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Old 04-25-2015, 08:16 PM   #1
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Fuse alternator battery lead?

I'm finishing up some rewiring in the engine room and am wondering about whether or not to fuse the cable from the 2nd alternator to the distribution panel. I've never seen a fuse from a primary alternator to the battery (many just go to the starter battery cable). There is a fuse at the battery bank. The 4' of cable between the alternator and the distribution panel is pretty well protected. If the cable were to short to ground, does it just blow the regulator?

Ted
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Old 04-25-2015, 08:20 PM   #2
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I don't fuse alt outputs on my installs. The internal diodes will act sorta like fuses. I don't know what the rules say, but most marine engines when shipped alt is hard wired to starter lug, no fuse.
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Old 04-25-2015, 09:06 PM   #3
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ABYC for good reason doesn't require a fuse in the wiring from the battery to the starter lug. As you and Ski note, most alternators are wired to that starter lug, so no fuse.

But for a secondary alternator that isn't wired to the starter lug, I would fuse it near the battery. You don't need it to protect the alternator. You need it to protect the wire which could short to ground and with a battery behind it could dump hundreds of amps into that wire, burn it and start a fire.

You should size the wire from the alternator to handle its output current so you don't need a fuse at the alternator end. You need a fuse at the battery end to deal with a short in the wire.

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Old 04-26-2015, 07:16 PM   #4
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Ted

Here is what the ABYC says - I think this confirms exactly what Ski & David have said.

11.10 OVERCURRENT PROTECTION

11.10.1 LOCATION OF OVERCURRENT PROTECTION DC CIRCUITS

11.10.1.1.1 Overcurrent Protection Device Location - Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point at which the conductor is connected to the source of power measured along the conductor. (See FIGURE 15.)

EXCEPTIONS:
1. Cranking motor conductors.

2. If the conductor is connected directly to the battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the battery, but not to exceed 72 inches (1.83m).

3. If the conductor is connected to a source of power other than a battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box or enclosed panel, the overcurrent protection shall be placed as close as practicable to the point of
connection to the source of power, but not to exceed 40 inches (1.02m).

4. Overcurrent protection is not required in conductors from self-limiting alternators with integral regulators if the conductor is less than 40 inches (1.02m), is connected to a source of power other than the battery, and is contained throughout its entire distance in a sheath or enclosure.

5. Pigtails less than 7 inches (175mm) in length are exempt from overcurrent protection requirements.
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Old 04-26-2015, 10:51 PM   #5
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Thanks for the responses. Pretty much confirmed what I was thinking. Have a fuse at the battery bank and all the wire between the fuse and the alternator is 4/0 (salvaged from the original installation). If there is a problem, either the fuse or the diodes should pop pretty quickly.

Ted
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