# KA surge protection of breakers



## guitarboyled (Jun 22, 2009)

Hi again, it's been a while

I was told that the breaker surge protection is like 10 or 15 KA for residential installations and 20-25 KA for commercial installations but how do you actually determine the right value?

Thanks


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## raider1 (Jan 22, 2007)

Are you asking about the amps interrupting capacity (AIC) of the breakers?

Chris


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## guitarboyled (Jun 22, 2009)

I'm not sure, I didn't now about this up until a few days ago. Still trying to figure out what the KA thing is about.


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## raider1 (Jan 22, 2007)

KA would be thousand-amps so my guess would be you are talking about the AIC rating of the breakers.

Breakers are rated in AIC which is the amount of available fault current that the breaker can withstand before there is the possibility that the breaker could catastrophically fail.

The minimum AIC rating for a breaker is 5000 amps.

Most utility companies will try to limit the available fault current for single family residents to less than 5000 amps.

Chris


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## brian john (Mar 11, 2007)

I think your question is confusing two items

the AIC rating of circuit breakers and fuses such as MOST residential CB's have a AIC rating of 10,000 amps that is available fault current.

Impulse ratings of TVSS is based on what you can afford for a TVSS and what mother nature can throw at you in the way of a lightning strike.


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## guitarboyled (Jun 22, 2009)

Sometimes I need to specify breakers... How do I know if it's 5, 10, 25, 50, 65 or 100 KA?

Do I always need to mention AIC or only when a transformer is involved?

I found this on the net:

Example design criteria:


Utility transformer rating: 2,500kVA
Utility transformer % impedance: 4.775%
Service conductors: 10 sets of 600 MCM copper
Available fault current at utility transformer secondary: 63,000A
X/R ratio at the utility transformer secondary: 11
Motor contribution: 400 hp
Ampacity of service conductors: 4,000A
Service gear tested X/R ratio: 4.9
A fault current of 62,321A is calculated at the switchgear. This value is based on the 2,500kVA utility transformer with 4.775% impedance and minimal impedance from the service conductors (11 feet of 10 sets of 600 MCM copper).
The simple form of this calculation, based on infinite bus theory, is indicated below:
2,500kVA÷(√3×480V)÷0.04775=62,975, or 63,000AIC at the utility transformer secondary

This 4,000A service switchgear requires AIC bracing and overcurrent protection in excess of 65,000AIC.


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## acrwc10 (Jan 28, 2007)

In most cases you would just contact the Utility provider and ask them for a "fault current letter". With that you will have your answer, but there is a lot more that goes into it then that. The farther you get away from the transformer the lower the fault current gets.


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## Shorty Circuit (Jun 26, 2010)

guitarboyled said:


> Sometimes I need to specify breakers... How do I know if it's 5, 10, 25, 50, 65 or 100 KA?
> 
> Do I always need to mention AIC or only when a transformer is involved?
> 
> ...


I just encountered this identical situation in a building. My guess is that the utility transformer is 5.75% impedence, that's usually typical. Available short circuit current is around 52,000+ amps. Ten sets of parallel 600 MCM run about 300 feet, series conductor impedence is negligable. Problem, there is no main breaker, four of five feeder breakers are MA36800 with AICs of only 30,000 amps. I've seen this kind of mistake many times.

Most residential installations have small transformers with available current well below the standard 10,000 AIC of QO breakers. Small transformers, high feeder impedence and if you're several hundred feet away with many users tapped in upstream of you, you'll see your lights dim in summer whenever someone's central AC condenser comes on. That's life.


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## guitarboyled (Jun 22, 2009)

If I understand correctly this AIC is dependant of the power source (provided by the utility company) and affects mainly the service entrance.


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## Shorty Circuit (Jun 26, 2010)

guitarboyled said:


> If I understand correctly this AIC is dependant of the power source (provided by the utility company) and affects mainly the service entrance.


It could be any source such as an on site generator, a UPS, or a solar cell array with an inverter. Also an on site transformer. The AIC rating of a downstream current interrupting device in each circuit, usually the first one must be able to interrupt the maximum current the source can deliver, usually that is with a short circuit at the load side of the breaker.

