Why 90 Amps for ACR breaker?

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JeffJuneau
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Why 90 Amps for ACR breaker?

Post by JeffJuneau »

Hi, I read through some of the strings here that mention ACRs, without coming across the exact answer to a question I have about the correct breaker size for my ACR and purpose. I have a Blue Seas 120 SI ACR that is probably from the original 26M install in 2009 - to battery 1 & 2 positive, and one lead to battery 1 negative. My battery charger had to be replaced, so I am not sure yet if the ACR is working. Last fall one of the Mac group members recommended that I add 90 amp breakers inline between the ACR and both battery's. I checked around the Blue Seas site, but wasn't able to find a simple explanation of what these breakers are actually protecting? Is it the electronics in the ACR?? Also, if my Suz DF70 alternator puts out 27 amps max, why is 90 amps the right number? Or is the protection needed when a battery is starting the engine? Sorry if the answer to this question should be obvious to me. My new ProMarine battery charger is protected with (4) 15 amp fuses on each of four leads, but I noticed they are rated at 250 volts. So, what is the correct voltage for the 90 amp fuses in the ACR? Finally, what is the best breaker assembly to use for this application? Who should I order it from and what exactly should I ask for?
Many thanks for any insight/advice on all these related questions. I am working though a long list of repairs and maintenance, and don't know what I would do without the amazing knowledge from this forum. Jeff
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Re: Why 90 Amps for ACR breaker?

Post by Starscream »

It was probably me that recommended the 90A fuse.

I have an Etec 90 that puts out a ton of charging amps. I sized the fuse based on the Blue Seas recommendations, see the link below. Also, if you check the installation manual for the ACR, there is a table that specifies "below 60 amps" the fuse size is "75-90".

https://www.bluesea.com/resources/578
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Re: Why 90 Amps for ACR breaker?

Post by Jimmyt »

JeffJuneau wrote: Fri Jun 11, 2021 1:58 am Hi, I read through some of the strings here that mention ACRs, without coming across the exact answer to a question I have about the correct breaker size for my ACR and purpose. I have a Blue Seas 120 SI ACR that is probably from the original 26M install in 2009 - to battery 1 & 2 positive, and one lead to battery 1 negative. My battery charger had to be replaced, so I am not sure yet if the ACR is working. Last fall one of the Mac group members recommended that I add 90 amp breakers inline between the ACR and both battery's. I checked around the Blue Seas site, but wasn't able to find a simple explanation of what these breakers are actually protecting? Is it the electronics in the ACR?? Also, if my Suz DF70 alternator puts out 27 amps max, why is 90 amps the right number? Or is the protection needed when a battery is starting the engine? Sorry if the answer to this question should be obvious to me. My new ProMarine battery charger is protected with (4) 15 amp fuses on each of four leads, but I noticed they are rated at 250 volts. So, what is the correct voltage for the 90 amp fuses in the ACR? Finally, what is the best breaker assembly to use for this application? Who should I order it from and what exactly should I ask for?
Many thanks for any insight/advice on all these related questions. I am working though a long list of repairs and maintenance, and don't know what I would do without the amazing knowledge from this forum. Jeffk
Starscream beat me to the draw... :D
See this info for the recommended connection schematic and wire/fuse selection. https://www.bluesea.com/resources/578

The fuses/breakers are there to protect the wire. The fuse/breaker should interrupt the current before the wire gets hot enough to be damaged. It looks like 60 amps is the minimum circuit that they recommend installing and a 75amp fuse/breaker is the minimum size recommended for that size wire (#6).

So, what size wire do you have on the ACR to the batteries? You should install fuses/breakers that will protect the installed wiring. If your wiring isn't #6 or larger, I'd install wiring according to the installation instructions.

Fuse current rating of 15 amps means it will limit the wire current to 15amps. The voltage rating means you can install that fuse safely in a circuit of 250 volts max. Any voltage below that is fine. Circuit breakers for that application are going to be $70-$120, so I'd stick with fuses.

Since blue seas recommends placing the fuse as close to the batteries as possible, I might use terminal fuses.

The way it's wired, and the fact that it states the device has start isolation, tells me that starting current will not be carried through the ACR.

