AIC and house bank fuse danger

[Mac2]

AIC: Ampere Interrupting Capacity.
I had no idea what this meant until recently. I am in the process of hooking up solar to my 1,200 ah house bank. The 6 8D batteries were replaced with LifePo4 batteries. The main class T fuse for the previous batteries was 300 amps. No need to replace right? Wrong! AIC is the most important rating, not amps. Seems really counterintuitive when considering the amp ratings. My thinking was 300 amp fuse should “trip” sooner than 1000 amp fuse. I didn’t understand how a short circuit would affect a fuse. Chat GBT definition of AIC: “The maximum short-circuit current a fuse can safely interrupt without exploding, arcing, or failing”. I didn’t realize my 300 amp fuse could vaporize or melt in a closed position and cause a fire. My system needs a 1000 amp class T fuse. There is a couple of threads going on about ABYC lithium installations. Not to mention names, but this was missed on a insurance company request of a survey on my boat.

 

[rslifkin]

AIC: Ampere Interrupting Capacity.
I had no idea what this meant until recently. I am in the process of hooking up solar to my 1,200 ah house bank. The 6 8D batteries were replaced with LifePo4 batteries. The main class T fuse for the previous batteries was 300 amps. No need to replace right? Wrong! AIC is the most important rating, not amps. Seems really counterintuitive when considering the amp ratings. My thinking was 300 amp fuse should “trip” sooner than 1000 amp fuse. I didn’t understand how a short circuit would affect a fuse. Chat GBT definition of AIC: “The maximum short-circuit current a fuse can safely interrupt without exploding, arcing, or failing”. I didn’t realize my 300 amp fuse could vaporize or melt in a closed position and cause a fire. My system needs a 1000 amp class T fuse. There is a couple of threads going on about ABYC lithium installations. Not to mention names, but this was missed on a insurance company request of a survey on my boat.

The 300A fuse rating isn't the issue. And a 1000A class T won't help you any, as it'll be too large to protect your wiring. The issue is that a typical class T fuse is rated for 20,000 amps AIC at 125V DC. No lower voltage ratings are provided (although AIC would likely be higher at a lower voltage). So per the current ABYC guidance, a class T fuse is only appropriate to cover 400ah worth of LFP.

In the case of Bussman Class T fuses, the 600A and smaller models are rated at 20kA DC for AIC, the 700A - 1200A models are rated at 100kA AIC. So the larger fuses have more AIC, but even 4/0 cable is only good for 445A without any bundling or engine room de-rates.

So a big bank like yours should have a class T fuse at each battery of suitable size to protect the wiring from that battery. Then an additional fuse to protect the wiring from the main battery bank bus bar (a class T at minimum, or a different type with higher AIC such as an NH00 or EF3 fuse).

 

[DDW]

You can buy a Mersen fuse that will have an appropriate AIC rating and fit in your existing holder. 100K AIC.

But I thought someone had discovered the weasel words in E-11 which let normal class T fuses off the hook.

 

[rslifkin]

You can buy a Mersen fuse that will have an appropriate AIC rating and fit in your existing holder. 100K AIC.

But I thought someone had discovered the weasel words in E-11 which let normal class T fuses off the hook.

That's a good find, looks like the Mersen A3T fuses are 50kA DC AIC, the A6T models are 100kA DC AIC. By ABYC those are good to protect 1000AH and 2000AH of LFP respectively. If the batteries are on a hard bus bar then one fuse for the bank is probably fine. If they're wired to a bus bar for the bank then I'd still want a fuse per battery plus the main bank fuse I think.

 

[twistedtree]

Well summarized by rslifkin. The latest version of E-11 addresses the need to break battery banks down into smaller chunks and fuse them individually to meet AIC requirements.

