AFC/AIC

[michael king]

I have a residential generator install I'm working on. In the permitting process, the city architect asked about the AFC/AIC. First time this has been brought to my attention in a residential application. They are currently using the NEC 2017, and in 110.9 has the interrupting rating guidelines. House has 200-amp service, and I'm looking to install a 200-amp transfer switch. The city wants the AIC of the transfer switch to be equal or greater than the transformer from the utility company. Which raises my question. Wouldn't I want to interrupt the current at a lower value than the potential from the transformer? Or better yet, can someone please give me the simplified example of why the AIC needs to be equal/greater than the AFC? Transformer is 58k and transfer switch is 22k. Possibly overthinking this, but I'm trying to wrap my brain around this one. Any and all help is appreciated.

 

[jim dungar]

110.9 and 110.10 have been in the NEC for more than 50 years.

Available Fault Current needs to be calculated at the point where your device is. The length an size of the service conductors has a tremendous effect and so should be included in your calculations.

Every device which closes onto a fault and then tries to open, like a breaker, needs to be have an Amps Interrupting Capacity based on the fault current at its line side terminal, per 110.9.
Every device that does not interrupt a fault, like a manual switch, needs to be able to tolerate the current flowing through it per 110.10.

Many service entrance ATS are built using circuit breaker and so need to meet 110.9.

 

[michael king]

110.9 and 110.10 have been in the NEC for more than 50 years.

Available Fault Current needs to be calculated at the point where your device is. The length an size of the service conductors has a tremendous effect and so should be included in your calculations.

Every device which closes onto a fault and then tries to open, like a breaker, needs to be have an Amps Interrupting Capacity based on the fault current at its line side terminal, per 110.9.
Every device that does not interrupt a fault, like a manual switch, needs to be able to tolerate the current flowing through it per 110.10.

Many service entrance ATS are built using circuit breaker and so need to meet 110.9.

So the AIC is a rating for that device to be able to open and close under a load? This is a new area of my electrical knowledge. Ratings and codes are what keep us safe, and our customers safe. The available fault current is the potential of the utility company? The ampere interrupting capacity is for the service devise we are using to disconnect the current under a fault? Don't we want to disconnect the power to the house as quick as we can in the event that a varmint chews into a transformer, or am I looking at this all wrong?

 

[tom baker]

What is calculated fault current at the line side of your service?
Where did the 58K come from, that seems high for residential, suggest you check with POCO

 

[michael king]

What is calculated fault current at the line side of your service?
Where did the 58K come from, that seems high for residential, suggest you check with POCO

Yes my utility planner gave me the calculated AFC value of 57,968 amps. Which he even stated is unusually large for a transformer but it handles a lot of houses.

 

[retirede]

Yes my utility planner gave me the calculated AFC value of 57,968 amps. Which he even stated is unusually large for a transformer but it handles a lot of houses.

It sounds like that number is at the transformer, the conductors between the transformer and your equipment will reduce that by quite a bit, depending on conductor size and length.

 

[suemarkp]

If you have a 25KVA or 50 KVA transformer feeding the house this wouldn't be an issue as the max available fault current from those is 5K to 10K amps. This must be a large transformer (150KVA or larger) to have that much available fault current. Most residential stuff is all rated at 10K AIC (Amp Interrupt Capability). Most newer main panels have a main rated around 24K AIC, and it will protect the smaller branch 10K breakers if they are the same brand (this is a series combination rating). Breaking things up and mixing brands messes up the series combination ratings and most manufacturers don't series rate with other brand breakers.

Your transfer switch needs to have an AIC rating equal to or greater than the available fault current. With that huge of a transformer, the city needs to be reasonable and allow the wires between the transformer and main disconnect to be included in the available fault current calculation. Finding residential main and transfer panels and breakers that can tolerate a 60KA fault current would be rather expensive and I doubt any of the houses on that transformer have such equipment. Hopefully, the conductors can drop it down into the 20KA range where you have a lot of choices at reasonable cost.

It may turn out that the existing main and branch breakers are limited to 10KA. I'm not sure whose problem that becomes if you install an appropriately rated transfer switch, as the existing old main and distribution breakers still need to be rated for the available fault current. Could be an expensive transfer switch install if you end up having to also replace their main/branch panel. Even worse if there is a subpanel with breakers of a different brand or type listed in allowed series/combination rating with the upstream main panel and there is still over 10K AFC at that sub panel.

 

[jim dungar]

So the AIC is a rating for that device to be able to open and close under a load?

No.
AIC is the amount of fault current the device can interrupt. The generic term Short Circuit Current Rating is used for devices that do not automatically open during a fault, like a manually controlled switch or a bus bar and lug termination.

 

[tom baker]

These terms are defined in Art 100 and suggest you review

 

[kwired]

So the AIC is a rating for that device to be able to open and close under a load? This is a new area of my electrical knowledge. Ratings and codes are what keep us safe, and our customers safe. The available fault current is the potential of the utility company? The ampere interrupting capacity is for the service devise we are using to disconnect the current under a fault? Don't we want to disconnect the power to the house as quick as we can in the event that a varmint chews into a transformer, or am I looking at this all wrong?

