What is a battery AIC safety overcurrent protection device?

[MerMike]

In the July 2025 update to ABYC standard E-11, there is now a requirement for a battery AIC safety overcurrent protection device in addition to the main overcurrent protection device.

11.10.1.4.1 For parallel battery banks with a capacity exceeding 500 Ah, a battery AIC safety overcurrent protection device shall be installed between each battery in the battery bank and the main overcurrent protection device (see DIAGRAM 18 ) that meets the requirements of E-11.10.1.3.

NOTE: This battery AIC safety overcurrent protection device is installed in addition to the main overcurrent protection device.


Can anyone shed some light on what an AIC safety overcurrent protection device is? Why it's needed? What devices meet the requirements to provide adequate AIC protection for a bank with 2 460AH batteries?

Please, please, please: I do not care about your opinion of ABYC or the rationality of this standard. I would love to hear from anyone with practical experience who has found a way to meet this standard. If you wish to blather off topic, please start your own thread.

[CharlieJ]

I would love to hear from anyone with practical experience who has found a way to meet this standard.

Install an OCPD with adequate AIC in each whip between the battery and the collecting bus. Until the Epoch 460V2 came to market, this was adequate. The Standard is already out of date.

E-11 was changed for two reasons:
1.Battery manufacturers, on the whole, do not provide the available short circuit rating for their batteries.
2. The existing guidance for determining the AIC of the first OCPD after the battery was sorely lacking.

To solve these issues a very conservative rule of thumb to estimate the ASCC for a lithium battery is 5kA/100Ahr. For your installation, the 460Ah battery has an ASCC of 23,000A. A Class T has an AIC of 20kA.

Since the ASCC for one of the batteries in your exceeds a Class T AIC. The Epoch 460Ahr V2 has a 400A internal fuse.

I personally would prefer to have a fuse open that I can easily replace. Therefore, even though it is marginally <required AIC, I recommend that each whip have a 200A Class T for a total at the collecting bus of 400A available. The “Main” OCPD's in the diagram would be sized <400A and <ampacity of the wiring to the loads.

Note that if one battery goes off line than there will only be 200A available at the collecting bus and operational reductions for the loads will have to be considered.

[wholybee]

The basic explanation of what AIC is:
Say you have a fuse of 15A. And some bad thing happens and you have 30A of current. The fuse blows, and everything is protected.
Now, imagine some really huge current going through the fuse, like 1000A. The fuse blows, but the current is so strong that it arcs across the gap and isn't stopped. The fuse didn't do it's job. That is the essence of what the AIC rating is. It's the MAX current a fuse can stop, and anything above that might not be stopped even if the fuse blows.
It is important with regards to large battery banks, especially Lithium, because they are capable of many thousands of Amps. So, you need to source a fuse that will have a high enough AIC for your installation. Having one fuse per battery instead of one fuse for the whole bank allows for using fuses with a lower AIC rating. As Charlie notes, the commonly used class-T is has an AIC of 20,000 amps and might not be enough for even a single 480Ah battery. I would check with Epoch if they have an ASCC of the battery instead of assuming one. Perhaps a class T will work. Also, the internal fuse if you have the model that includes it would likely have high enough AIC, so check with them on that also.

[Montanan]

Snipets from the internet.

What is AIC?

Ampere Interrupt Current

The AIC (Available Fault Current) rating is a measure of the short-circuit current that a circuit can handle safely. It is used in electrical systems to ensure that overcurrent protective devices (OCPD) are rated to handle the available fault current without causing damage. The OPCD (Overcurrent Protective Device) is the device that protects electrical circuits from overcurrent conditions, such as short circuits. In a series rated system, the AIC rating of the upstream device must be equal to or greater than the available fault current, while the downstream device's rating is less than the actual available fault current



 One of the key items to consider when selecting a transfer switch is fault current capabilities
 Overcurrent Protection Devices (OCPD) clear faults
• Fuses
• Circuit breakers
 OCPDs have an Ampere Interrupting Capacity (AIC) rating
 The AIC rating is the maximum available fault current that an OCPD will safely clear when a fault is applied at the load side of the OPCD
 Transfer switches are not rated to clear faults and therefore don't have an AIC rating

[MerMike]

Thanks, all. I understand AIC, but the purpose of the additional safety OCPD escapes me. I'm installing a pair of the Epoch 460's and the highest AIC rating of a Class T fuse is 20,000A.

@CharlieJ, I don't understand what will be accomplished with a lower rated fuse in the whip. The AIC is the same. My understanding of AIC is that it is the current that the fuse will safely interrupt. Lowering the amperage of the fuse would not change that. What am I missing?

Since the Epochs already have an internal fuse, does that meet the requirement of an individual fuse between the battery and collector bus?

Thanks,

Mike

[MV Intrigue]

Just one thing to note about the epoch 460 V2 with the internal 400 amp SET fuse. That fuse was spec'ed by the engineers for the safety of the battery for a complete internal failure. Essentially to extinguish any current path in an internal catastrophe, including BMS failure. It was employed internally without any consideration for ABYC or any other standard.

Even though this is the case, there is obviously some overlap between what the Epoch engineers did and what ABYC calls out for. Unfortunately, the fuse they selected was of a proprietary nature and despite efforts to get details the fuse maker has not responded.

In light of the above, even though I suspect that the 460 SET fuse likely has sufficient AIC, it cannot be confirmed, so ABYC standards should still be employed.

MerMike, Charlie J's assumption on the 200amp fuse likely is due to selected wire size of the feeder wire. Even though AIC hasnt changed, the basic current rating of the class T in the whip will be for wire size. Maybe you are using all 4/0 all the way?

