Circuit Protection, Interrupt Ability, and ABYC

[sailingharry]

I have a question for you electrical experts, especially those who know ABYC. I've been reading and learning and getting ready to clean up some wiring, and I would like to follow ABYC, or at least know when I'm breaking the rules!

I'm trying to learn about, and select, Over Current Devices, or OCD's. My battery bank will consist of two T-105's and a Grp 27. Trojan doesn't list the CCA's of the T-105's, but the similar Lifeline batteries are rated at around 1000 CCA. The start battery hasn't been selected yet, but will run around 600-800 CCA. Net is over 1500 CCA when the 1-2-Both switch is in Both. ABYC requires that, for any bank over 1100 CCA, that my main breakers must have an interrupt capacity of 5000A, and the branch breakers must have 2500A.

I understand the idea of the ones directly connected to the unlimited power of the batteries, and there's lots of merchandise appropriate to the job.

My concern has to do with smaller circuits downstream of the main battery breakers. My interpretation of the ABYC regulations is that a "main" is connected to the battery directly, and a "branch" is anything else. That, to me, means that if I have a breaker panel that selects "Nav Station," it is a branch. If at the nav station, I have a sub distribution panel that feeds the radio and the GPS, both of those circuits are also branch circuits. Now for the problem -- 2500A is beyond the scope of virtually any fuse! The plastic ATC "blade" style fuse has an interrupt capacity of 1000A and the glass tubular type is even lower.

Am I reading this right? Am I to understand that all those little in-line fuses hanging out the back of a piece of equipment are illegal? And, the nice little in-line fuse that Xantrex and Balmar includes in the sense lines that they want me to connect directly to the battery are doubly illegal?

I'm having troubles convincing myself I'm not missing something. My voltage regulator or battery monitor (don't have that yet, but it's on the list) should be connected very close to the battery terminal -- the instructions and gossip say it should be directly to the battery -- yet the vendors include a neat little in-line fuse and make no mention of the fact that there is NO circumstance where that is legal (fuses directly connected to batteries below 650CCA require 1500A interrupt). My reading is that a LinkLite requires 3 circuit breakers to meet ABYC, which certainly isn't what is shown on the installation instructions.

Someone, please correct me!

Harry

[johnar]

I would send what you wrote to The ABB Group - Automation and Power Technologies and they will be happy to help you.

In the past they have help me alot.

Best of luck,

Dutch

[hellosailor]

"ABYC requires that, for any bank over 1100 CCA, that my main breakers must have an interrupt capacity of 5000A, and the branch breakers must have 2500A."
It gets worse, Harry. Typical DC breakers will arc, fuse, and fail at about 3300A load. They are not really intended for service over 3000A loads, so they should not be used on batteries. You really want a FUSE at each battery, unless you spend $100++ for the 5000A rated breakers.

"Now for the problem -- 2500A is beyond the scope of virtually any fuse!" Correct, but at what time would you want your battery bank to supply 2500A without blowing a fuse? Unless you are on a steamship, there's damn little that would pull over 100A combined loads, and that's easily fused.

The battery primary fuse should be rated to blow before the capacity limit of the battery cables. And if those cables are able to carry more than the maximum power that your breaker panel (all devices, plus the starter) can carry, you would reduce the size of the main fuse to perhaps 25-50% more than the maximum power that your systems will ever consume in normal operation. You want that fuse to be the weakest link. You want every fuse (or breaker) to be the weakest link, so that it blows/trips BEFORE the wire it is connected to an overheat.

Or, so it blows when any device pulls more power than it was rated for.

For instance, you may have a wire run for your cabin lights, typically one runs up portside and the other up starboard and they're on separate breakers. And you might run 8AWG cable all the way in order to prevent voltage loss in the wire, even though you've only got 4 10-watt bulbs on each side. 40 watts (3-4 amps) of load, with a cable that can carry 23 amps of current all day and night. American Wire Gauge, AWG Cable Size Description for Copper Wire Cable is one of many calculators and charts you can find on the web for this.

So you've got a 23-amp capable cable but only a 4 amp load. What breaker do you use? A 5 or 7.5A breaker, NOT a 25A or 30A. You want that breaker/fuse to trip as quickly as possible if there is an overload, so you size it 25-50% above the largest or safest (whichever is smaller) load that should run on that circuit.

Those piggyback fuses are there to make sure the one device they are connected to, is protected even when it is attached to a larger circuit run. When they just hang in space, I hate them. They need to be secured someplace, preferably right behind what they protect. We lost a guage on a boat once and found out the breaker was good, the circuit was good...the guys who built the boat had left that extra glass fuse hanging about three feet into a cavity behind a bulkhead. I only found it because I had climbed in looking for loose wires behind the guage. I'd rather add a properly mounted fuse holder (the kind with a twist-off cap that installs in one drilled hole) adjacent to whatever the gizmo was, or else move it back to the main panel.