 

[DDW]

E-11 specifies one fuse per battery over a certain size, and also one for the bank. One wonders when the multiplicity of fuses itself becomes a fire hazard. In then end I decided to use one Mersen for the bank (600A as it has to run the thruster) and one Bussman/BlueSeas 250A for each 300AH battery. I wanted to avoid a situation where one of the three batteries might go offline while trying to use the thruster causing fuses to blow like a zipper from one end to the other.

 

[Mac2]

The 300A fuse rating isn't the issue. And a 1000A class T won't help you any, as it'll be too large to protect your wiring. The issue is that a typical class T fuse is rated for 20,000 amps AIC at 125V DC. No lower voltage ratings are provided (although AIC would likely be higher at a lower voltage). So per the current ABYC guidance, a class T fuse is only appropriate to cover 400ah worth of LFP.

In the case of Bussman Class T fuses, the 600A and smaller models are rated at 20kA DC for AIC, the 700A - 1200A models are rated at 100kA AIC. So the larger fuses have more AIC, but even 4/0 cable is only good for 445A without any bundling or engine room de-rates.

So a big bank like yours should have a class T fuse at each battery of suitable size to protect the wiring from that battery. Then an additional fuse to protect the wiring from the main battery bank bus bar (a class T at minimum, or a different type with higher AIC such as an NH00 or EF3 fuse).

Each battery has a 200 amp breaker built in (front panel along with a internal bms rated for 200 amp continuous duty). The batteries are hooked to a common bus bar. A class T fuse holder is attached to the 4/0 cable about 12 inches from the battery. This is getting more confusing reading the above posts. I will look at the Mersen fuses. I appreciate the information.

 

[Mac2]

Well summarized by rslifkin. The latest version of E-11 addresses the need to break battery banks down into smaller chunks and fuse them individually to meet AIC requirements.

Would you consider the breakers on each battery the same lingo as individually fused batteries?

 

[SteveK]

Each battery has a 200 amp breaker built in (front panel along with a internal bms rated for 200 amp continuous duty). The batteries are hooked to a common bus bar. A class T fuse holder is attached to the 4/0 cable about 12 inches from the battery. This is getting more confusing reading the above posts. I will look at the Mersen fuses. I appreciate the information.

So far it does not matter what is built in.
You need one class T fuse for every 400Ah of batteries, the size of the fuse ie 300A is to protect the wire under normal shorts. Each class T is rated for 20KA.
The weasel words @DDW mentioned are that class T so far is rated 20KA @ 125V, so it is probably greater at 12V, but not proven.

 

[Diep]

Think of it this way:
The fuse rating is how much the fuse will allow to pass. The AIC is how much the fuse is able to stop.

You need a fuse rating sufficiently low to protect your cabling. You need an AIC sufficiently high to stop whatever might hit it.

The new E-11 requirements call for a "battery AIC safety overcurrent protection device". This would be provided on each battery in a parallel battery bank of 500 Ah or more. So you need a device, which could be a breaker or a fuse. The AIC rating of the device needs to be 5,000A per 100Ah BATTERY capacity. So you can figure it for whatever your individual batteries are. See if there is any info on the device or in the specs or anything identifying it that you can use to research it.

Then you need a "main overcurrent protection device" on the combined bank. This needs to be sized to protect your cabling downstream. It also needs to have an AIC based on the BANK size (5,000 A per 100 Ah). This is where it gets monstrous if you have a big bank. It seems like the Mersen A6 can get you covered up to a 2000 Ah bank. Not sure what the strategy would be after that. I was able to use a Mersen A3 since my bank was under 1,000 Ah.

 

[Mac2]

So far it does not matter what is built in.
You need one class T fuse for every 400Ah of batteries, the size of the fuse ie 300A is to protect the wire under normal shorts. Each class T is rated for 20KA.
The weasel words @DDW mentioned are that class T so far is rated 20KA @ 125V, so it is probably greater at 12V, but not proven.

This would be 3 class T fuses for my setup. This means 3 cables going to my inverter. Not sure the stud is big enough-maybe a bus bar attached directly to the stud. I’ve never seen, or heard of a setup like this.

