alternator connection to primary battery bank

[J_Sail]

As is often the case, everyone is partly correct in what they are saying.

When sizing fuses, in general it is best practice to size the fuse to the wire's safe ampacity (safe current carrying capacity) rather than the load it will be carrying. There are occasional exceptions (such as protecting stalled electric motors from burning out their windings), but usually one uses the wire's ampacity.

The downside of selecting a smaller fuse based on the expected load is the increased probability of nuisance blows due to mis-estimating or due to the addition of future loads. Nuisance blows can indirectly cause safety issues if the load is important (like running a bilge pump or starting an engine), so it's important not to introduce such risks unnecessarily. Thus the best practice recommendation.

It can be hard to accurately estimate loads, while it is easy to look up the wire's ampacity (though be sure to use the correct table and column for insulation temperature rating, service location, and bundling). Secondarily, fuses add a tiny voltage drop that increases with smaller fuse ratings, and many types of slo-blow fuses suffer long-term degradation when operated near their rated value, again resulting in nuisance blows down the road. So it's not that a given sized wire "requires" a fuse of a certain min size, only that best practice says to use a fuse based on the ampacity so that you can take full advantage of the wire's capacity without nuisance blows.

In the specific case of fusing the wire for the alternator, the max operating current is guaranteed to be no higher than the alternator can generate, therefore at least for the present there is no nuisance-blow risk to selecting a fuse based on that (with at least 20% margin, so 70A preferred over 60A). Selecting one based on the rating of the largest alternator you might upgrade to would provide future proofing. That said, there is no problem with sizing the fuse using the best practices approach (wire size). A short inside of the alternator would blow a smaller fuse quicker, but the difference is extremely unlikely to have a safety impact. In either case the alternator housing should contain the damage. 

And, of course, in ALL cases the fuse must be no larger than the wire's ampacity.

Jeremy

P.S. As stated previously, so long as the wire is heavy enough to safely carry the max output of the alternator, there is no need for a fuse at the alternator end.

PPS. I amend my earlier statement to read, "...anywhere between the max output of the alternator (plus 20% margin) and the ampacity of the wire used..."

[KWKloeber]

Jeremy (It's great to have a name!)

Honest, unloaded question.  What is the battery fuse rating (250a /300a controversy) based on?  ie, how was that number arrived upon?

Back to my "60."  Owner has a 51a alt; presuming no change (because, as of yet, it has not been "decided" he will later upsize,) I added 15% factor (= 58 amps), so I picked the trouble-causing, rabble-rousing "60".  20% would be 61 amps (I figured 60 was still close enough.)  I didn't go to 70 -- maybe should have -- but if it's an MRBF, the next higher is 75, not 70.  So, I figured the chance of a nuisance blow is minimal and, in my mind anyway, stuck w/ 60 amp fuse.  I might have wanted to jump higher, but it wouldn't have been a life-threatening faux pas.

I agree regarding the critical circuits, but as always there's no magic bullet.  A bilge pump is critical, but also one of the ones to have a high chance of a locked rotor -- so it's a double edged sword how far to upsize.

No matter the cable size or the equipment, there's no requirement,, nor a "common sense" argument, that I can conjure up, to fuse at the max cable capacity unless that's what's needed (considering nuisance blows of course.)  Parallelling Mark's post, if there were an abrasion on the charge cable, an intermittent short, that might doesn't carry 250 or 300 amps, I sure want MY FUSE to blow with any short, even if it's 1/4 the cable capacity.  Likewise, SBSC (Stu's boat, Stu's choice) if but if a connection comes off on MY BOAT, I want to cut current at "60", rather than the lug welding itself to the alternator frame or engine block with 300 amps behind it.  Ouch.

Ken

PS - Remember, regarding "everybody's correct", that  this was re: specific application, a specific alternator, not a generic discussion about fusing cables in general and whatnot.  I believe what I proposed was "right on" (or close enough) for the stated conditions. Conditions change, then answer changes.