Let's see what the rest of the gang has to say... :)
Last edited by Jimmyt on Fri Jun 11, 2021 2:31 pm, edited 1 time in total.
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Re: Why 90 Amps for ACR breaker?

Post by Be Free »

The short answer to the original question is that 90 amps is the appropriate size protection for the recommended wires for the ACR you purchased. An ACR that is designed to carry a different load would recommend a different wire and a different protection device. I know Blue Seas goes larger and maybe a little smaller as well (around 60 or 65 amps IIRC).

The start isolation on the Blue Seas ACR is to disconnect the non-starting bank from the starting bank during cranking so you do not get a voltage drop on the isolated bank during starting ex: your chart plotter is connected to the house bank and it turns off at 11.5 volts. Your battery bank briefly drops below 11.5 volts while cranking and your chart plotter turns off.

I don't think that any of us have alternators that can put out enough amperage to worry about (if you do, I envy you) but there are a couple of specific conditions that can keep the battery banks connected through the ACR while starting the engine.

The ACR will only connect the banks when the voltage is above 13V for a minimum time so the only time your ACR should see the high amperage starting current would be if one of the batteries was above 13V. This would normally only happen while you are charging the batteries or just after disconnecting the charge source (surface charge not yet down below 13V). An example would be you are using solar to charge your house bank and the ACR has connected your start battery to allow it to be charged as well or you briefly stop your engine and then restart it (the start battery would probably still be above 13V and the ACR still connected to the house bank). In either of those cases the positive cables between the ACR and the batteries would see a brief high amperage condition (the negative wire on the ACR only sees the current needed to operate the ACR itself). That is what the recommended fuses are protecting: the positive cables between the ACR and the batteries.

When sizing the cables you need to be aware of what your high amperage loads are going to be. For most of us that is going to be the outboard starter but it could be the alternator or perhaps a large inverter or some other large power sucking device. The cables connecting your batteries need to be able to support this load and those cables need to be fused appropriately. Don't forget to build in a safety factor in your design.

If you use the same size cables connecting the ACR to the batteries that you have connecting the batteries to the high amperage load (usually the starter) AND you have those cables (the ones not connected to the ACR) properly fused, then (and only then) do you not need additional fuses for the ACR wires.

tl:dr The wires on the ACR should be sized for maximum current expected (plus a safety factor). This will be either the alternator or the starter in most cases. There should be fuses protecting these wires. The size of the fuse depends on the size of the wire.
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Re: Why 90 Amps for ACR breaker?

Post by OverEasy »

Hi

The Battery and ARC Fuses and Circuit Breakers are primarily system designed to be in place to prevent catastrophic short damage to wiring which could result in fire. Often there are long leads which run in/on/under structure. These leads are subject to potential shorting due to a variety of causes. The closer (within reason) one can isolate the source of electrical power the better.

DOT regulations for Cars and trucks have fuse links or circuit breakers at or near the batteries.
FAA regulations for aircraft have fuse links or circuit breakers at or near the batteries.
Commercial and residential have main breakers where the power comes into use and distribution.

The same generally applies to individual use circuits which is why Fuses and Circuit Breakers are located at the positive bus bar where those circuits tap off from. Now that is not to say one can’t deliberately undersized a circuit fuse or breaker to better protect and end use device, but the general purpose is to prevent current overload of the wires delivering power.

Note: Devices can have their own function protective fuses and circuit breakers to avoid damage internal to the device.

Boat electrical systems need the same protections. Unfortunately many DIY projects don’t know or understand or appreciate the risks or consequences of electrical shorts.

Below is a portion of the National Electrical Code (NEC) wire size vs. max amperage capability.
Ambient operational environment temperatures also come into play.
Fuses and/or circuit breaker current ratings should always well below the wire current rating.

Hope this helps.

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Re: Why 90 Amps for ACR breaker?

Post by OverEasy »

What’s the difference between fuses and circuit breakers?

There are two phrases “Pay Me Now Or Pay Me Later” & “Penny Wise But Pound Foolish” in product design. Often times what appears to be cheaper doesn’t actually work out that way…especially in a crisis or functionally or over time. Many times a perceived negligible cost difference decision changes what could have been a minor inconvenience into a major (avoidable) crisis.