 

[SteveK]

Think of it this way:
The fuse rating is how much the fuse will allow to pass. The AIC is how much the fuse is able to stop.

You need a fuse rating sufficiently low to protect your cabling. You need an AIC sufficiently high to stop whatever might hit it.

The new E-11 requirements call for a "battery AIC safety overcurrent protection device". This would be provided on each battery in a parallel battery bank of 500 Ah or more. So you need a device, which could be a breaker or a fuse. The AIC rating of the device needs to be 5,000A per 100Ah BATTERY capacity. So you can figure it for whatever your individual batteries are. See if there is any info on the device or in the specs or anything identifying it that you can use to research it.

Then you need a "main overcurrent protection device" on the combined bank. This needs to be sized to protect your cabling downstream. It also needs to have an AIC based on the BANK size (5,000 A per 100 Ah). This is where it gets monstrous if you have a big bank. It seems like the Mersen A6 can get you covered up to a 2000 Ah bank. Not sure what the strategy would be after that. I was able to use a Mersen A3 since my bank was under 1,000 Ah.

It is the one fuse AIC that adds up the entire bank that I find confusing. So why is that not enough for the bank, why the individual fuses which already protect the AIC bank parts.

 

[SteveK]

This would be 3 class T fuses for my setup. This means 3 cables going to my inverter. Not sure the stud is big enough-maybe a bus bar attached directly to the stud. I’ve never seen, or heard of a setup like this.

This is what is new 2025. But it does not end there as they also want the three cables to go to one fuse and then to the inverter. Post 12

 

[Diep]

It is the one fuse AIC that adds up the entire bank that I find confusing. So why is that not enough for the bank, why the individual fuses which already protect the AIC bank parts.

I don't have an answer for this - I am only stating what I have read and understand the rules to be.

I have never heard an explanation of why there is an AIC requirement for the bank when each battery is protected as described. I don't see how thousands of amps can be delivered to the bus if each of the sources (the batteries) would be interrupted at a few hundred A each and each of those fuses will not fail to interrupt the short-circuit capacity of the battery.

I would love to hear the logic behind the rule. Either it is mostly overkill or I am not understanding something. The latter is certainly possible and if so, I would appreciate the correction.

 

[Diep]

This would be 3 class T fuses for my setup. This means 3 cables going to my inverter. Not sure the stud is big enough-maybe a bus bar attached directly to the stud. I’ve never seen, or heard of a setup like this.

Mac - what is your battery Ah and how many do you have? Are they combined into a single bank going to the inverter?

 

[rslifkin]

If there's a spec available for the breakers built into the battery modules then you could determine if the internal breakers count as sufficient protection.

Personally, my design for the system would be each battery through a Class T going to a common hot bus for the bank. Then from that bus, a Class T to battery switch to inverter. And if there's a DC system fed from these batteries, then another Class T and another battery switch from the battery hot bus to feed the DC system.

If you have 3 battery modules wired to the hot bus, each with a BMS capable of (and wiring sized for) putting out 200A, you'd use a 200A Class T on each battery. However, with 3 modules that would mean you could pull 600A from the battery hot bus without blowing the fuses. So that's where the second layer of fuses comes in to protect the cables feeding from the battery hot bus to other equipment.

An 8KW inverter at 48V would likely only need a fuse in the 200 - 300A range, although the wiring might support more depending on the length of the run and resulting size for appropriate voltage drop. But it's unlikely that wiring could support 600A, and the inverter manufacturer also likely specs a maximum fuse size. And you wouldn't want to just downsize the battery fuses to accommodate, as that increases the risk of a cascading failure if one battery drops offline for some reason.

 

[Mac2]

Mac - what is your battery Ah and how many do you have? Are they combined into a single bank going to the inverter?

I have 6 200 amp 24 volt batteries in parallel. Each battery goes to a common bus. From there, 4/0 cable to class T fuse holder , then to on/off switch then to 4kw inverter.

