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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #11  
Old 09-12-2011
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Quote:
Originally Posted by Maine Sail View Post
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.
Thanks Maine Sail, that makes perfect sense. Thanks!
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  #12  
Old 09-13-2011
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Quote:
Originally Posted by Maine Sail View Post
80%..? Jon A. at ABYC says, and we've specifically discussed this, the ampacity chart already accommodates for ANL, Class T etc. and MRBF is really quite similar to an ANL.
Not really. ANL fuses are unlike any other CPDs, in that they blow at a much higher than nominal rating, whereas most other fuses and breakers blow at about 130% of their rating.

ANL blowpoints can exceed their ratings by 140% to as much as 266%. A 100A ANL won't blow until current reaches 175A or more for 500 seconds -- over 8 minutes!

The ampacity tables cannot take the differences between CPDs into account; you must do that yourself, or find a table in which the specific CPD is referenced.

I believe that MRBFs are like most other fuses found in the marine environment -- relatively fast blowing at 130% of their nominal rating. ANLs are very different.

Here are the curves for the MRBFs: http://bluesea.com/productline/specs/379#td

Bill

Last edited by btrayfors; 09-13-2011 at 12:55 AM.
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  #13  
Old 09-13-2011
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Quote:
Originally Posted by btrayfors View Post
Not really. ANL fuses are unlike any other CPDs, in that they blow at a much higher than nominal rating, whereas most other fuses and breakers blow at about 130% of their rating.

ANL blowpoints can exceed their ratings by 140% to as much as 266%. A 100A ANL won't blow until current reaches 175A or more for 500 seconds -- over 8 minutes!

The ampacity tables cannot take the differences between CPDs into account; you must do that yourself, or find a table in which the specific CPD is referenced.

I believe that MRBFs are like most other fuses found in the marine environment -- relatively fast blowing at 130% of their nominal rating. ANLs are very different.

Here are the curves for the MRBFs: Detailed Specifications for Terminal Fuses (MRBF - Marine Rated Battery Fuse) - Blue Sea Systems

Bill
Bill,

Yes really. The ABYC and Blue Sea make no exceptions or derating suggestions for short circuit protection for ANL fuses vs. other types. Now if you're using it for an inverter many manufacturers suggest using Class T because they blow slightly faster but that would not be a short circuit protection issue.

You only need to look at this Blue Sea .pdf document:
http://bluesea.com/viewresource/1453

of which I have photographed and posted the important information.



And you'll notice that Blue Seas own fuse sizing chart actually up-sizes on occasion to the next size fuse, which the ABYC recommends against:


You notice that the company you've referenced makes no changes in the ANL column for short circuit protection and the fuses rated value is what is used.


The face value rating of an ANL is perfectly acceptable to use with the ABYC ampacity chart for short circuit protection and Blue Sea makes it easy to understand in the above doc though I would down size rather than up size if pushing up against the 150% rule at least.

As I mentioned I have had this very conversation with John A. at ABYC, and Eric J., before he left, and they both made the point that the E-11 ampacity chart already takes all this into account.

Derating an ANL is certainly fine but there is no need for all that extra math if using it for short circuit protection and you can simply use the face value rating of the fuse...
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Last edited by Maine Sail; 09-13-2011 at 09:06 AM.
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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
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Old 09-13-2011
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Quote:
Originally Posted by btrayfors View Post
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
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....
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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

Last edited by btrayfors; 09-13-2011 at 11:02 AM.
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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.
Cheers,
J
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Quote:
Originally Posted by btrayfors View Post

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
That was my whole point....
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Originally Posted by josrulz View Post



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.

Quote:
Originally Posted by josrulz View Post
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...
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Quote:
Originally Posted by Maine Sail View Post
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.

Stick with the 250...
Thanks Maine Sail!
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