You can gang fuses, within reason, or run one for each device AND one for each shared circuit run. Either way works, but once fuses get like roaches (all over the damned place and you can't keep track of them or the spares) that's just another way to make life too complicated.

[sailingharry]

Hellosailor, you've given helpful responses to me in the past, and I was hoping you'd chime in here as well -- Thanks.

You've given me real pause for thought on the main breakers. My intention has changed slightly since we last talked about OCD's, when I was going to have a dedicated house battery and a dedicated start battery (and use them that way), and then the house battery would have a battery mounted fuse. I'm keeping the 1-both-2 switch, but using the batteries as "the battery" and "spare." Hence, both are cranking batteries. My current intention is to keep "battery cable" wires from battery to switch to starter, all unfused. Then, drop off the switch with three heavy (#6, or maybe #4) with some very short leads (maybe not the ABYC 7" limit, but certainly under 12") to three 100A breakers, say Blue Seas series 187 (5000A interrupt, $50 each), and then connect everything but the starter to those. You think that they should be fuses instead?

I hadn't really thought about setting fuses by the LOAD, not the WIRE, but it makes sense. I have noticed that, across the board, wiring design is generally limited by voltage drop than by ABYC ratings, and so your point is quite appropriate.

My real problem though is the smaller OCD's. The 5, 10, 20A units. Or, perhaps, my interpretation of what is a "branch" circuit in the literal reading of ABYC. That 2500A interrupt limit that I read for ABYC is required for any branch breaker, and those small sub-distribution blocks (or individual panel mouont fuses by the protected units) are under ABYC "branch circuits" and required to meet that limit (and none do).

A second question is a cross of the two -- the suitability of those inline fuses that Balmar, Xantrex, and others provide that are intended to mount directly to the battery terminal on items such as battery chargers, battery monitors, and alternator sense wires. Seems that they are unsuitable as "branch circuits" -- and they are functioning as "mains!" Most people that I've talked to have done just that, and it may be the norm, but it is an error that is encouraged by the very companies that write ABYC.

Thanks again for your input.

Harry

[bloodhunter]

Harry,
I think you are misreading the meaning of 'mains'. In most wiring codes the mains circuit is defined as that which brings the power from the power source to the boards or panels from which other circuits branch. A line from your bilge pump directly to the battery is not considered a main because while it is connecting directly to the power source, it is not bringing pwoer to the subsidiary (branching) circuits. Naturally there would be a breaker or fuse on the hot line betweenm the battery and the bilge pump but this line would be considred a branching line for purposes of the code. It is the same for lines that run direct for the power sources to meters or sensors. They are directly connected to the power source but they are not distributing power to the branches so they are not considered mains.

[Kefaa]

Originally Posted by hellosailor

"When they just hang in space, I hate them. They need to be secured someplace, preferably right behind what they protect."

Is there an advantage to putting them behind the units themselves rather than a central board? I am currently hunting down all the fuses that have been left hidden in cupboards, behind shelves, or anywhere else they seemed to "naturally" occur on the vendor supplied wire.

My plan was to move them to a panel (much like my house and RV). I was trying to avoid tearing apart the console to get to the fuse behind the GPS(for example).

[hellosailor]

Kefaa-
If you're using, let's say a 2.5A fuse to protect some gizmo, you could certainly just put a 2.5A fuse/breaker in the main panel and run the wire all the way from there. And that fuse will protect both very nicely. OTOH, many folks eventually run out of room. Either no slots on the panel, or the panel is in some "protected" spot where you can't expand it. So rather than have odd fuses in inline fuse holders (where the mfrs often have put them simply to ensure there IS a fuse somewhere) I'd change that to a fuse next to the gizmo--if I couldn't add it to the master panel, or for whatever reason didn't want to. But, I'd rather put it in the master panel, yes.
In a time when mfrs are doing everything they can to cut the last penny out of costs, it is somewhat perplexing that they still often waste money on an inline fuse holder that only "should" be cut out and tossed in favor of a master panel. Go figure.