The decision as to whether to use a static fuse or a resettable circuit breaker is often times more than just a perceived initial component cost. One should evaluate the overall integration cost of a decision into a design (what additional items are required to make it work and function) as well as the operational consequences.

Fuses tend to have an lower initially less expensive cost (but one needs remember to include the cost of the holder as well as the actual fuse) and are bonehead simple primarily consisting of an accurately sized conductor that will melt at a specified current flow. They tend to be physically robust and typically are not impacted by general vibration loads. The drawback it they are one shot items and a specific size shape geometry replacement is needed to restore power once the cause of the overload has been mitigated. Fuses also require some form of fuse holder to safely retain the fuse element. Blade and barrel cartridge Fuses typically don’t incorporate readily identifiable means to determine condition of the fuse. One typically needs to remove the fuse to visually inspect. So generally one would want to have appropriate spares available nearby with you. This requires prepurchase of the relevant fuses, accessible storage, access as well as means to effect the replacement.

Circuit breakers tend to have an initially more expensive cost and have somewhat more complicated internal structures. Most circuit breakers work on a spring/catch thermal deflection basis that interrupts the current path by physically moving contacts out of contact. They tend to be generally physically robust but could be subject to vibrational environmental loads depending upon specific design. Many circuit breaker designs are available either as an all inclusive mount variety or can be mounted in a housing structure. Most circuit breakers have positive physical trip indications and are easily manually resettable for hundreds (if not thousands) of cycles without any tools. No additional spares are required. Physical and visual access to the circuit breaker is needed to verify and reset after the overload has been mitigated.

Which one to use, fuse or circuit breaker, isn’t mainly a matter of choice and convenience for the majority of applications.

At the low end of consequences might be a cabin light circuit. If a fuse blows and you don’t have a spare fuse you could use a flashlight 🔦 (if you brought one).

At a little bit higher level of consequence might be the VHF radio circuit. If a fuse blows and you don’t have a spare for most of the time and situations one could do without but should the conditions be that of an emergency 🆘 that could be a different story…even if one had a spare fuse would there be time to change it?🤔

On a similar level one could be dealing with a navigation GPS/Chart-Plotter/Depth-Sounder. On your favorite familiar lake it might not be an issue but if there are currents and changeable sand bars and /or an unfamiliar harbor with rocks/reefs/obstructions it might be a different story where the time finding and installing a fuse (possibly in the dark) can change outcomes.

If the situation is that of an engine start circuit it might be that you don’t get to use your engine that day or if the wind has died out and the tide is flowing out and you find yourself staring at an approaching low bridge or reef or……. Then the time spent fumbling about changing a fuse (if one is available) can change out comes.

Take the above circumstances and now replace a one-shot fuse with a readily accessible simple push to reset circuit breaker (with nearly an infinite number of retries).

Having extensively used both options in literally hundreds of products, vehicles and installations used in a wide variety of applications with a wide array of end users I’ve found that the relatively small overall operational cost difference between fuses and circuit breakers tends to favor the use of circuit breakers in the overwhelming majority of cases nowadays.

As always, this is something that is your choice to do on your vessel ⛵️.

Hope this helps.

Regards,
Over Easy

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JeffJuneau
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Re: Why 90 Amps for ACR breaker?

Post by JeffJuneau »

Thank you Over Easy, Be Free, Starscream, and JimmyT for your very complete answers. Sill digesting this to make sure I understand your information and see if I have more questions. But I think you guys covered everything that was adding to my confusion. I now understand that my ACR needs a fuse or breaker around 90 amps on each positive wire, and that the protection is needed to mostly protect the wires on the ACR. I am checking into my wire size, and will compare it to the amp requirement table to make sure. I have a better understanding some of the tradeoffs between use of a breaker vs a fuse (re: Over Easy), but assume that installing a breaker means finding a location to put another breaker panel. I don't think that would allow the breaker to be close to the terminals of the two batteries in my situation, which I understand is desirable. Also, I don't think that blowing a fuse on one of the positive legs to the ACR would be that big a problem when starting or running off the outboard, as long as charging to one of the batteries was still happening. I should still be able to start the engine with one battery if the other was too discharged. I might be wrong about that, and am happy to be corrected. I will probably pick up a couple of inline fuse housings with 90 amp fuses and that match my wiring gauge in the Western Auto marine section, unless someone can suggest a better supplier and product. :) Many thanks again, Jeff
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Re: Why 90 Amps for ACR breaker?

