What Size Alternator Fuse ?

[AaronJ]

Hi, I have a 35A alternator on my Yanmar 2GM20F, according to the manual. It has a 30A inline fuse from the output to the battery. My question is why hasn't the 30A fuse blown? Does the alternator not actually put out 35 amps; is it weakened when converted from AC to DC?

Thanks,
Aaron

[CharlieJ]

Every fuse and circuit breaker is designed to open after a prescribed overload condition for a certain length of time. Here is an example: Detailed Specifications for ANL Fuses - Blue Sea Systems Note that in this example, the fuse will not open if the load is < 140% of the fuse rating. And this is only an example. Other fuses and circuit breakers may be designed to be much more responsive.

Over current protection devices (OCPD) are installed to protect the circuit; i.e., the conductors. Unprotected conductors become strip heaters in a ground fault condition.

By the way, the ABYC Standards require an OCPD where a conductor connects to a source. The alternator output is one source and the alternator's conductor is connected to a battery/bus bar/battery switch at the other end which could become a source if the alternator failed. That other end needs to be protected by an OCPD also. That's right; each end of the alternator's output conductor requires an OCPD.

[AaronJ]

If the 30A fuse is between the alternator and battery, why do I need another fuse in that wire?

[David M]

In that circuit you have two sources of current, the battery and the alternator. The idea is to put a fuse as close to the source as is practical to protect the wire further down the circuit. Given you have two sources of current in that circuit, you need two fuses.

[AaronJ]

Ok, I see. Because current flows from both directions, so the other fuse would be inline with the battery cable. It must be a big fuse, because the starter draws over 100 amps, right?

[S&S]

Interesting. I rarely see a fuse between the alt and battery.

[David M]

Yes, its not a real common thing but a good idea. If the starter draws its current through that same circuit, then yes, fuse it for the starter motors draw at the battery end of the circuit. Most engines combine the starter in to that circuit and some do not.

[sailvayu]

ABYC allows you to run the main cable to the starter without a fuse, but you do need a disconnect switch as close to the battery as practical. Most times the Alt output wire is run to the same positive connection on the starter rather than all the way back to the battery. It is good practice to have a fuse close to the Alternator output as possible to protect it. ABYC now requires this but in the old days it was rarely done.

Wayne Canning, AMS
projectboatzen.com

[AaronJ]

Mine does have a 30A fuse on alternator output, but why is it only 30A if the alternator outputs 35A?

[CharlieJ]

David has it right...two sources...two OCPDs. Granted, the battery only becomes a source if there is a fault to ground failure in the alternator, but what a source it would be!!!

Here are the pertinent citations from the ABYC Standards, E-11:

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.

11.10.1.1.2 In addition to the provisions of E-11.10.1.1.1 the ungrounded conductors to a battery charger, alternator or other charging source shall be provided with overcurrent protection within the charging source, or within seven inches (175mm) of the charging source, based on the maximum output of the
device.

Regarding the combined alternator/starter circuits, consider changing the wiring so that you can install the required OCPDs in the alternator to battery conductor and establish a separate circuit for those times when the house bank needs to be used to start the engine.

There should be ONLY one unprotected circuit on a vessel and that is the engine starting circuit. Every other circuit needs to protected by a properly sized OCPD with an adequate trip curve and the correct ampere interrupt capacity (AIC) for the battery bank serving the circuit. To NOT properly protect every circuit is false economy and pure folly.

[sailvayu]

Most alternators almost never put out anywhere near there rated load and if they do it is only for a short time. and like CharlieJ said most fuses only blow at over there rated load. If it works then life is good.

Wayne Canning, AMS
projectbaotzen.com

[GordMay]

I’d use a minimum wire size of #8 AWG (>8mm) for a 35 A alternator, which ABYC rates to 68A & over (NEC & I etc only rate it to 40A).
In my view (not necessarily that of other authorities) the prime purpose of fusing an alternator output is short circuit protection, which is a very high catastrophic fault current.

Accordingly, I be happy with a fuse, at the alternator, sized at anywhere between 40A and 60A, for a 35A alternator /w #8 wire.

Although I understand Charlie’s concern, I (FWIW) don’t view the battery as a power source to the alternator, and wouldn’t require a fuse at the load end (batt).

[CharlieJ]

Gord-
I became a believer when delivering a 49' power boat north on the ICW. We had suffered a lightning strike that destroyed most everything electronic and had replaced most of it during an interim stop. As a sidebar, it was beneficial that I had done all of the electrical upgrading on the boat and my mate had assisted as a sub-contractor! But I digress.

Knowing that lightning does strange things, I stress tested almost everything electrical, including running the mains and working the alternators very hard. Alternators passed their test.

Long story a bit shorter; the Mate came up after one of the half hour engine room checks to report that the circuit breaker (original equipment in this circa 1985 vessel) on the battery end of the alternator to battery conductor had tripped and the toggle would not move. I am not talking about resetting; it would not move.

Investigation showed that the port alternator, fed the battery switch complex with 1/0 wire and was protected by the aforementioned CB on the battery end. The alternator developed a ground fault, the battery bank responded with a bunch of amps and the CB opened, and gave up its life since it had insufficient AIC to survive.

On reflection, I am very happy that this CB did its job. The 1/0 run was about 15' long, ran in the wire race in the overhead and, had there been no CB on the battery end, the conductor would have become a very high capacity, high temperature strip heater with molten copper dropping on the port engine, taking out other wiring, possibly starting a significant fire that we would not have discovered for another half hour.

My recommendation to those following this thread is to include properly sized OCPDs on each end of the conductor per the ABYC Standard period. All of my designs and installations include them and with the development of the Marine Rated Battery Fuse (MRBF) why quibble over a $30 fuse and fuse holder?

[GordMay]

Charlie:
Wouldn't the fuse at the alternator have opened, virtually as quickly, under those short circuit conditions?
The breaker at the battery end protects the intervening cable, which wasn't the issue.

[CharlieJ]

The whole idea of the ABYC 7" rule is to minimize the length of conductor that is unprotected should a source "see" a fault to ground. You are probably right, the OCPD at the alternator conductor would probably open just as fast when exposed to a dead short. But why take the chance when for $30 you can comply with very a very well reasoned approach to protecting this critical conductor from massive amounts of short circuit current.

The real, and underlying issue is, that most folks do not understand how much energy we are packing in the large house banks prevalent in today's boats. Short circuit currents > 5,000 amps per battery are not uncommon. I really believe that this is one place where prudence is the better part of valor.