[mark_53]

Great discussion. I think you also have to consider the consequence of a blown fuse and running alternator is likely blown diodes.

[mark_53]

I wanted to see how significant voltage drop between 10ft (each way) of AWG1 and AWG8 was. I used an online calculator with the following parameters.
10amp load
14 Volt input (alternator charge)
Single set of conductors.

Voltage drop for AWG8 0.13v
Voltage drop for AWG1 0.025v
Difference 0.105 v or .07%

Seems insignificant to me.

Now if you battery is at 50% SOC (or 12.06v) and your alternator output is 14v then the voltage contributing to battery charging is 1.94 volts.  Now, 0.105v of 1.94v is a .054 improvement using AWG1 vs AWG8

[KWKloeber]

Great discussion. I think you also have to consider the consequence of a blown fuse and running alternator is likely blown diodes.

Absolutely,   
And content-rich and fact-based discussions are always interesting!

kk

[KWKloeber]

mark

True, 8 awg (theoretically) would have some 1.08% v drop, and 1 awg a 0.21% v drop. 
So bulk charging at say 14.5v, the bank sees 14.3v, vs 14.5v.

But consider a more-depleted bank (and higher alt charge rates.)
At 40 amps, the bank sees 13.8v vs. 14.4v.

And it's more critical with the OEM, internally-sensed, single-stage, low-setpoint, non-adjustable, motorola (automobile) alternators that don't charge at high enough voltage!

kk

I wanted to see how significant voltage drop between 10ft (each way) of AWG1 and AWG8 was. I used an online calculator with the following parameters.
10amp load
14 Volt input (alternator charge)
Single set of conductors.

Voltage drop for AWG8 0.13v
Voltage drop for AWG1 0.025v
Difference 0.105 v or .07%

Seems insignificant to me.

[J_Sail]

Ken,
I will try to respond to each item. Pls forgive me if I miss anything.

What is the battery fuse rating (250a /300a controversy) based on?  ie, how was that number arrived upon?

I'm not sure what specific controversy you are referring to, but there has been some published controversy surrounding how to best balance protection against nuisance blows. Lots of folks mis-interpret the peak current draw of a starter motor and assume that the peak would blow any fuse smaller than the peak. In fact, a 250A fuse can routinely carry a short term peak of 750A for 1 second and over 1,500A for 0.1 sec!

MaineSail did some nice instrumented experiments showing that the current-draw profile for common small 4-cylinder diesels was well within the operating curve for a 200A MRBF fuse. Obviously one needs to pick a fuse that safely protects the wire against a short, but for anything other than severely undersized starter cables it should be possible to spec a fuse that protects the wire and doesn't nuisance blow.

See
http://www.pbase.com/mainecruising/battery_fusing

You can even get by with 150A if needed:
http://www.cruisersforum.com/forums/f14/revised-diagram-cables-and-fuses-correct-83430.html#post968888

Back to my "60."  Owner has a 51a alt; presuming no change (because, as of yet, it has not been "decided" he will later upsize,) I added 15% factor (= 58 amps), so I picked the trouble-causing, rabble-rousing "60".  20% would be 61 amps (I figured 60 was still close enough.)  I didn't go to 70 -- maybe should have -- but if it's an MRBF, the next higher is 75, not 70.  So, I figured the chance of a nuisance blow is minimal and, in my mind anyway, stuck w/ 60 amp fuse.  I might have wanted to jump higher, but it wouldn't have been a life-threatening faux pas.

Yup, 60a probably would be fine for a 51 amp alternator.

I agree regarding the critical circuits, but as always there's no magic bullet.  A bilge pump is critical, but also one of the ones to have a high chance of a locked rotor -- so it's a double edged sword how far to upsize.

For bilge pumps, I prefer using a highly visible easily resettable circuit breaker to protect the motor, sized at the normal load of the pump (e.g. 5-10a). Since the wiring upstream of the breaker still needs protection at the source, I spec heavy gauge wire and high-value fuse (e.g. 40a) at the source end of the wire. Then if debris clogs the pump the breaker trips before the motor burns out (and before the fuse blows). The breaker is visible and easily reset. The fuse blows only if there is a wiring short (or big overload on anything else powered by the same wire). I believe Jon may have gone that route.