Post by Starscream »

JeffJuneau wrote: Sat Jun 12, 2021 1:36 am Thank you Over Easy, Be Free, Starscream, and JimmyT for your very complete answers. Sill digesting this to make sure I understand your information and see if I have more questions. But I think you guys covered everything that was adding to my confusion. I now understand that my ACR needs a fuse or breaker around 90 amps on each positive wire, and that the protection is needed to mostly protect the wires on the ACR. I am checking into my wire size, and will compare it to the amp requirement table to make sure. I have a better understanding some of the tradeoffs between use of a breaker vs a fuse (re: Over Easy), but assume that installing a breaker means finding a location to put another breaker panel. I don't think that would allow the breaker to be close to the terminals of the two batteries in my situation, which I understand is desirable. Also, I don't think that blowing a fuse on one of the positive legs to the ACR would be that big a problem when starting or running off the outboard, as long as charging to one of the batteries was still happening. I should still be able to start the engine with one battery if the other was too discharged. I might be wrong about that, and am happy to be corrected. I will probably pick up a couple of inline fuse housings with 90 amp fuses and that match my wiring gauge in the Western Auto marine section, unless someone can suggest a better supplier and product. :) Many thanks again, Jeff
No new breaker panel required, just use two of these mounted near the ACR.

I also like the discussion on breakers vs. fuses, and use breakers whenever I can, for convenience. I used another one of these, but 30A, when installing the second switch-panel on the starboard side of boat.

The design rule that I follow is to have element in the circuit (wires, breakers, terminals, connectors, etc.) rated for at least 1.25 times the expected full-load amps (FLA) for that circuit (this is called Minimum Circuit Ampacity, or MCA). The breaker can be sized anywhere from a fraction over the full-load amps to 2.25 times the full load amps of the largest circuit load plus the sum of all the other loads on the circuit. The breaker size is called the maximum overcurrent protection, or MOCP. Sizing the breaker too close to the full-load amps can result in nuisance trips, and sizing the breaker above 2.25 times the full load amps doesn't give the desired level of circuit protection.

In general, for a given circuit:
Full Load Amps = FLA = steady state amp draw of the circuit with all loads running
Minimum Circuit Ampacity = MCA = 1.25 * FLA
Maximum overcurrent protection = MOCP = 2.25* largest load + sum(all other loads)


https://www.amazon.ca/Erayco-Circuit-Tr ... ljaz10cnVl
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Re: Why 90 Amps for ACR breaker?

Post by Jimmyt »

Good discussion.

If that breaker is any good, I would certainly prefer that over a fuse for roughly the same price - if the application allows breakers. I missed that source - may have been looking at name brands or different sources. The only issue would be mounting as close to the battery as possible. This is an issue if your ACR is not mounted very near both banks of batteries. Depending on where your batteries are located, a suitable location very close to each battery might not be available. That's where terminal fuses shine. You can't get much closer to the battery than mounted on the terminal.

A breaker or fuse is there to provide protection during a failure or other such event. You need to find the problem and fix it before resetting the breaker or replacing the fuse. Continually resetting the breaker after a trip, without fixing the problem, while convenient, can result in the very issue it's there to prevent - wire/equipment damage or fire. If everything is sized right and working properly, you should not have any nuisance breaker trips or blown fuses.

I do agree that a breaker, at the same price, installed in the proper location in the circuit, is more convenient and more cost effective than a fuse. :wink:

To the original post question; the instructions for installation, for that ACR, I referenced only mentioned fuses (and specific allowable fuse types) for the ACR system - not breakers. Has anyone seen breakers referenced in the installation instructions for that device? :o
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Re: Why 90 Amps for ACR breaker?

Post by Be Free »

Jeff:
If the breaker or fuse opens the circuit on the ACR wires it will not affect any of the devices on the boat. They are being fed from the wiring that you had before the ACR was installed. All that will be affected is the ability to connect the banks together through the ACR for charging. It is never (or not for very long) going to connect the batteries together while you are not charging one or both banks.

Jimmy:
I don't remember any mention of breakers in the ACR literature. It was always fuses. As long as the breaker is sized correctly I can't think of any reason why the two should not be interchangeable for this application.

