fuses

[bcbristoll]

Hey everyone,
I took out a friend yesterday and while looking over my boat he told me I needed to have fuses on the positive terminals of both batteries within 7" of the batteries. It's an atomic 4. Can anyone tell me what amp rating the fuses should be ??

[tenders]

Whoops - correction - you don't need to put in a fuse for the starter side of the circuit. This has been discussed here before:

http://www.moyermarineforum.com/forums/showthread.php?t=9509

I don't believe boats are required to fit or retrofit to any ABYC standards (only Coast Guard standards), but the relatively current ABYC standards are discussed here:

https://www.sailfeed.com/2015/05/boat-wiring-reterminating-cables-and-adding-big-fuses/

A 1990 version of ABYC showing this more officially, pages 18-22:

https://law.resource.org/pub/us/cfr/ibr/001/abyc.E-09.1990.pdf

This is one of many great ABYC engineering ideas that, sorry, just isn't getting installed on my boat. The positive leads are all fused at the circuit panel, which is maybe 24-36" from the batteries via the 1-all-2 Perko switch. That will have to be good enough.

[Al Schober]

Start battery doesn't need (or want) a fuse.
House battery - probably a good idea. As for the size of the fuse, you're protecting the cable going to your DC panel - what size is the cable?

[bcbristoll]

Thanks for the info. I won't worry about the starter circuit and size the fuse based on the gauge of the wire.

[joe_db]

If you do want to fuse the starter, mine draws abut 125-127 amps. I would use a 200 amp fuse if I did this.

[sastanley]

I have a 100 amp breaker on my BlueSea battery switch. Prolonged cranking has tripped this breaker, when I forget to do things like prime the mech fuel pump after a long layover. I was planning on adding a 150amp breaker and a dedicated start circuit from the battery to the starter, but just haven't gotten around to it. Joe's numbers seem about right to me.

[toddster]

I have these on my house and start batteries. Pretty painless to install, though it does add an inch to the height of the battery. I don't know why you wouldn't want to protect the cable going to the starter. Seems like a prime candidate for vibration and chafe. Lots of boats are lost due to "electrical fire of unknown origin." IIRC, I used a 150 amp fuse and figured I'd go up if needed. Haven't needed.



Actually, I have "main" and "emergency backup" batteries. Doesn't seem like the A4 has great enough cranking demands to require a dedicated start battery. Smaller loads like the panel, windlass, and SSB are branch circuits off the main switch with their own smaller breakers. There is more than one way to do this. Doubtless I'll rearrange it all when I add more charging sources.
I've thought of giving the engine its own disconnect switch or breaker, so that I could do work on it without disconnecting the rest of the boat systems, but haven't done so as yet.

[edwardc]

... I don't know why you wouldn't want to protect the cable going to the starter. ...

The problem is that motor startup can produce huge transient current surges that will pop a breaker or blow a fuze. One rule of thumb is to size the breaker/fuze on a motor circuit at 6 times the normal operating current in order to avoid nusiance trips! In the case of a starter motor, this would be so large as to provide no protection to the starter wires.

[joe_db]

The problem is that motor startup can produce huge transient current surges that will pop a breaker or blow a fuze. One rule of thumb is to size the breaker/fuze on a motor circuit at 6 times the normal operating current in order to avoid nusiance trips! In the case of a starter motor, this would be so large as to provide no protection to the starter wires.

I doubt an A4 would blow a 150 or 200 amp slow-blow fuse. Unless it is too quick for the meter to follow, I have never seen over 130 amps on startup. Someone on here is using a 100 amp breaker on their starter and only blowing it some of the time.

[tac]

The inrush current to a motor on starting is the same as “Locked Rotor” current found on manufacturer’s drawings and data sheets (if you can find one for consumer goods). As Ed said, it can be about 6 times the motor’s full load current. Locked rotor is the current a motor draws when the rotor is locked, that is, not moving. In normal situations on startup the motor shaft is not moving (locked rotor) for only an instant (milliseconds) before the shaft starts to turn. As the shaft turns faster and faster, the current drops off appreciably, until at some higher speed it is at rated full load. In general, a slow blow fuse is not needed.

A fuse/breaker’s rating (e.g. 175 amps) is NOT the point the device trips. In fact, a fuse/breaker located in an ambient temperature of about 70ºF will take days, if ever, to trip with 175 amps (100% of rating) going through it.

The MRBF (https://www.bluesea.com/products/518...al_Fuse_-_100A), like most fuses and thermal breakers, has time-current curves that show its trip characteristics. The trip time of these curves is roughly proportional to the inverse of the current squared - doubling the current decreases the trip time by a factor of four. This is because thermal elements are resistive, and heat generated in a resistance is proportional to the square of the current.

Keep in mind you’re fusing the cable, not the motor. If the starter cable is a #2 with 90ºC insulation, it is rated, in the engine room, at 148 amps. If fused at 100%, use a 150 amp fuse. At 135% of the cable rating, use a 200 amp fuse. I’ve used a 200 amp fuse for 20 years, and never blown one yet.

As to using a fuse at the battery on the starter motor feed:
If the starter shaft won’t spin (locked rotor) because the engine is frozen, would you rather,
a) Blow a fuse after a short time, and replace it for $18.
b) Melt the starter cable, start a fire, and buy a new boat.