No matter the cable size or the equipment, there's no requirement,, nor a "common sense" argument, that I can conjure up, to fuse at the max cable capacity unless that's what's needed (considering nuisance blows of course.)  Parallelling Mark's post, if there were an abrasion on the charge cable, an intermittent short, that might doesn't carry 250 or 300 amps, I sure want MY FUSE to blow with any short, even if it's 1/4 the cable capacity.  Likewise, SBSC (Stu's boat, Stu's choice) if but if a connection comes off on MY BOAT, I want to cut current at "60", rather than the lug welding itself to the alternator frame or engine block with 300 amps behind it.  Ouch.

I believe the argument that is behind the, pretty much universal, recommendation by the relevant professionals is:
a) exact load amperages are often hard to predict (either due to lack of sufficient specs or due to changes in configuration over time), whereas the ampacity of the wire is easy to know.
b) there is little benefit to undersizing the fuse; the scenarios you describe come out remarkably close with either way of sizing the fuse. That's because the fault currents for short circuits are so vastly higher than the fuse rating in either case. The fuse does not "cut the current" at its rated value, it simply blows after a certain time period that is a function of the time*current integral and is calculated to be soon enough for the wire's insulation to not melt or start a fire. Until it blows, the current is limited by the total resistance of the wiring and battery, not the fuse. In the case of a short from the alternator lead to ground, the current could easily be 1,000A or more for a fraction of a second. The marginal short from a lightly abraded wire will blow (or not) both fuses in roughly similar manner and the welding result from a hard short will be similar.

[KWKloeber]

Thanks Jeremy.   

The controversy I meant was when you said "Lots of discussion between 250 or 300 A."

If there's no reason to "undersize" a fuse/breaker then what's the benefit or reason to "oversize" it (taking into account nuisance)? I suppose I'd prefer to "err" on the undersize, than oversize.

I'd have to see, with my own eyes, or documented, that a 60-amp fuse would blow exactly the same as a 300-amp fuse, given an intermittent or "leakage" short.  I ain't buying that at face value.  BUT even if true, there's no reason to oversize it, other than to just "do it."

kk 

[KWKloeber]

Harry,

Let me clarify how an energy budget can help you size not only the battery bank, but the other side (alternator, solar, etc.) as well.

Premise:  The budget, like a household budget considers two sides.  Expenses (a visa card,) and income replenishment (a paycheck) on the other..  if you don't consider both sides and they aren't in equilibrium, it's not a budget, it's called a "pipe dream."  Or the Treasury Department (but we sailors can't borrow amp-hours from China.) 

Esoterically, what's a budget?
Merriam-Webster  defines a financial  budget as b :  a plan for the coordination of resources and expenditures <develop a budget for her company> i.e., TWO sides.

Our resources are our batteries (our wall safe) and alternator, shore charger, solar (our paycheck) and the expenditures are our amp-hour needs -- per hour, day, or week.  Defining a "budget" as only our wall safe (i.e., an energy-use spreadsheet) is only half the story.   SB,SC but it's not MB,MC.

"Budgeting," is akin to solving a few simultaneous equations:

I have $$ income and $$$$$ expenses -- that's a deficit. BUT can I cover that additional $$$ by working some $$ overtime or a 2nd $ job?  Or a better $$$$$ job, but which pays no overtime.  What about combining a different job with some lower-pay, but continuous guaranteed income?  All variables.

So goes it with spending energy (amp-hours,) and putting them back in (by shore, alternator, solar, and genset charging.)
 
So the equations to solve are: spending AHs during the day and evening while not charging, etc. -- versus the "income side" of replenishing those AHs while motoring, or shore charging, or w/ solar while sailing or on a hook, or w/ a genset., and what combination of those are in your wheelhouse. 

