Fusing question

May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
when I was picking up info on wiring two axioms stuck with me
1) fuse early fuse often
2) start at power source fuse for smallest wire between it and the next fuse point or equipment

So seeing a 12AWG wire come off a 120A breaker gives me pause. But I know what to do

The question is on fusing alternators and battery chargers. I know I need to fuse the battery end of the wire. But what about the alternator and battery charger end. Should I run the alternator/BC output to a fuse rated at say 20% over the rated capacity or at wire capacity??

Thanks

Les
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
when I was picking up info on wiring two axioms stuck with me
1) fuse early fuse often
2) start at power source fuse for smallest wire between it and the next fuse point or equipment

So seeing a 12AWG wire come off a 120A breaker gives me pause. But I know what to do

The question is on fusing alternators and battery chargers. I know I need to fuse the battery end of the wire. But what about the alternator and battery charger end. Should I run the alternator/BC output to a fuse rated at say 20% over the rated capacity or at wire capacity??

Thanks

Les

Battery charger and alternator fuses should always be at the battery end of the positive wire and sized for 150% of battery charger or alternator max output. If the charger wire is sized properly for voltage drop, 150% is easily within the wires max ampacity.

Chargers, alternators and solar are considered "current limited" sources so they do not require a fuse at the charging device end of the positive wire, just the battery end.
 
May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
Perfect - now to design and see what I have to fix. Beyond the above they have a 10' run between battery sets - no fuses on either end of those wire - I think I'll be buying some MRBF fuse.

Les
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Chargers, alternators and solar are considered "current limited" sources
MaineSail you have all the correct words too describe what is happening with this magic electricity stuff. While I arrived at the same end, understanding the fuse point is nearest the biggest risk source, Battery vs charger. And all of the circuit designs show this configuration identifying a charger as a "current limited" device simplifies all the issues.
Thanks.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Les... Those suckers are not cheap. Sort of $15 to $20 a "pop".
 
Jul 8, 2012
126
Catalina 28 North East
Battery charger and alternator fuses should always be at the battery end of the positive wire and sized for 150% of battery charger or alternator max output. If the charger wire is sized properly for voltage drop, 150% is easily within the wires max ampacity.

Chargers, alternators and solar are considered "current limited" sources so they do not require a fuse at the charging device end of the positive wire, just the battery end.
Question on "alternator max output" - my M3-20 has a 51 amp alternator - is that 51 Amps per hour, 51 Amps constant or a potential for 51 Amps depending on battery condition, engine speed etc?
I wired to the house battery with a 60 amp maxi fuse as a service cutoff in the engine compartment, should that be fused at 75 amps or replaced with a switch and add a terminal fuse at the buss bar to the house bank?
 
Last edited:
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
Question on "alternator max output" - my M3-20 has a 51 amp alternator - is that 51 Amps per hour, 51 Amps constant or a potential for 51 Amps depending on battery condition, engine speed etc?
I wired to the house battery with a 60 amp maxi fuse as a service cutoff in the engine compartment, should that be fused at 75 amps or replaced with a switch and add a terminal fuse at the buss bar to the house bank?
The 51A is the alternators SAE J56 "rating" and when cold it can produce more than 51A but when hot considerably less. Alternator ratings are based on either ISO test standards or SAE test protocols. For SAE J56 the "rating" is at 6000RPM and 13.5V in an ambient room temp of 73F. In the real world, on a boat in a hot engine room, you will rarely see the full output, except when cold. This is especially true with an automotive based unit such as the 51A alt found on many Universal engines.

Expect about 25-35A, as installed, as your max bulk output for a 51A alt.. It's still best to size your fuse at 150% of "rated" so you don't inadvertently cause a nuisance trip, which could blow your alternators diodes..

In terms of units there really is no term "amps per hour". The unit of current is called amps or amperes. If your alternator could run at 51 amps continuously for 1 hour straight, and it can't, you would have delivered 51 ampere hours to the battery, but not 51 amps per hour or amps/hour.

I would still know what you are saying but an EE might look at you like you have eight holes in your head if you said that.

Amps or Ampere are a measure of electrical force at a single point in time.

An Ah or ampere hour is a measure of energy use over time.

1 amp for 1 hour = 1 Ah

"My alternator is putting out 51 amps." Is correct..

If you ran if for one hour it would have delivered 51 ampere hours but what if you only ran it for 10 minutes? If it was delivering 51 amps but for only a 10 minute duration it would be:

51 amps X 0.17 hours = 8.67 Ampere hours

"My alternator is putting out 51 Amps per hour." Is incorrect because amps is an instantaneous measurement..

A battery for example is a storage device and as such contains XXX number of ampere hours of energy or Ah or amp hours of energy capacity. Of course that stored Ah capacity is only true at 77F and at the 20 hour discharge rate.

A battery for example is not a "100 amp battery" it stores 100 amp hours, at 77F, at the 20 hour discharge rate. It would be referred to as a 100Ah battery.

What about a starter motor that draws 286A, seems like battery killing discharge amperage right?? Starter motors only run for very short duration's so the ampere hours consumed are negligible.

Cranking Amps Converted to Ampere Hours:

0.75 Seconds is approx 0.0002 hours - 286A X 0.0002 = 0.06Ah's

1 second is approx 0.0003 hours - 286A X 0.0003 = 0.086 Ah's...

2 Seconds is approx 0.0005 hours - 286A X 0.0005 = 0.14 Ah's

3 Seconds is approx 0.0008 hours - 286A X 0.0008 = 0.23 Ah's

4 Seconds is approx 0.001 hours - 286A X 0.001 = 0.28 Ah's

5 Seconds is approx 0.0014 hours - 286A X 0.0014 = 0.40 Ah's
 
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