 

[Mac2]

So far this is looking like the best solution for me without reconstructing my setup.

 

[twistedtree]

Would you consider the breakers on each battery the same lingo as individually fused batteries?

I'm not sure what breaker or fuse you have that's part of the battery. Can you elaborate? For ABYC, the BMS disconnect function does not take the place of fusing requirements. I have seen a number of batteries that include an MRBF fuse in the battery, but that doesn't meet the AIC requirements in many (most) cases. You need to look specifically at the fuse's AIC rating at whatever voltage the battery operates. If the battery includes a Class T fuse, then it likely meets the fusing AIC requirments. Also note that if you parallel smaller batteries, you need to fuse them as a group to meet their combined AIC requirements. So if a single 400Ah battery needs it's own fuse, then two 200Ah batteries in parallel need fuse dedicated to those two batteries.

 

[Frosty]

SteveK:
It is the one fuse AIC that adds up the entire bank that I find confusing. So why is that not enough for the bank, why the individual fuses which already protect the AIC bank parts.

I wasn't sure about this at first either (made a looong thread). But in a nutshell it's because for example:

Two batteries and one fuse has enough AIC and an amp rating to protect your downstream system. Why the need to fuse each battery? What I found out is that is protecting against one battery discharging into the other battery. With just say one Class T fuse downstream of the bank, there is no protection against that scenario.

Hence above a certain bank/battery size, ABYC specs a fuse on each battery (then you would probably have one more downstream to protect your wire, but that would not need the AIC rating if the two individual ones have that covered).

Victron calls for a fuse on each battery once you have more than one in a bank regardless of size of battery, so a bit stricter of a guideline than ABYC.

 

[DDW]

I remain skeptical that the many cables and many connections that this adds reduces fire risk on average. Each cable and each connection is itself a fire risk.

I believe there are many more fires started by partial shorts, or high resistance (loose) connections, than by a fuse's AIC rating being exceeded. I've seen several cases of the former myself, and never a case of the latter. By fusing each battery in a bank, we are doubling the number of cables, and tripling the number of connections, each of which is a point of failure. A four battery bank has 16 potential points of failure in the positive lead to the busbar. If each is fused there are 40 points of failure.

I'd love to see the statistical analysis done by the ABYC to justify the spec.

 

[SteveK]

SteveK:
It is the one fuse AIC that adds up the entire bank that I find confusing. So why is that not enough for the bank, why the individual fuses which already protect the AIC bank parts.

I wasn't sure about this at first either (made a looong thread). But in a nutshell it's because for example:

Two batteries and one fuse has enough AIC and an amp rating to protect your downstream system. Why the need to fuse each battery? What I found out is that is protecting against one battery discharging into the other battery. With just say one Class T fuse downstream of the bank, there is no protection against that scenario.

Hence above a certain bank/battery size, ABYC specs a fuse on each battery (then you would probably have one more downstream to protect your wire, but that would not need the AIC rating if the two individual ones have that covered).

Victron calls for a fuse on each battery once you have more than one in a bank regardless of size of battery, so a bit stricter of a guideline than ABYC.

Frosty, I think we are almost saying same thing. If every 400Ah of battery has a class T fuse then multiple 400Ah batteries are already protected from each other. If a class T fuse (wire rating A) 20KA is all that is needed to protect the wire, then each 400Ah battery has such a Class T fuse, how could multiple batteries, each individually protected add up their 20KA AIC into the wire, one or more fuses should have blown.

 

[Wdeertz]

You can buy a Mersen fuse that will have an appropriate AIC rating and fit in your existing holder. 100K AIC.

But I thought someone had discovered the weasel words in E-11 which let normal class T fuses off the hook.

Note the Mersen fuses > 400A are a slightly bigger size so need a different fuse holder. Originally I had my house bank fused with a Mersen 400A fuse which was the standard T-fuse size. I upgraded to a Mersen 500A fuse and had to get a different fuse holder. Just a fyi if anyone is going above the 400A threshold.