But not all of these are equal, so let's forget solar and a genset to keep it simple for now.

variables:
Maybe I expect  be on a shore charger only 2 days out of 2 weeks. 
Or I'll be in an anchorage (or slip w/o power) and won't run the engine for 8 hours to replenish what I could have done with my shore charger. 
How long will I be motoring and charging? -- two hours only, getting out of and then into the next marina/anchorage? 
How long will I have available to recharge if not motoring? -- sail for 2 hours and have 10 hours to run the engine whenever I care to?  Or can I only charge for 1 hour per day? 

Sure, there's practical limits and bounds to above, and everybody's situation is somewhat different.  But it doesn't matter available charge time defines what size alt you need.

Let's just say that I'll use 100 AHs per day, and so I know, per previous posts, what bank size I need.  BUT, I can charge for only "X" hrs a day.  THAT determines what alternator output I need to replenish those 100 AHs.  To say that my energy budget and bank size, doesn't affect my alt size, is like saying that I can rely on only my 51 amp alternator, no matter what my cruising situation is.  The three "sides" are interrelated.  I may never have to upsize from my 51a, external sense (and 8-awg cable.)  Or I might need to install a 120 amp/serpentine belt (and 1-awg cable.)

So, you calculate how many AHs you'll spend -- really, that's the easiest part -- and of course it's an estimate, but can be very close depending on how diligent you are.

The hard(er), but not impossible side, is to determine how you will recharge, and over what time period -- then that determines what alt you need to accomplish that. 

Consider -- an admittedly out-of-the-box example, only to demonstrate a point.   

I use 100 AH each day, BUT I'll recharge only every other day.  Oh, so my bank account suddenly has to be 200, instead of 100?  And my alt may also have to be larger if the recharge time is limited.  This is ONLY to make my point that the three are interrelated variables, not stand-alone constants.

My diagram below shows this, in a VERY simplistic way. 

Assume the AHs used is set (constant, one day's energy use.) For the budget to work (i.e., be in equilibrium.) the relationship between charge time and charge capacity (and so minimum cable size) are variable and depend on one another.  Increase one, decrease the other, and vice-versa.

kk

Harry,

Can I clarify my reply TO YOU right below, since it seemed to cause some confusion.....?

Ken, In response to your earlier question..I currently am tied up and shore charging each evening..but plan on more extensive cruising (1-2 weeks) in the future
Harry

Harry so as Stu alludes to, what will your recharge needs be then, so you don't have to re-do wiring again.  When you say 2-week cruise, that means 2- weeks away from a charger?  So you're needing to recharge only with batts?  Or w/ solar also?  I would start with a power budget and that will help define what alt you may want to upgrade to for cruising.  Then we can plan on that for the new cable -- otherwise you may have to re-do this again.

A simple sketch of what you're doing and what you're replacing might help.  How many feet is the cable run?

below not to meant be curt, harry -- simply brief....
1) I don't have a C34 - don't know the run length (I didn't measure it when I chartered) -- so I asked you.

2) There's sometimes a propensity to "over do", "just because", add "bells/whistles", because "they did it."  I like to know exactly what and why I'm doing something -- does it meet or exceed my needs (and budget.)  A wire upgrade is a one-time cost, but there's still no need to overspend for what I absolutely need.

3) I don't know whether you'll cruise for 2 weeks, relying on the engine to recharge.  "Cruising" isn't well-defined -- could be "transient-ing" and @ a different slip every night ........ or on a hook for 2 weeks.  BIG difference in recharging/alt needs.  So a budget will tell how you'll spend your energy (negative cash flow) and how quickly you'll need to replenish the batts (checking account,) because recharging is in chunks (like a paycheck, not a constant positive cash flow offsetting each purchase.) 