[MerMike]

Originally Posted by MV Intrigue

MerMike, Charlie J's assumption on the 200amp fuse likely is due to selected wire size of the feeder wire. Even though AIC hasnt changed, the basic current rating of the class T in the whip will be for wire size. Maybe you are using all 4/0 all the way?

Yes, 4/0 will be employed from the batteries to the main distribution bus.

[s/v Jedi]

With 4/0 cabling everywhere you can simply fuse each battery with it’s dedicated fuse and skip the main fuse. As long as the fuses used have an appropriate AIC rating for the battery.

Apart from class-T you also have MRBF fuses that eliminate one cable for each fuse, but their AIC is a bit lower so it doesn’t work for every battery.

Also, when a battery has a BMS that interrupts the current when it exceeds a certain value, this does not replace the fuse, so you must still install the fuse.

I also recommend to install an isolation switch for each battery. If there’s only two batteries, a 4-position switch is good as well.

Finally, I also recommend a Victron Smart battery monitor for each battery, with its shunt connected in the negative line of each battery like the fuse is in the positive. Then, on the other side of the shunts, connect them to the “battery side terminal” of a Smart Shunt.

Now you can get detailed data on each battery with a programmable relay for each battery, plus you have data of the whole bank.

[MerMike]

Originally Posted by s/v Jedi

With 4/0 cabling everywhere you can simply fuse each battery with it’s dedicated fuse and skip the main fuse. As long as the fuses used have an appropriate AIC rating for the battery.

The Epoch 460's don't have a published short circuit current, so I have to use the the ABYC estimate of 5KA per 100 AH of storage. The requires a fuse with an AIC rating of 23,000A, which is beyond that provided by a Class T.

I'm vaguely aware of a fuse that has a higher AIC rating and could be used in this type of installation, but I don't recall what it is.

[CharlieJ]

@MerMike #9

The Epoch 460's don't have a published short circuit current, so I have to use the the ABYC estimate of 5KA per 100 AH of storage. The requires a fuse with an AIC rating of 23,000A, which is beyond that provided by a Class T.

As I discussed in my original response. I also stated that E13 is already out of date. However, Class T is the best we can do in the USA at the moment. BTW, the Class T AIC rating is at 125VDC and AIC increases as voltage decreases, we just don’t know how much as the manufacturer has not performed the necessary testing. So we do have some headroom.

@CharlieJ, I don't understand what will be accomplished with a lower rated fuse in the whip. The AIC is the same. My understanding of AIC is that it is the current that the fuse will safely interrupt. Lowering the amperage of the fuse would not change that. What am I missing?

As I originally said, if there is a fault to ground that is going to open an OCPD, I would prefer to prefer it opens one that I can replace so I want the external (in the case of the Epoch 460) fuse to open first.

I typically have installed 2/0 or 4/0 whips, hence my 200A recommendation for a whip.

@Jedi #8:

With 4/0 cabling everywhere you can simply fuse each battery with it’s dedicated fuse and skip the main fuse. As long as the fuses used have an appropriate AIC rating for the battery.

Not and be compliant with E13. The sticking point became the “Main” OCPD requirement as defined in E11. The AIC safety overcurrent protection device concept was developed to work around this requirement without reducing safety.

An installation is going to cost some thousand’s of dollars. Class T fuses are about $50 as are their fuse holders. Wrap the whips with fire retardant split loom and they can be 72” long before the fuse. It really has not been a problem on the systems we have installed.

[neflier]

How does this new July 2025 requirement affect existing installations? I have a single 300A class T fuse protecting two 420 Ah LFP house batteries in parallel. This was ABYC compliant when installed earlier this year. Does this new requirement mean prior installations must be rewired to be considered ABYC compliant (insurance concerns), or would they be grandfathered as meeting ABYC requirements at the time of installation?

[CharlieJ]

If your install was compliant before the revised E13 was promulgated there is no requirement to back fit. However, it may be worthwhile to take a hard look at the revisions and upgrade anyway.

[MerMike]

Originally Posted by CharlieJ

@Jedi #8:
Not and be compliant with E13. The sticking point became the “Main” OCPD requirement as defined in E11. The AIC safety overcurrent protection device concept was developed to work around this requirement without reducing safety.

@CharlieJ, if the whips are 4/0, and each battery is fused with a 400A Class T, is an additional "Main" OCP still required? What purpose does it serve?

Thanks,

Mike

[PippaB]

Originally Posted by MerMike

@CharlieJ, if the whips are 4/0, and each battery is fused with a 400A Class T, is an additional "Main" OCP still required? What purpose does it serve?

Thanks,

Mike

What size is the red wire leaving your main bus? It is not protected with OCPD until you reach 800A in the sketch provided.

[s/v Jedi]

Originally Posted by PippaB

What size is the red wire leaving your main bus? It is not protected with OCPD until you reach 800A in the sketch provided.

Well of course these 4/0 cables must go straight to the main busbars. Everything that connects to that busbar must be fused again.

For the negative, 4/0 cable from each battery to a shunt for each (a dedicated battery monitor) and then continue and both terminate on the same battery-side terminal of a SmartShunt. Then from the load side of the SmartShunt, -two- 4/0 cables to the main negative busbar.

These main busbars must be capable of 800A. BlueSea has popular 1,000A busbars for this.

This doesn’t work for more than two batteries unless you use smaller fuses, which would be the wrong thing to do. The right thing would be to increase system voltage.