 

[Frosty]

I remain skeptical that the many cables and many connections that this adds reduces fire risk on average. Each cable and each connection is itself a fire risk.

By fusing each battery in a bank, we are doubling the number of cables, and tripling the number of connections, each of which is a point of failure.

I hear you. What I don't know (and would like to know!), is how great is the risk of one battery shorting/discharging all its amps into another battery in the bank. I don't have a feel for whether that is something that "happens" or is more theoretical but no-one has seen it happen in captivity.

I find it interesting that Victron has been showing (in diagrams) each battery being fused for ages; but up until recently they were fine with using Mega fuses for the whole bank (very low AIC - like 2,000 or 2,500 amps IIRC). I surmise the individual fuses is because one BMS takes care of the whole bank. Nothing explains to me why they didn't seem to know about AIC until like last year (when the Class T Power In came out). Many of their schematics still show the Megas only.

Right now I have one battery. But I think I'm going to add a second (matching) battery this winter. Technically, I believe I'm below the ABYC standard of needing a fuse on each battery because they give a pass if the bank is under 500ah total (mine would be 400ah). The single Class T fuse I have now would cover the AIC of the new pair handily.

That's not to say I want to do the bare minimum. Also, Victron always shows one fuse per battery..... so I have not decided yet.

On the one hand, if I don't add fuses I'll have one single Class T, so fewest connections. It has the AIC capacity for the bank, and it also protects my downstream cabling.

If I go to a fuse per battery, I'll have two Class T fuses, and then a following ANL (or whatever) to protect my downstream cabling. Feels so cluttery and full of connections. (Or I could put two MRBF's on the incoming post of the Class T fuse, but they are marginal for my battery size at juuuuuust the AIC rating necessary at precisely 12 volts, plus.... I just don't love them.)

Not sure yet which way to go. Either way will satisfy ABYC though (and, you know, be "professionally" done by the one person who cares most about the boat).

 

[Delta Riverat]

It is extremely difficult to start and maintain an arc at 12 VDC.

Go watch some utubes on "battery welding" and they start off with 3 in series for 36 volts. And even then it ain't easy.

 

[SteveK]

I hear you. What I don't know (and would like to know!), is how great is the risk of one battery shorting/discharging all its amps into another battery in the bank. I don't have a feel for whether that is something that "happens" or is more theoretical but no-one has seen it happen in captivity.

I find it interesting that Victron has been showing (in diagrams) each battery being fused for ages; but up until recently they were fine with using Mega fuses for the whole bank (very low AIC - like 2,000 or 2,500 amps IIRC). I surmise the individual fuses is because one BMS takes care of the whole bank. Nothing explains to me why they didn't seem to know about AIC until like last year (when the Class T Power In came out). Many of their schematics still show the Megas only.

Right now I have one battery. But I think I'm going to add a second (matching) battery this winter. Technically, I believe I'm below the ABYC standard of needing a fuse on each battery because they give a pass if the bank is under 500ah total (mine would be 400ah). The single Class T fuse I have now would cover the AIC of the new pair handily.

That's not to say I want to do the bare minimum. Also, Victron always shows one fuse per battery..... so I have not decided yet.

On the one hand, if I don't add fuses I'll have one single Class T, so fewest connections. It has the AIC capacity for the bank, and it also protects my downstream cabling.

If I go to a fuse per battery, I'll have two Class T fuses, and then a following ANL (or whatever) to protect my downstream cabling. Feels so cluttery and full of connections. (Or I could put two MRBF's on the incoming post of the Class T fuse, but they are marginal for my battery size at juuuuuust the AIC rating necessary at precisely 12 volts, plus.... I just don't love them.)

Not sure yet which way to go. Either way will satisfy ABYC though (and, you know, be "professionally" done by the one person who cares most about the boat).

Frosty, you have said this before, please explain why you believe that you can place 500Ah on one Class T fuse.