The only thing that will complicate using a breaker vs. a fuse is following ABYC rules pertaining to the placement of fuses and breakers. (Sorry, I choked on my coffee while I was writing that. Our boats were wired with LAMP CORD!!!) I'm OK now.

The ABYC rules regarding the distance between the battery and the fuse/breaker is "a 7 inch maximum if the conductor is not housed in a sheath or enclosure in addition to the wire insulation, 40 inch maximum if the conductor is housed in a sheath or enclosure in addition to the wire insulation, 72 inch maximum if the conductor is connected directly to the battery and housed in a sheath or enclosure in addition to the wire insulation".

Without running the cables through a "sheath or enclosure" it may be difficult to mount the ACR and the breakers and stay withing the distance limits.

When I installed an ACR in my boat I used the same size wires for the ACR that I already had on the batteries (#6 battery cables from your friendly neighborhood auto store). That way I would not have to install/mount any additional devices in an area that is getting a little crowded after adding the fifth battery.
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JeffJuneau
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Re: Why 90 Amps for ACR breaker?

Post by JeffJuneau »

Thanks again everyone, I think I have sufficient information to protect the wiring for my ACR now. As JimmyT and Be Free noted, in general, it is going to be very difficult to find a place to mount two breakers in a Mac when it has to be 7 inches or less from the battery terminal. For the way I have arranged my 2 batteries that barely fit in the battery compartment, I can attest to the fact that there is simply no place to put breakers in that location within 7" of the terminal, partially because it is packed with batteries as well as a mass of wiring. As OverEasy noted, it can a bit of a cost/benefit analysis to assess when it is better to invest the effort/expense in a breaker. In this case, if the fuse trips, the system just defaults to the previous wiring. I am happy with that. The ACR was installed in the original Blue Water Yachts electrical install had no fuses at all, and probably worked fine. So the benefits of using the breaker over a fuse in this specific application seems to me to be zero, while the cost in my time to install two breakers is substantial, especially since there is no place to put one. Appreciate all the assistance again. Jeff
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Re: Why 90 Amps for ACR breaker?

Post by OverEasy »

Hi JeffJuneau

Just a couple of options found on a quick look-see via Amazon

Image

Image

Image

Image

There are probably more out there and at a variety of Amperage limits but these are a couple of options to show that the Circuit Breakers are not very expensive.

What I’ve done in times past where the distance was an issue has been basically three options:

A)Take a short strip of heavy gauge copper, drill two holes in it to accommodate the battery terminal screw post and the circuit breaker post and mount it that way at the battery post.

B) If possible use one of the mount holes (it may need to be enlarged depending upon the battery terminal) then make a short “U” shaped jumper to reach the circuit breaker terminal and clamp the circuit breaker to the battery terminal with one end of the jumper.

C) (My preferred option when I can plan the space) Make a small strip panel to mount the circuit breakers within the distance to terminal constraints then run the short jumpers to the battery terminals.

In the case of the Mac26X that can be the outer surface of the battery compartment.

A gain, this is your vessel to do what you feel comfortable with.
If going with fuses I would recommend getting at least one spare from the get-go and keeping it nearby or mounted in the battery compartment within sight so you know where it is should you ever need to replace it.

Happy Sails! :)

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Re: Why 90 Amps for ACR breaker?

Post by JeffJuneau »

Hi OverEasy, Well, I stand corrected. You have pointed out some solutions for directly connecting a breaker to the + terminal for my ACR. I wish you or some of the other electrical specialists on this group were my next-door-neighbor. You could help me troubleshoot all my boat issues and install solutions. Since I have some other wires that need to go to the positive terminal of the house battery mostly, could I install both a 90/100 amp terminal fuse like one you show in the first of your series, and a second direct terminal fuse-connected bank of four to handle those lower amperage lines? Like a Blue Seas ST-Blade Battery Terminal Mount Fuse Block kit? If so, would they both be attached to the threaded terminal or would the 4-fuse block kit be added on top of the 90/100 amp fuse block terminal? While were at it I might as well ask what gauge of wire is appropriate for the 90/100 amp terminal fuse for the ACR. I now has a 10 AWG wire on it. Would 4 AWG be too much? Thanks, Jeff
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