AIC - is the amount of current the device can safely interrupt per testing standards without physically destroying the device.

Available fault current is how much current can potentially flow during a fault condition. This value varies at different points in the circuit.
The highest amount will be at the source terminals, as you get further from the source you must factor in impedance of conductors and how much that will lessen what is available at the source.

So if you have available fault current of 21K amps, a device rated 10K is subject to possible destruction should it try to interrupt a fault, but a 25K rated device is supposed to be able to handle that amount without damages.

 

[electrofelon]

Transformer is 58k

What is calculated fault current at the line side of your service?
Where did the 58K come from, that seems high for residential, suggest you check with POCO

Yeah something seems wrong. About the biggest lowest impedance transformer I imagine feeding this (assuming it's not a network area) would be like a 166 at 1.7%z, and even that is only 40.5K. however....... the line to neutral current is typically higher than the line line current for single phase center tap transformers, and typically you would multiply the line line AFC times 1.5, so it is plausible.

Can you look at the actual transformer and get the data off of it? I have pumped out UG vaults and gone in, and gone up poles to get the data plate Data before, when I get stupid ridiculous values from the power company.

 

[Steve16]

If you have a 25KVA or 50 KVA transformer feeding the house this wouldn't be an issue as the max available fault current from those is 5K to 10K amps. This must be a large transformer (150KVA or larger) to have that much available fault current. Most residential stuff is all rated at 10K AIC (Amp Interrupt Capability). Most newer main panels have a main rated around 24K AIC, and it will protect the smaller branch 10K breakers if they are the same brand (this is a series combination rating). Breaking things up and mixing brands messes up the series combination ratings and most manufacturers don't series rate with other brand breakers.

Your transfer switch needs to have an AIC rating equal to or greater than the available fault current. With that huge of a transformer, the city needs to be reasonable and allow the wires between the transformer and main disconnect to be included in the available fault current calculation. Finding residential main and transfer panels and breakers that can tolerate a 60KA fault current would be rather expensive and I doubt any of the houses on that transformer have such equipment. Hopefully, the conductors can drop it down into the 20KA range where you have a lot of choices at reasonable cost.

It may turn out that the existing main and branch breakers are limited to 10KA. I'm not sure whose problem that becomes if you install an appropriately rated transfer switch, as the existing old main and distribution breakers still need to be rated for the available fault current. Could be an expensive transfer switch install if you end up having to also replace their main/branch panel. Even worse if there is a subpanel with breakers of a different brand or type listed in allowed series/combination rating with the upstream main panel and there is still over 10K AFC at that sub panel.

The available fault current of 5-10k for a 25 or 50kva transformer is not exactly accurate. Most utilities show upwards of 15k or higher for a 50kva depending on impedance. On top of that you are constantly seeing, and will be seeing larger residential transformers due to PV and EV chargers so the utilities are likely to throw larger fault current numbers out to contractors inquiring to cover themselves.

60kva is definitely brutal to try and find for a residential transfer switch breaker.

 

[electrofelon]

The available fault current of 5-10k for a 25 or 50kva transformer is not exactly accurate. Most utilities show upwards of 15k or higher for a 50kva depending on impedance. On top of that you are constantly seeing, and will be seeing larger residential transformers due to PV and EV chargers so the utilities are likely to throw larger fault current numbers out to contractors inquiring to cover themselves.

60kva is definitely brutal to try and find for a residential transfer switch breaker. You could (correct me if I'm incorrect) have a fused disconnect with type R fuses rated higher 100K or higher AIC upstream from the transfer switch.

In my experience, most utilities do not provide accurate data for the AFC. It is often 1.5 - 2 X the value you would get by using transformer data and the assumption of infinte primary. Providing some extra cushion may be prudent, depending on the circumstances.

He could certainly find a 65k or higher breaker or fuse, but the key would be getting the transfer switch breaker to series rate with it AND the beanch breakers to rate with it OR there is a triple series rating for the three. For a 200A frame it is typicalaly not a probelm to series rate from 65k to 10 k branches. In other words if A is the breaker closest to the utility, B is the transfer switch, and C are the branches, then he would need a series rating for A-B and a series rating for A-C. I know for siemens that would be fine with their QR frame breakers at 65k. Michael is going to have to do some investigating. Although we stil have seen the calc for the conductor impedance, maybe its a non issue!

 

[Steve16]

In my experience, most utilities do not provide accurate data for the AFC. It is often 1.5 - 2 X the value you would get by using transformer data and the assumption of infinte primary. Providing some extra cushion may be prudent, depending on the circumstances.

