock, (meaning those fuses are too big!) but I'm working my improvements slowly downstream. As you said, a battery fuse alone is a HUGE improvement over most stock systems. Note that, as you pointed out in your original post, a 200A fuse is amply large even for a starter. Discounting inrush (which you clearly demonstrated is NOT an issue), my ancient Volvo MD11C cranking current is right around 100A -- and your curves tell me I can crank at 300A for almost 10 min without blowing. A 200A fuse is plenty large -- but will blow in a heartbeat on a serious short.

Harry

Harry,

I believe you have a Bass panel with standard Cooper Bussman breakers. They should be around 3000 AIC. Those breakers have remained fairly consistent for years and everyone uses them. After the battery OCP the rest of the fuses or breakers are at reduced AIC. Any fuse used after a breaker, like an in-line for instruments etc., is already protected by the breaker protecting the wire so AIC for those does not matter. The only time I wire direct to a battery is for a bilge pump, battery monitor, or alt sense. It is pretty tough to meet the AIC requirements with these items so I often mount them off an OC protected positive buss bar or at least go to an AIC rated fuse like an ATO/ATC as opposed to glass. With these small wires a dead short would simply use the wire like a fusible link so I worry less about AIC on these than on a 2/0 battery cable...
 
Feb 8, 2009
117
Sabre 34 MK-1 Annapolis, MD
Main Sail,

You pegged my house panel right. And I intend to keep that panel pretty much in tact. Where I ended up delving into AIC and such was with things like the bilge pump, battery monitor, etc. My power to those will be:
* 200A ANL battery fuse
* 20A surface mount breaker (not purchased, size still pending) (this serves to protect the fuse block and as a "main breaker" for that bock).
* 6 fuse ATC block -- feeding all those "constant on" vampire loads

This system will keep all those critical loads very close (16" or so) to the battery, fed mostly by VERY large wires with very low voltage drops, with minimal chance for any accidental trips or turn-offs, yet also provide for easy, consolidated maintenance. It will all be well protected, even if not absolutely up to ABCY AIC requirements.

Both the fuse and the breaker provide high AIC protection. But the ATC fuses don't meet the clear ABYC requirement for branch breakers -- and that's where the frustration arises. If you assume ABYC means what they say, and fuses don't require AIC ratings, then it's easy. If you assume ABYC means breakers AND fuses when they talk about AIC, it's impossible. If you assume ABYC means some hybrid interpretation, then I'm really confused.

But, the real thing is that when an ABYC certified mechanic like you, one who is clearly more knowledgeable than most, one who is very safety conscious and aware of the issues, and one who knows ABYC board members by name -- gives up and just does the best you can, well, I decided a while back that the arrangement I listed above was not going to keep me up nights!
 

dhays

.
Aug 2, 2010
93
Catalina C400 Gig Harbor, WA
Question on fuse sizing....

I have 2/0 cables from my batteries to the switches and they run through the engine compartment. That would indicate that I need a 300amp fuse. However, my battery is really a bank of 6v with 1/0 cable interconnections. Since this 1/0 is outside the engine compartment could I still go with the 300amp or should I go with a 250 amp fuse which is what would be appropriate for a 1/0 that went through the engine compartment?

I currently have a 250 amp fuse coming, but am wondering if the 300 would be a more appropriate choice?

Second question: I am assuming if the alternator is connected directly to the house bank, then it should be fused as well with a wire that is appropriate to the size of the alternator wire running through the engine compartment? In this case a 6 AWG wire and a 100 amp fuse?

Dave
 
Sep 15, 2009
6,241
S2 9.2a Fairhope Al
i am thinking that #6 is only good for 50amps so you may have too much fuse .....
 
Sep 15, 2009
6,241
S2 9.2a Fairhope Al
after reading some of the charts on amps for wire size i am confused i have gone all my life knowing that 12 ga is 20 amps ..10 gauge is 30 amps.... 8 gauge is 40.... and 6 gauge is 50...and now the charts are showing 6 gauge at 60 and 80 amps depending on whose chart you are reading :confused:
 
Sep 28, 2008
922
Canadian Sailcraft CS27 Victoria B.C.
6 gauge boat cable is rated for 114 amps in an engine space and 134 outside engine spaces - Blue Seas chart. The larger issue is voltage drop over the distance. I would size the alt output wire for as little drop as possible - less than 2% for more effective charging. Fuse can be 150% of max alt output.
 
Sep 15, 2009
6,241
S2 9.2a Fairhope Al
6 gauge boat cable is rated for 114 amps in an engine space and 134 outside engine spaces - Blue Seas chart. The larger issue is voltage drop over the distance. I would size the alt output wire for as little drop as possible - less than 2% for more effective charging. Fuse can be 150% of max alt output.
thanks for the reply i guess i need to get out more often lol...i am used to ac so i guess as long as i use my ac knowledge when sizing wire for dc i will be be in overkill mode most of the time...this is going to be a good day...since i just found out i am in better shape than i thought i was in
 

dhays

.
Aug 2, 2010
93
Catalina C400 Gig Harbor, WA
6 gauge boat cable is rated for 114 amps in an engine space and 134 outside engine spaces - Blue Seas chart. The larger issue is voltage drop over the distance. I would size the alt output wire for as little drop as possible - less than 2% for more effective charging. Fuse can be 150% of max alt output.
I used the Blue Seas chart as well. The 6AWG wire is a lot bigger than the wire that came stock with the boat, I think 10 awg. The alt max output is 80 amps. Now, the chart I used showed 100 amps for the fuse but it was incremented in available fuse sizes so that is maybe why the 100 amps instead of 114.

Any ideas on the main battery fuse? 300 vs 250? Or will it matter?
 

dhays

.
Aug 2, 2010
93
Catalina C400 Gig Harbor, WA
Thanks once again. I have the 250 amp fuse so will start with that.

dave
 
Jan 18, 2013
29
Cape Dory 30 Solomons, MD
How would I go about getting the battery covers on 2 batteries that are in a cockpit locker. The batteries are not in a dedicated battery compartment. Thanks!
 

NYSail

.
Jan 6, 2006
2,317
Beneteau 423 Mt. Sinai, NY
Maine Sail...... it continually amazes me how smart and on-top of all this you are! After reading this and a few other of your posts here I am seeing that my new to me Beneteau, a 2005 423 has the house batteries in a horible place (front engine compartment) and is wired crazy (all original factory). Not only do I have the typical Beneteau battery switches, the fuses are at the switches and not within the 7 inches. I like the idea of the Blue Sea MRBF fuses and they appear simple to install once you have sized them correctly. If installed would you remove the fuses at the battery switches? Also, I have read about your fix for the Beneteau wiring. My regret is that YOU are not within my area to let you do a complete electric power make-over on my boat..... Do you make house calls (LI, NY)???? Or better yet do you do consultations and draw schematics for people like me that can do certain things but need to pay for the expertise of a person to help layout a system??
 
Feb 27, 2004
91
Hunter 335 North East, MD
So Maine, I got these fuses to install on my batteries- which I did and none of my electrics worked after hooking everything up. Everything works fine without the fuses- bad fuses or operator error (probably the latter). My hook up looks like your first picture
 
Feb 6, 1998
11,094
Canadian Sailcraft 36T Casco Bay, ME
So Maine, I got these fuses to install on my batteries- which I did and none of my electrics worked after hooking everything up. Everything works fine without the fuses- bad fuses or operator error (probably the latter). My hook up looks like your first picture

Going to need some more info on this one... Fuse size, load size, engine starting etc.? Have you continuity tested the fuse?