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Battery Fuse Sizing - How, What...?

46K views 28 replies 8 participants last post by  josrulz 
#1 · (Edited)
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 should not exceed the ampacity rating. The OCPD is there to prevent the wire from overheating, melting and starting a fire.



#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 200ms to 250ms long and not long enough to blow a properly sized fuse. FUSES ARE NOT SIZED FOR INRUSH they are sized to the wire they are protecting..

This video below shows the absolute peak inrush as captured by a Fluke 376 meter. The engine is an older 2QM20 Yanmar. The absolute 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.

YouTube - ‪2QM Yanmar Inrush Starting Current‬‏



#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 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 draws 225.5 amps peak inrush and all engines are slightly different. Some draw more and some draw less but the inrush 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 the battery cable and this inrush 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.

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...



#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
 
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#7 ·
I have a follow-up question, which I believe is directly related to Maine Sail's post. Perhaps someone here can help.

Our boat has no fuses on primary battery cables, as was common in 1984, but I'd like to remedy the situation. I plan to install a negative busbar and a positive bus bar adjacent to the house bank, and use 1/0 cable for all the battery cables. If I protect the house bank between the POS bus and the house batteries with an MRBF fuse, do I need a fuse in the cable between the POS bus and the battery switch? The cable will be the same, 1/0. Since it's the same size wire, wouldn't the MRBF fuse at the battery protect it?

Thanks!
-J
 
#10 ·
I have a follow-up question, which I believe is directly related to Maine Sail's post. Perhaps someone here can help.

Our boat has no fuses on primary battery cables, as was common in 1984, but I'd like to remedy the situation. I plan to install a negative busbar and a positive bus bar adjacent to the house bank, and use 1/0 cable for all the battery cables. If I protect the house bank between the POS bus and the house batteries with an MRBF fuse, do I need a fuse in the cable between the POS bus and the battery switch? The cable will be the same, 1/0. Since it's the same size wire, wouldn't the MRBF fuse at the battery protect it?

Thanks!
-J
Yes it will. If the wire is all the same then one fuse can protect from the switch to the battery BUT if you pull other loads off that buss bar then they need to be protected for the wires ampacity rating. If you lowered the ga of the wire from the buss bar to the switch from 1/0 to 1ga then you'd need a smaller fuse unless the fuse protecting the 1/0 was not to the max of the 1ga wires ampacity limit.
 
#14 ·
Well, yes and no. Although we may be splitting hairs a bit here, please read the following statement from Blue Sea Systems regarding selection of their ANL fuses (bolding mine):

"Because ANL fuses behave differently than all other circuit protection devices, that is, their blow point value ranges from 140 to 266% of nominal value, the 80% rule doesn't work. It is necessary to use a different procedure to select a suitable ANL fuse. To choose a suitable ANL fuse, refer to the ANL Fuse Blow Point table above. According to the table, a 100A ANL fuse has a blow point of 175A. Therefore, a 100A ANL provides suitable protection for the 4 gauge AWG circuit in this example."

This is particularly relevant to the smaller ANL sizes, where the blowpoint -- after 8 minutes -- can be as much as 266% of the nominal rating.

Bill
 
#15 ·
I suspect that statement is not for short circuit protection which is what the ABYC E-11 ampacity table deals with.. Can you post a link to that?

What is the 80% rule they are referring to? When posted out of context with the rest of it sometimes it's hard to decipher that statement.

If you look at Blue Seas own short circuit protection suggestions they are not following their own advice, and that is why I suspect that statement is for device protection not short circuit..

Gotta run off to replace more dead three year old Lifeline's....:mad:
 
#16 · (Edited)
They're talking about high-amperage circuit protection. Here's the reference to their primer.

The "80% rule" is described midway down and below.

Note that with regard to short-circuit protection, they say that "Precise sizing of short circuit protection is not critical," and that fuse ratings up to 150% of the wire rating may be used.

Choosing Circuit Protection - Resources - Blue Sea Systems

Have fun today.

Bill
 
#18 ·
#17 ·
This stuff is extremely helpful. Thanks to you both!

On our Sabre 34, we have a Westerbeke W27, the house bank will have two Group 31 flooded deep cycles, and the reserve a Group 24.

I'm planning to use 1/0 cable for batteries (I think the current cable is 1 AWG). Runs are short since the battery compartment is adjacent to the engine compartment as is the battery switch.

By the chart, the proper MRBF fuse for 1/0 in engine spaces is 250 amp.

So, first, is 1/0 wire a good choice on this boat (Maine Sail, I think you used 1/0 on an S34 recently, but correct me if I'm wrong)?

Second, is the 250 amp a good choice for MRBF fuse on the 1/0 wire, or should I drop down to 225? And will the 225 or 250 avoid nuisance blows when starting?

Thanks, as always, for the input. I owe you guys, and a lot of others on Sailnet, a cold one. :D
Cheers,
J
 
#19 ·
So, first, is 1/0 wire a good choice on this boat (Maine Sail, I think you used 1/0 on an S34 recently, but correct me if I'm wrong)?
Yes I usually use either 1/0 or 2/0 wire for battery systems on sailboats. The Sabre 34 I did this spring was 1/0.

Second, is the 250 amp a good choice for MRBF fuse on the 1/0 wire, or should I drop down to 225? And will the 225 or 250 avoid nuisance blows when starting?
Stick with the 250...
 
#22 ·
OK one more question, but it's still related for the most part. Hope you don't mind!