He could certainly find a 65k or higher breaker or fuse, but the key would be getting the transfer switch breaker to series rate with it AND the beanch breakers to rate with it OR there is a triple series rating for the three. For a 200A frame it is typicalaly not a probelm to series rate from 65k to 10 k branches. In other words if A is the breaker closest to the utility, B is the transfer switch, and C are the branches, then he would need a series rating for A-B and a series rating for A-C. I know for siemens that would be fine with their QR frame breakers at 65k. Michael is going to have to do some investigating. Although we stil have seen the calc for the conductor impedance, maybe its a non issue!

Utilities embellishing something?!?!?!?

Could he used a fused disconnect with fuses rated for the fault current upstream from the transfer switch, or would that still need to be protected in series with the transfer switch for the available fault current?

 

[electrofelon]

There is a caveat to what I said and I realized it is not entirely correct, regarding A rating with B and A rating with C. Here is the wording from Eaton on this:

Breakers A, B, and C are in series respectively from main to branch. Breakers A and B series rate together. Breakers A and C series rate at the same interrupting level (or higher). It is allowable to use A, B, and C together at the A-B series rating

 

[electrofelon]

Utilities embellishing something?!?!?!?

Could he used a fused disconnect with fuses rated for the fault current upstream from the transfer switch, or would that still need to be protected in series with the transfer switch for the available fault current?

The first device (closest to the utility) needs to be fully rated. Any downstream device needs to be either fully rated, or have a series rating with the device ahead of it. With three devices it is tricky and many people do it wrong and combine the series ratings of three devices improperly - hell I just messed it up!

 

[michael king]

If you have a 25KVA or 50 KVA transformer feeding the house this wouldn't be an issue as the max available fault current from those is 5K to 10K amps. This must be a large transformer (150KVA or larger) to have that much available fault current. Most residential stuff is all rated at 10K AIC (Amp Interrupt Capability). Most newer main panels have a main rated around 24K AIC, and it will protect the smaller branch 10K breakers if they are the same brand (this is a series combination rating). Breaking things up and mixing brands messes up the series combination ratings and most manufacturers don't series rate with other brand breakers.

Your transfer switch needs to have an AIC rating equal to or greater than the available fault current. With that huge of a transformer, the city needs to be reasonable and allow the wires between the transformer and main disconnect to be included in the available fault current calculation. Finding residential main and transfer panels and breakers that can tolerate a 60KA fault current would be rather expensive and I doubt any of the houses on that transformer have such equipment. Hopefully, the conductors can drop it down into the 20KA range where you have a lot of choices at reasonable cost.

It may turn out that the existing main and branch breakers are limited to 10KA. I'm not sure whose problem that becomes if you install an appropriately rated transfer switch, as the existing old main and distribution breakers still need to be rated for the available fault current. Could be an expensive transfer switch install if you end up having to also replace their main/branch panel. Even worse if there is a subpanel with breakers of a different brand or type listed in allowed series/combination rating with the upstream main panel and there is still over 10K AFC at that sub panel.

Yes this is a larger transformer, 100KVA, and the utility planner stated that this is unusually large. The city architect said that he will accept calculations for the AFC at the meter can. Through this fantastic website, we were able to utilize a spreadsheet for the new AFC. I have learned first hand on how drastic of a drop in the value you get through the conductor length and size. All this being said, I still have the question as to what the AFC is, what the AIC is, and what are the hazards associated with them. I've read the definitions, and the NEC for the book meanings. But what do they actually do? If the AIC is lower can the device be welded shut and never trip open? The transfer switch has a 200-amp breaker style disconnect, so wouldn't that trip long before seeing this much higher AFC? I'm assuming the AFC is just the utility companies power current, or potential. So is the AFC/AIC more for the line side, and the 200-amp more on the load side? We found our solution through calculations, I would just like to know the "why".

 

[augie47]

This is one of many references you might find interesting:

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/technical-literature/bus-ele-br-10757-fuseology.pdf

AIC ignored =

 

[michael king]

AIC - is the amount of current the device can safely interrupt per testing standards without physically destroying the device.

Available fault current is how much current can potentially flow during a fault condition. This value varies at different points in the circuit.
The highest amount will be at the source terminals, as you get further from the source you must factor in impedance of conductors and how much that will lessen what is available at the source.

So if you have available fault current of 21K amps, a device rated 10K is subject to possible destruction should it try to interrupt a fault, but a 25K rated device is supposed to be able to handle that amount without damages.

Okay, so now I'm seeing the picture. I couldn't wrap my brain around this for some reason. The AIC is the amount of fault current before failure of the device in the event of like a racoon or something. Not like it's really related to a breaker tripping under a heavy load, but the device itself being able to withstand a fault. Awesome!!

 

[michael king]

There is a caveat to what I said and I realized it is not entirely correct, regarding A rating with B and A rating with C. Here is the wording from Eaton on this:

Yeah one of my options would be to install a fused disconnect before the transfer switch, and install fuses rated higher than the AFC. The more I'm learning on this, with support from all of you, I believe that the calculations we made have us safe for the 22k transfer switch. With help from the utility company too.