 

[ksanders]

Is there anything written that you need to protect each battery?

Or, do you need to protect each battery in a large bank to meet the AIC rating of 5000 AIC per 100AH with existing fuse technology?

Then you run into the issue of a pretty typical 900AH battery bank, compromised of three 300AH batteries in parallel

Is there a commercially available fuse available that would meet the 5000 AIC per 100AH for a 900 AH bank? That would require 45000 AIC as a main bus protection fuse. Is that even available?

I cannot imagine creating a standard that technology today does not offer a compliance solution

 

[DDW]

The new E-11 does not say anything about protecting the battery, but does require a fuse per battery above a certain size. The assumption is it is to protect batteries from batteries. E-11 doesn't say (apparently - it is secret). I am skeptical that it provides any protection from any real hazard. I'm skeptical that it provides any protection even from an imaginary hazard. I've got 250A fuses. You would need a very peculiar fault to have one battery discharge into another, with both BMS failing simultaneously, and maintain enough current to blow one of two fuses in series (around 400A for 250A fuses). If the fault was only 300A there would still be a lot of heat, but no protection.

I don't have insurance against a meteor strike either, but I don't lose sleep over it.

As mentioned above, there are class T fuses with an AIC of 50K and 100K and 200K DC. My bank of 900AH requires and AIC of 45K, and a Mersen 600A provides this at 50K. The Bussman that are more common do not. Mersen are not as widely distributed but can easily be found (Amazon for example).

 

[twistedtree]

SteveK:
It is the one fuse AIC that adds up the entire bank that I find confusing. So why is that not enough for the bank, why the individual fuses which already protect the AIC bank parts.

I wasn't sure about this at first either (made a looong thread). But in a nutshell it's because for example:

Two batteries and one fuse has enough AIC and an amp rating to protect your downstream system. Why the need to fuse each battery? What I found out is that is protecting against one battery discharging into the other battery. With just say one Class T fuse downstream of the bank, there is no protection against that scenario.

Hence above a certain bank/battery size, ABYC specs a fuse on each battery (then you would probably have one more downstream to protect your wire, but that would not need the AIC rating if the two individual ones have that covered).

Victron calls for a fuse on each battery once you have more than one in a bank regardless of size of battery, so a bit stricter of a guideline than ABYC.

Assuming I'm following correctly.....

If you have two 400Ah batteries, each with it's own Class T fuse, then connect to a common bus bar, then I think you are done and have met the AIC requirements for the batteries and can now connect fused loads to the bus bar. I don't think you need to bring the two fused batteries together to another single fuse before using the power.

One of the many challenges with writing E-11 (or any standard) is that whoever wrote the particular section had one or more example implementations in mind. To make matters worse, different people debating the topic often have different implementations in mind, sometimes unstated. So everyone is interpreting the words with their needs in mind, but often not thinking about other possible implementations where the wording might not make sense, or might be confusing.

If this new language is unclear or seems inapplicable or incorrect in certain circumstances, I would suggest submitting a common an E-11 the next time it's up for review.

 

[twistedtree]

Is there anything written that you need to protect each battery?

Or, do you need to protect each battery in a large bank to meet the AIC rating of 5000 AIC per 100AH with existing fuse technology?

Then you run into the issue of a pretty typical 900AH battery bank, compromised of three 300AH batteries in parallel

Is there a commercially available fuse available that would meet the 5000 AIC per 100AH for a 900 AH bank? That would require 45000 AIC as a main bus protection fuse. Is that even available?

I cannot imagine creating a standard that technology today does not offer a compliance solution

It's meant to protect against a short down stream of the fuse. That could conceivably be a dead short inside a paralleled battery, but I don't think it really matters. A short is a short, and you want the fuse to blow.

The new language is meant to cover your exact example. Maybe there is an exotic single fuse, but it would be exotic. The practical solution is to fuse the bank in segments so that off the shelf parts can be used.