4) An ulterior motive is that everyone *should* know more about their vessels and systems (especially electrical.)  You seemed as interested in learning about alts, cabling, as you were to "just give me an answer."   Looking at all of one's electrical uses, recharging, etc. is a a way to learn and understand more about everything 12v.  It can open eyes about usage (waste?) where one may not otherwise realize it -- e.g., should I buy all LEDs?  Not if I daysail and am on a charger every night, but maybe so if i will be on a hook for 2 weeks.  Maybe one never uses the fridge -- I know several "purist" sailors who don't.  I'm a "I like to prove it to myself" guy, rather than take things on their face --- it *helps* keeps my brain working.  If you're not so inclined, then that exercise may not be applicable.  To each his/her own.

5) I believe it's NEVER a waste to revisit something, yes something may be tried and true, not changed for 5 years, but I yahnevah know what tidbit I might pick up that could get one out of a jam in an emergency or possibly what lightbulb turns on upstairs when I work thru things myself.  Quoting a very knowledgeable forum member who has posted similar recommendations herein more than 25 different times, as recently as last month, it's just math and "An "energy budget" is the first place to start," and I thought possibly, per reasons in 1,2,3,4 above, that approach might also be useful to you (if you have the time.)  To each his/her own.

Hope that helps yah some, Harry!

Ken

[J_Sail]

Thanks Jeremy.   

The controversy I meant was when you said "Lots of discussion between 250 or 300 A."

If there's no reason to "undersize" a fuse/breaker then what's the benefit or reason to "oversize" it (taking into account nuisance)? I suppose I'd prefer to "err" on the undersize, than oversize.

I'd have to see, with my own eyes, or documented, that a 60-amp fuse would blow exactly the same as a 300-amp fuse, given an intermittent or "leakage" short.  I ain't buying that at face value.  BUT even if true, there's no reason to oversize it, other than to just "do it."

kk

I believe it was Jon who said "Lots of discussion between 250 or 300 A." not me. I'm not sure what those discussions were.

Regarding sizing based on ampacity rather than predicted load - again, it's hard to always know what the actual worst case load will be, so I wouldn't call sizing a fuse to the wire's ampacity oversizing. The purpose of the over-current protection is simply to protect the wire against causing a fire. If you can do that, why go smaller when you can't be certain about load changes and resultant nuisance trips? In your particular case you may be able to determine a worst case load, and as the boat's owner, ensure that it doesn't change. The industry guidelines, though, are to ensure continued performance across ownership changes etc.

Regarding the proposed scenarios, you are trying to solve a hypothetical problem that's beyond the purpose of the fuse. I know you love to go against the established industry guidelines, but even though they are not always right they do embody many thousands of man-years of collective effort and experience. Again, on your own boat you may be able to constrain some of the variables, but it's wise not to assume everyone else can.

[KWKloeber]

Jeremy,

MEA CULPA. it was Jon.  Apologies, I'd like to see that discussion also.

NO, I prefer "standards" -- it's easier and (usually) proven.  The exceptions are when a standard doesn't apply to specific situations, I fall back to common sense approach. 

Again, what I am saying (which may be getting buried in more esoteric explanations) refers to only ONE specific situation -- a current limited 51-amp alternator, not "under sizing" everything we wire up on our boats.

Where does one find current/blow time ratings for different overcurrent devices?

kk

[Jon W]

I was referring to the discussions in my 19 page electrical upgrade post from a year ago. There were also e-mails and phone calls. Sizes from 150 to 300 were discussed for battery fusing.

[KWKloeber]

I was referring to the discussions in my 19 page electrical upgrade post from a year ago. There were also e-mails and phone calls. Sizes from 150 to 300 were discussed for battery fusing.

OK, thanks Jon.  Just as a thumbnail, don't need the full monty, what was the final decision of 250a based on, or alternately, why was 300a jettisoned?

ken

[Jon W]

Nothing is simple on this subject. Most discussed was cable size used and whether 90 or 105 deg insulation, derating for engine compartment temperature, and using 150% margin, and mainesail's rule of thumb to use 250A fuse at the battery as minimum. Graphs, external links, and lots of text in the thread.

[Noah]

Mine is 300, but what's 50 amps among friends?