Over Current Protection On Pos Battery Connection - SailNet Community
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post #1 of 38 Old 02-23-2012 Thread Starter
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Over Current Protection On Pos Battery Connection

Should all + battery post connections be fused at the battery post, or what is the max distance a positive conductor can run from the battery post to say a panel board that contains the branch circuit breakers?
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post #2 of 38 Old 02-23-2012
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Looking at some automotive sites, they recommend no more than 18 inches from the battery post. Since the fuse is there to protect the battery and the battery wires from damage in a short circuit situation, the closer to the battery the fuse is, the better. Placing it anywhere between the positive battery terminal and the panel board (or wherever the postive cabling terminates) should work in theory. Only consideration I can think of is the resistance of the wires. If you were very far from the battery with your fuse, it might take longer to build up enough thermal energy to break the fuse. During this longer time period, extra damage could occur. Am thinking this extra time is nanoseconds, but it could make a difference.

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My understanding is that you want to provide short circuit protection for the positive lead, in case it gets shorted to a negative conductor. The longer the positive lead travels from the battery post to a fuse, the more chance that lead has of say the insulation failing and the lead short circuiting. I think the AYBC says no more than 7 inches for leads that go to power generation equipment like alternators, but not shore about leads that would go to a boats main breaker panel.
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True - the longer the unfused length is, the more opportunity you have for a short circuit situation to develop. If one does develop, the more wire could be damaged, too. Close is better, but I don't know what the maximum distance should be. No more than 18 inches, if you apply the automobile standards.

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post #5 of 38 Old 02-23-2012
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I'm getting ready to add a fuse to my setup. Not sure your exact application but MRBF fuses from Blue Sea look pretty easy to add to an existing setup. Their site also says 7" is AYBC requirement. Terminal Fuse Block, 1 Terminal Stud - PN 5191 - Blue Sea Systems
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post #6 of 38 Old 02-23-2012
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These are great devices, and before cars used a Fusable Link, they used a similar terminal fuse that was approximately 500 amp max current. The one linked to above is a 300-amp model that fits directly on the battery terminal and I highly recommend this device, especially if you have multiple battery hookups in the battery compartment.

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post #7 of 38 Old 02-23-2012
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Positive leads are required to be fused. Over current protection is required to be within 7 inches of the source of power unless the wire is contained through its entire length in a sheath. Then it can be up to forty inches. That is both an ABYC and USCG standard.

I might add, if you run the lead to a panel and then have branch circuits off that panel, the panel is the source of power for the branch circuits and each branch circuit must have over current protection withing 7 inches or 40 inches if in a sheath.

The exception is the lead to the starter for the engine. It is not required to be fused, but there are now fuses available that can handle the power surge, and some builders are fusing this wire too.

See http://newboatbuilders.com/pages/elect.html
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Last edited by peikenberry; 02-23-2012 at 11:32 PM.
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post #8 of 38 Old 02-24-2012
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Quote:
Originally Posted by jkemp101 View Post
I'm getting ready to add a fuse to my setup. Not sure your exact application but MRBF fuses from Blue Sea look pretty easy to add to an existing setup. Their site also says 7" is AYBC requirement. Terminal Fuse Block, 1 Terminal Stud - PN 5191 - Blue Sea Systems
That's what I have on my boat and on my travel trailer, and I chose a 70 amp fuse, for the boat and 50 amps for the trailer. Both have # 4 wire.

Dick

Last edited by Flybyknight; 02-24-2012 at 07:50 AM. Reason: Verify wire ga.
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post #9 of 38 Old 02-24-2012
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Quote:
Originally Posted by casey1999 View Post
The longer the positive lead travels from the battery post to a fuse, the more chance that lead has of say the insulation failing and the lead short circuiting.
This is really the bottom line. If the cable passes through something, or could rub against something, or could be exposed to heat, chemicals, whatever, then there is a possibility of the insulation failing. You want it to be fused BEFORE that point.
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post #10 of 38 Old 02-24-2012
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The simple point is, the fuse is there to protect the wire. The number one source of fires on recreational boats is electrical fires. More often than not it is a wire that fries and starts the fire. So having a long length of unprotected wire is just asking for trouble. That's why the fuse or circuit breaker has to be close to the source of power. AdditionallY:

Quote:
If the cable passes through something, or could rub against something
If this is the case the requirement is to have grommets, or some other kind of chafe protection so the wire insulation will not be damaged by sharp edges and rough surfaces.

Frankly I suggest you read up on the electrical standards before doing any electrical work on your boat. You can download free an old copy (2008) here of ABYC E-11. http://www.kroescorp.com/Other/e-11.pdf It hasn't changed all that much in 4 years.

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