My alternator is a 90 amp Balmar with ARS-5 external regulator. I don't know what gauge the charging wire is currently. Regardless, what should it be for a 5 foot run to the house bank?

According to the Blue Sea chart that Maine Sail posted, that looks like 4 AWG. But when I use the circuit wizard on the Blue Sea site, it comes out as 6 AWG, even up to 100 amps. Of course it will usually be a lot lower than 90 amps from my alternator.

So how do I size the alternator wire? I know the fuse will be MRBF and I'll size it according to the alternator wire size.

Thanks for the additional help!
-J
 
#23 ·
OK one more question, but it's still related for the most part. Hope you don't mind!

My alternator is a 90 amp Balmar with ARS-5 external regulator. I don't know what gauge the charging wire is currently. Regardless, what should it be for a 5 foot run to the house bank?
You actually have a 10 foot run, there and back. Remember all DC circuits are a loop. There should be a negative wire that is a minimum of 4ga and a positive wire that is 4ga. This will still result in a 2% voltage drop or 0.3V drop @90A. So at 14.6 you'd be seeing about 14.3v at the battery. 2ga would be better than 4ga.

In either case the alternator should be grounded direct to the house bank, or a closely located buss, for best performance. It should not just be connected to the engine ground then back to the bank. Most Balmar's are isolated ground not case grounded. This will help reduce any more VD than is absolutely necessary. Both of these wires, neg & pos, should be led to the house bank for the best performance.

Above all else your ARD-5 regulator should sense the house batteries not the alternator but this needs to be done with care and wired properly. This will eliminate any VD issues as the regulator will compensate the voltage. You'll still have VD in the wire but the banks won't see it because the regulator will compensate for it.

So how do I size the alternator wire? I know the fuse will be MRBF and I'll size it according to the alternator wire size.
You can size it lower than the wire size and still be fine. The fuse should be as close to the batteries as possible. We are trying to get as little VD as possible and that requires large wires. With battery sensing you can get away with smaller wires but bigger is still almost always better.
 
#24 ·
Thanks Maine Sail. I just stopped by the boat, and found some interesting details:

First, the current alternator "out" wire appears to be a #10 AWG, and it wraps around the block, and then connects to the starter solenoid. From the starter solenoid, there's a 1 AWG cable that goes to the battery switch on the center post.

Is there any good reason why the alternator would go through the solenoid, or was this more likely a matter of convenience with the wiring harness?

Second, my alternator is only 75 amp, not 90. Though by Blue Sea chart, that's on the line between 4 AWG and 6 AWG, so might as well go with 4.

Third, I can't find a ground wire attached to the alternator that runs back to the ground. I can find a couple wires that supply ground to other things like gauges and the voltage regulator, but it appears that the Balmar is grounded by its mounts. Is this even possible?

If this is getting too far off the topic of fuses, please let me know and I'll start another thread.
Thanks!
-J
 
#25 ·
Thanks Maine Sail. I just stopped by the boat, and found some interesting details:

First, the current alternator "out" wire appears to be a #10 AWG, and it wraps around the block, and then connects to the starter solenoid. From the starter solenoid, there's a 1 AWG cable that goes to the battery switch on the center post.
Typical factory installation..

Is there any good reason why the alternator would go through the solenoid, or was this more likely a matter of convenience with the wiring harness?
Only that it is cheap and easy for the manufacturer and when boats came with only two batteries and a 1/2/BOTH/OFF it allowed the user to select where the charge current went. Bottom line it is cheap and easy and builders still do that to this day.

Second, my alternator is only 75 amp, not 90. Though by Blue Sea chart, that's on the line between 4 AWG and 6 AWG, so might as well go with 4.
The only thing that hurts by going bigger is the wallet..:D

Third, I can't find a ground wire attached to the alternator that runs back to the ground. I can find a couple wires that supply ground to other things like gauges and the voltage regulator, but it appears that the Balmar is grounded by its mounts. Is this even possible?
Some older Balmar's had a case ground option but most still shipped with a ground stud.. If there is a ground stud on the back of the case make good use of it....
 
#27 ·
Maine Sail, just a couple of quick questions...

First, I have the bus bars and MRBF terminals in hand now, so I've been scoping out where to place them. I've seen that pic of the S34 battery compartment, where you have the NEG bus and shunt along the after bulkhead of the battery compartment. Do you mind me asking--where did you put the POS bus? Is it opposite? That looks like a tight spot on our boat.

I'm thinking of putting the MRBF fuses on the bus bar, only because the clearance above the batteries is tight. They appear to fit, but they are very close.

Second, the POS studs on my batteries are 5/16, not 3/8. Is this going to be an issue with the MRBFs since they're made to mount on 3/8 studs?
Thanks!
-J
 
#28 ·
Do you mind me asking--where did you put the POS bus? Is it opposite? That looks like a tight spot on our boat.
Water heater compartment works well.. Just drill the wires through the bulkhead.

I'm thinking of putting the MRBF fuses on the bus bar, only because the clearance above the batteries is tight. They appear to fit, but they are very close.
With some batts they will clear but with most they won't. The buss bar is fine joust route the POS into the water heater compartment.

Second, the POS studs on my batteries are 5/16, not 3/8. Is this going to be an issue with the MRBFs since they're made to mount on 3/8 studs?
Thanks!
-J
Many are and Blue Seas does not make a 5/16" MRBF, been round and round with their engineering department on this.

Blue Seas says it is perfectly safe and "acceptable" if your post has a wide enough base to use a 3/8" on a 5/16" stud.. Personally I would rather see a 5/16" MRBF but they simply don't exist..
 
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