Harry-
I confess, I don't have a set of ABYC specs although I have some recent drafts they made available squirrelled away somewhere. I suspect bloodhunter is right about mistaking "mains", as best I know "mains" always refers to the main power line coming in from a power company's street power, etc. Dunno how they define it on a ship or boat.
Keeping one battery as a spare is a mixed blessing. It still needs to be charged at least once a month, or it will take damage from just sitting there. And, if your batteries are combined, you can get better life from them because you will be discharging the combined bank less than you would discharge just one battery. (i.e. one battery to 50%, dual bank is still 75% for the same power drawn out.) Battery life goes up fast when the discharge rate isn't as great. If it is 1000 cycles at 50%, it may be 1500 at 70%, each vendor claims different numbers but the same trend for most.
The 2500 amp for branches is probably why conventional breakers can take 3000+. I'd guess that's the intent, and then the only question is putting primary fuses right at the battery post--or within however many inches of it.
Consider if you will, sometime when your hands are busy and there's salt water spraying around and someone manages to fling some anchor chain or a wrench across the back of the battery switch and a ground. Ooops, 4000A dead short and no fuse to protect it and now you've got a problem. Even a good size car battery can put out 3000+ amps into a dead short from a wrench. The USN used to have a web page on one of their sites showing the "traumatic amputation" of a finger when some seaman let his wedding ring short out a battery post. It gives you a new respect for "its only twelve volts".
Glass cartridge fuses have their drawbacks. Small wires sometimes come unsoldered or break from vibration over time, and glass itself can shatter when the fuse blows or you remove it. Some of the new fuse types are more robust, actually can respond and blow faster, and cause less shrapnel if they do. But then again...some of them are ten times the price of large glass fuses, too. And of course, it doesn't make any sense to install a fuse unless you also buy a couple of spares.
If you're looking at parts from top-name vendors like Blue Seas, the best thing is often to give them a call or email, and ask them about suitability and conflicts. That's exactly how I found out conventional breakers can fuse and fail when used on batteries. I hate fuses, but I'm not ready to spend $100 on a fuse!

[CharlieJ]

SailingHarry-I have to commend you for delving into the AIC arena. This is usually an area where the Owners' eyes roll back in their heads!

The short answer to your dilemma is to put a Cooper-Bussman Marine Rated Battery Fuse (MRBF), repackaged by Blue Sea and sold at West. The BS pn for the fuse holders are 5191 and 2151. Here is the link: Terminal Fuses (MRBF - Marine Rated Battery Fuse) - Blue Sea Systems These fuses have a rating from 30 to 300 amps (for conductors from AWG 12 to 00 inside engine spaces) and an AIC of 10,000 amps at 14 volts.

Now for the design philosophy part. I design systems so that the conductor is never stressed above its temperature corrected ampacity and the OCPD that I specify will open before this point is reached. I also design to ensure that the OCPD is never (or rarely ever) in a system where it has to pass >80% of its rated load. And finally, especially with large circuit breakers, the prudent designer must adhere to the specifications regarding the size of the conductor being connected to the circuit breaker. The reason this spec is provided is that breakers (and switches) are rated with the conductor providing a heat sink.

Regarding the AGC (glass) fuses on instrumentation conductors and their connection to the battery. They have an AIC of 1,000 amps, but they are protecting AWG 16 or smaller conductor that normally carries zero current as the conductors are only for voltage sensing. Short the downstream ends of these conductors and the fuse will open, and in all probability, remain intact. Contrast that with a heavy load device that sees a bolted fault and the reasoning for an AIC is clear.

FYI: The Cooper-Bussman Series 187 are relatively inexpensive circuit breakers with an AIC of 5,000 amps at 12 volts: 187-Series Thermal Circuit Breaker Panel Mount - Blue Sea Systems

And finally, you are correct in your assessment of main and branch circuits. From the 2009 ABYC Standards:

TABLE IV - A – CIRCUIT BREAKER MINIMUM AMPERE INTERRUPTING CAPACITY FOR SYSTEMS UNDER 50 VOLTS

*NOTE: The main circuit breaker(s) shall be considered to be the first breaker(s) in a circuit connected in series with the battery. All subsequent breakers, including sub-main breakers, connected in series with a main circuit breaker shall be considered to be “branch circuit breakers.”

Hope this helps.

[bloodhunter]

Kefaa,
I have multiple instruments in a pod at the helm and running wires for separate circuits for each one would have been a nightmare. Instead there is one wire that runs from my breaker panel to the pod and all of the instruments connect to it.
The breaker for this circuit is big enough to allow all the instruments to run without blowing. However, each instrument is individually protected by it's own in-line fuse. I keep spare fuses in a little waterproof box inside the pod. This is one case where the in-line fuses fill a definite need

[hellosailor]

Charlie, are the 187 series breakers new, or have they just been reduced in price? Still not cheap, $83 + shipping + tax from West, under $50 from Defender--maybe--but Defender's web site is offline this afternoon.

AGC...Better to use the larger AGU size, which I'm told stop at 80A but I thought I'd seen in 100A ratings. AGC is still surprisingly common, I guess some vampires never get staked. Once I switched to ATC (? the plastic ones) well, there's still legacy stuff that still needs the AGCs.

Bussman's ANL/ANE fuses, Littelfuse's MegaBolt, are both readily available from car stereo shops and should work well. And at low cost. Hopefully one doesn't blow the battery primary fuse terribly often so the added convenience and expense of a breaker should be mainly expense. And I've never liked thermal breakers, as opposed to magnetic ones. They can wear out in one lifetime, easier to change a fuse that's clearly blown.

