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Old 05-06-2014, 10:52   #1
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Battery Fuse Size?

When fusing a battery at the terminal how do you figure out what size fuse to install. I'm assuming that the start battery and the house battery fuse sizes will be different.

Also is there a equation/graph/chart or anything to help determine fuse size for batteries or circuits in general?

Thanks all!
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Old 05-06-2014, 11:01   #2
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Re: Battery Fuse Size?

Blue Sea Systems - Innovative electrical systems €” Built to last, reference resources tabs under SUPPORT very good wire size & fuse sizing chart

You fuse for the wire size.
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Old 05-06-2014, 11:06   #3
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Re: Battery Fuse Size?

Quote:
Originally Posted by ontherocks83 View Post
When fusing a battery at the terminal how do you figure out what size fuse to install. I'm assuming that the start battery and the house battery fuse sizes will be different.

Also is there a equation/graph/chart or anything to help determine fuse size for batteries or circuits in general?

Thanks all!
Fuses are based on the size of the wires connecting them. You don't determine fuse size by load. The whole point of the fuse is to protect the wires.

Wires should be sized based on distance, load and voltage drop.

Here are a couple of good resources:
  • First and foremost, anything written by Maine Sail on this forum or on sailboatowners.com where he has his own section. Also, he has his own website where he publishes a ton of useful information.
  • Blue Sea Systems puts out a very useful graphic on wire and fuse sizes (warning, large pdf in link. Source is here).
  • Lastly, you own a Catalina. There is likely an active Catalina owners association in your area. Look for them and join. Lots of helpful fellow Catalina owners.
Good luck and fair winds,

Jesse
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Old 05-06-2014, 11:18   #4
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Re: Battery Fuse Size?

West Marine has a number of advisory articles which will be of help. See here: West Advisor Articles | West Marine

Also, any good text on marine electrical wiring would be useful. One of the best and easiest to understand is Charlie Wing's book.

In general, fuses are to protect the wiring, so they must be sized to the ampacity of the wiring. It is OK to use smaller fuse sizes than the wiring dictates, so long as enough amperage can be carried to run connected equipment.

There is no need under ABYC specs to fuse the start battery. However, lots of marine electricians believe it's a good thing to do on smaller to medium size boats. Mine is wired with a fuse. For a 30-footer, a 250 to 300 amp fuse would be enough.

For the house loads, you'll want a smaller fuse.

Note that the only fuses approved for attachment directly to house batteries are ANL or MRBF (terminal type), as well as the much larger Class-T fuses. These should be installed close to the batteries.

That's it in a nutshell. Would need more details on your boat setup, equipment, type of sailing you do, etc. for a more detailed response.

Bill
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Old 05-06-2014, 12:43   #5
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Re: Battery Fuse Size?

Quote:
Originally Posted by btrayfors View Post

There is no need under ABYC specs to fuse the start battery. However, lots of marine electricians believe it's a good thing to do on smaller to medium size boats. Mine is wired with a fuse. For a 30-footer, a 250 to 300 amp fuse would be enough.

For the house loads, you'll want a smaller fuse.


Bill
If the battery is, and will never be called upon, to ever start the motor then this is true. For most boats the house bank could be used for motor starting, either regularly or in an emergency. I always fuse both banks for highest potential load and wire... As we get into much larger engines, that can't be reasonably fused, I wire the starter cables in loom to protect it.

Folks should keep in mind when sizing fuses that no fuse will always result in worse outcomes in a catastrophic short than a fuse even if that fuse is slightly larger than the ABYC max ampacity tables.

I have personally tripped hundreds of fuses OCP testing including a 300A fuse on 15' of 8GA wire directly connected to a 400A lithium battery bank. The wire never even got warm to the touch and the fuse tripped in micro seconds, yet 300A is 375% larger than the ABYC allows for with 8GA wire... In a dead short this is what you want to happen...

If I had the choice of no fuse at all or a 300A fuse on 8GA wire I would take the 300A fuse every time because at least I would stand a "chance". Without the fuse the wire and boat will eventually ignite....


Max Ampacity Chart Non-Bundled Wire:
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Old 05-06-2014, 13:59   #6
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Re: Battery Fuse Size?

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Originally Posted by Maine Sail View Post
If the battery is, and will never be called upon, to ever start the motor then this is true. For most boats the house bank could be used for motor starting, either regularly or in an emergency.
Thank you all for the responses!!!!!!

I am in the crowd that uses my house bank for everything. I start the engine and run all loads off of it. My "start" battery is my emergency back up hopefully never have to use battery (yes I do still start my engine with it once a month just to test it)

So I am running 1/0 cable off the batteries so from what I am reading off the charts everyone has provided I could run a 250 - 300 amp terminal block fuse on both banks and be good?
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Old 05-06-2014, 20:41   #7
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Re: Battery Fuse Size?

This, you and I agree.

A fuse can play two rolls:

1. A current limiter. ie. a properly designed circuit the fuse pops when excess current above the design circuit avails its self.

2. A dead short. ie a positive into a ground fault. The fuse should trip immediately. Now if we are talking a low current fault maybe not. It depends on the circuit design.

And battery fuses, as you have noted here and before should meet the AIC ratings for direct to battery.

Lloyd


Quote:
Originally Posted by Maine Sail View Post
If the battery is, and will never be called upon, to ever start the motor then this is true. For most boats the house bank could be used for motor starting, either regularly or in an emergency. I always fuse both banks for highest potential load and wire... As we get into much larger engines, that can't be reasonably fused, I wire the starter cables in loom to protect it.

Folks should keep in mind when sizing fuses that no fuse will always result in worse outcomes in a catastrophic short than a fuse even if that fuse is slightly larger than the ABYC max ampacity tables.

I have personally tripped hundreds of fuses OCP testing including a 300A fuse on 15' of 8GA wire directly connected to a 400A lithium battery bank. The wire never even got warm to the touch and the fuse tripped in micro seconds, yet 300A is 375% larger than the ABYC allows for with 8GA wire... In a dead short this is what you want to happen...

If I had the choice of no fuse at all or a 300A fuse on 8GA wire I would take the 300A fuse every time because at least I would stand a "chance". Without the fuse the wire and boat will eventually ignite....


Max Ampacity Chart Non-Bundled Wire:
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