Fusing

[Dsanduril]

Originally Posted by Ecos

The idea of fire scares me. A direct short from the alternator frame to the block with no fuse and 4 8D's feeding the flames seems a little out of control. My boat is flamable.

That's what the fuse at the battery end of the connection is for - it protects both the wire and the alternator from excessive current should a short try to drain the batteries.

While some disagree, a second fuse at the alternator output just adds a point of failure with (my opinion) no real benefit.

[Paul L]

Originally Posted by Ecos

How about the plastic insulator at the + lug? A loose connection will heat it right up. The diode pack could melt to ground. The bearings/bushings could fail and the rotor could explode. I don't know, I would rather replace a diode pack than the boat. My boat has a bunch of amps and that could cause a big fire.

How will that create an output greater than the alternators 100 amps capable of blowing the proposed 150 amp fuse?

[piberman]

Here’s how Uncle Sam’s Navy does it. Every circuit is fused both positive and negative.
Recreational boats just fuse positive leads (hopefully). Yes its true some alternators will self destruct before large loads are generated. But fuse blocks are cheap and will impress future owners and surveyors. Here’s another tip from Uncle Sam’s Navy. Use neoprene welding cable rather than plastic auto style battery cables. Neoprene lasts forever even in wet conditions. And if the cable lug ends are soldered on to the thousands of fine conductors you have a bullet proof connection. Be forewarned. Neoprene welding cable is considerably more expensive than the commonly used rubber welding cable that is not impervious to water. And after 50 years the neoprene will still be flexible. Its’ why its used the world over for mining cables.

[model 10]

Originally Posted by Paul L

How will that create an output greater than the alternators 100 amps capable of blowing the proposed 150 amp fuse?

It's not a matter of the alternator putting out to many amps, it's the alternator shorting out to ground. With no fuse, you could have your entire house bank going through your alternator to ground.

[Wotname]

Originally Posted by Ecos

It's not a matter of the alternator putting out to many amps, it's the alternator shorting out to ground. With no fuse, you could have your entire house bank going through your alternator to ground.

Which is why there should be a fuse at the house bank end of the wire (but not at the alternator end)!

[smac999]

Originally Posted by NYSail

Its a mental block I guess I am having when it comes to picking the appropriate size fuses. The batteries currently have nothing. I am leading my new alt directly to the house bank... its a 150 amp alt so at the battery I will put a 200 amp terminal fuse. However I am a bit confused with from battery to battery switch. I have 2 lifeline agm 4D's with 2/0 battery wires.

Greg

200a is perfect on the house batt for the alt.

It depends on your engine and starter draw. I would start with 300a on the batteries feeding switch. . If you blowit. You need to step up to bigger anls. Also depends on switch type, and if both batt can start engine

[smac999]

Originally Posted by mickt243

So, what’s the question?

To be safest, you need an appropriate fuse at every current generating device... ie alt and batt.... the fuse at the battery end needs to be sized so that a short will blow it with battery current, but without nuisance blowing.... so if your battery can generate 1000 amps, but the wire is good for only 300, then the fuse needs to be about 275 amps. The fuse at the alternator end needs to be sized to protect both the wire AND the alternator- say the alternator is good for 100 amps, same 300amp wire, you probably want a 110 amp fuse.

Alts are fused at battery end at 150%

Limited current sources. (Alts, chargers, solar etc) is not fused at source.

[NYSail]

Isn’t that why we put a fuse at the battery side of the alt feed?

Greg

[NYSail]

Originally Posted by smac999

200a is perfect on the house batt for the alt.

It depends on your engine and starter draw. I would start with 300a on the batteries feeding switch. . If you blowit. You need to step up to bigger anls. Also depends on switch type, and if both batt can start engine

I am thinking anl 300 for battery leading to the arc (which in turn leads to starter) and an anl 300 for wire leading back to battery switch. This is a beneteau and protection is away from batteries. They have fuses after the switch before the panel of 100 amp that have been fine with current set-up..... all heavy stuff..... bow thruster, windlass come off battery switch and are fuses separately.

Thanks
Greg