[CharlieJ]

Charlie, are the 187 series breakers new, or have they just been reduced in price? Still not cheap, $83 + shipping + tax from West, under $50 from Defender--maybe--but Defender's web site is offline this afternoon.

They have been on the market for at least five years. ANL's are a good choice as they are relatively inexpensive and have an AIC of 5,000 A. They are also ignition protected.

[sailingharry]

CharlieJ,

Thanks for the reply -- you seem to know the material well! I'm an engineer, so the stuff makes a lot of sense. My degree is mechanical, but they cover a lot of EE in there as well.

I agree with your design philosphy, and I think it is right on. The bit about sizing for the ampacity is, from what I can tell, not really an issue, since every circuit I've looked at the numbers on is limited by voltage drop, not ampacity.

The MRBF is truly an outstanding product, at least from a value and convenience perspective. I'm in the middle of a major re-wire (actually, no, I'm in the middle of a new battery box, and THEN a major re-wire) of my primary distribution system -- battery cables through first distribution point -- and if I decide to put fuses in my battery cables (which are also starter cables), then I'll probably be using the MRBF's. Otherwise, it will be a 100A 187 right off the battery switch to run all my lesser loads.

But, my real question isn't "what makes sense" or "what most people do" but rather "what does ABYC require." I probably won't get to 100% ABYC, because it is very difficult to re-design a 30 year old boat to be perfect. For instance, that 187 I mentioned will probably be closer to 12 inches from the switch, not the required 7 -- but now we are picking nits and I don't think anybody will ever complain about that. Still, before I go violating a rule, I like to know what the rule really is.

A careful reading of 11.10.1 is actually confusing. It clearly requires that any branch breaker must meet the interrupt requirements. This means that if you have a nice 50A breaker feeding your house panel, that all the breakers in that panel must meet that 2500A interrupt capacity, which seems kind of nuts. However, and here is the interesting part -- FUSES have no such criteria spelled out. We know that fuses have lower interrupt capacity, as a general rule, so it can't be that the thought is "don't worry, when they open, they open" (it makes sense -- a fuse can't weld shut, although I gues it can vaporize and conduct like a fluorescent tube). If we can ignore the interrupt requirement for fuses (I don't think we can), then it is perfectly fine to run a 20A branch circuit over to your Nav Station, and then put a little 6 fuse ATC panel to feed each of your instruments, providing protection, convenience, and fault isolation. However, the decision to literally interpret the fuse section (11.10.1.6) as being silent on interrupt capacity means you can also ignore it on a main battery fuse -- not something I'd recommend! Conversely, if you believe ABYC meant to apply interrupt rules to fuses, then that ATC fuse panel is out.

I can't believe I'm the only one confused. Or perhaps I'm missing the words that make it clear. I do have the July 2008 standard here on my computer, so feel free to reference specific sections.

Harry

[hellosailor]

Harry-
"(it makes sense -- a fuse can't weld shut, although I gues it can vaporize and conduct like a fluorescent tube). "

But first, you ahve to pump in 50,000(?) volts to form a plasma in the fuse, the same way a fluorescent tube starts. If you've got 50,000 volts going through your 12v panel, you've got bigger problems than ABYC compliance.

And you know, somewhere out there? I'll bet there's someone just clever enough to find a way to make that happen to the poor "next owner" who buys his boat.

[sailingharry]

HelloSailor,

You've hit on the subject I keep pondering. I really don't understand the failure mode of a fuse with regard to AIC. I can certainly see it in a breaker, but not a fuse.

If you read the entire E-11, there appears to be NO mention of interrupt capacity ANYWHERE, except for section "11.10.1.5 Circuit Breakers", with no similar discussion in the fuse section. The table, IV-A, is titled "TABLE IV - A – CIRCUIT BREAKER MINIMUM AMPERE INTERRUPTING CAPACITY."

As bizarre as it sounds, it actually seems clear that ABYC doesn't have any opinion on interrupt capacity for fuses. This means that the main battery fuse does not need to be sized in accordance with the table. A SEA fuse or even a Maxi if they are large enough for your purposes are perfectly acceptable fuses for the protection on the circuit going to your house panel, and even the glass AGC fuse on the alternator "sense" line connected straight to the battery meets ABYC. And even the "auto sound shop" ANL fuses, which are cheap and mighty attractive but include no specs on AIC, are fully appropriate for use (they probably are the exact same item Blue Sea sells anyway).

Am I right? This last paragraph seems to be the direct answer to my initial question.

Harry

[GordMay]

Interrupting Capacity is the maximum power in the arc that a circuit breaker or fuse can successfully interrupt without restrike or violent failure. Fuses can literaly "blow up" when trying to interrupt currents above their AIC rating.