In the mid 1980s I had a problem with a data center where there was no longer redundancy from a 1500 KVA x 2 double ended substation. A consultant suggested replacing the 1500 KVA transformers with 2500 KVA units. The breakers were GE AKD-6 with an AIC of 36,000 amps. Discussing this with GE engineers they advised that while the combination of the AIC of the main secondary breaker for each transformer and feeder breaker for each circuit would be greater than the available AIC, this arrangement which I think is called cascading is not FM approved and they strongly recommended against it. I have taken that opinion to be definitive even if it is technically legal in NEC (I haven't looked it up.)


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## acrwc10 (Jan 28, 2007)

guitarboyled said:


> If I understand correctly this AIC is dependant of the power source (provided by the utility company) and affects mainly the service entrance.


What you need to know is, what is the available fault current from the utility. With that you can start selecting your down stream devices. These may need to be current limiting fuses, if the fault current is to high. All of your breakers downstream need to have a high enough AIC rating also. If your main breaker is ,for example, 30k AIC you can't use 10K AIC breakers in the downstream equipment, without engineering the system to have selective coordination. What "selective coordination" does is keep the down stream breakers closed for a fraction of a second longer then the main breaker. So in a dead short, the main will open before the downstream breaker opens. This is important to keep the high fault currents from causing a catastrophic failure in the lower rated breakers.


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## Shorty Circuit (Jun 26, 2010)

acrwc10 said:


> What you need to know is, what is the available fault current from the utility. With that you can start selecting your down stream devices. These may need to be current limiting fuses, if the fault current is to high. All of your breakers downstream need to have a high enough AIC rating also. If your main breaker is ,for example, 30k AIC you can't use 10K AIC breakers in the downstream equipment, without engineering the system to have selective coordination. What "selective coordination" does is keep the down stream breakers closed for a fraction of a second longer then the main breaker. So in a dead short, the main will open before the downstream breaker opens. This is important to keep the high fault currents from causing a catastrophic failure in the lower rated breakers.


I think you are confusing two things. A coordination study compares the time current curves of different overprotection devices in a distribution network and selects or adjusts them so that when there is an overload, say due to a single component failing, only the next higher circuit breaker or fuse opens. This minimizes unnecessary disruption to circuits not affected by the fault. For example, a burned out motor on a branch circuit from an NQ panel will take out only the branch circuit breaker, not the main or upstream feeder breaker killing the whole panel. A short circuit study looks at a worst case catastrophic failure and determines whether a fault under such conditions will be cleared at all. You could say a coordination study is performed to prevent nussiances, a short circut study is performed to prevent catastrophes.


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## BBQ (Nov 16, 2010)

Shorty Circuit said:


> I think you are confusing two things. A coordination study compares the time current curves of different overprotection devices in a distribution network and selects or adjusts them so that when there is an overload, say due to a single component failing, only the next higher circuit breaker or fuse opens. This minimizes unnecessary disruption to circuits not affected by the fault. For example, a burned out motor on a branch circuit from an NQ panel will take out only the branch circuit breaker, not the main or upstream feeder breaker killing the whole panel. A short circuit study looks at a worst case catastrophic failure and determines whether a fault under such conditions will be cleared at all. You could say a coordination study is performed to prevent nussiances, a short circut study is performed to prevent catastrophes.


I know what you are trying to say and I agree with it .... kind of.

A 'coordination study' as you put it is in fact a "Short circuit coordination study"

A "Short circuit coordination study" studies the coordination of breakers under short circuit conditions.

This is above and beyond determining how much available fault current is available at any particular point in the system.


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## Shorty Circuit (Jun 26, 2010)

BBQ I don't think you quite understand the distinction between a short circuit study and a coordination study. Here's an actual example of each. In an office complex in Livingston, shortly after it was finished, a large coffee maker the cafeteria concessionaire had brought in had a short circuit. Instead of just the branch circuit breaker tripping out, three quarters of the complex went black. A breaker far upstream had tripped out taking down many panelboards with it. One clue is when all of the adjustable instantaneous trip settings are at minimum which is how the breakers are shipped for manufacturer liability reasons, there had been no coordination study performed. The upstream breaker tripped out faster than the downstream breaker, their time current curves crossed. In a different incident in Redbank, there was an insulation meltdown as far as we could figure out. All of the insulation of the THW wire packed tightly in the incoming raceway of a large chiller. (It should not have gotten a UL listing but it did anyway.) This created a dead short on the load side of the feeder breaker. The breaker settings were correct which in no way affected what happened. The reason the breaker didin't explode or the contacts fuse is that it was rated at 100,000 AIC, having been selected over a 50,000 AIC equivalent because the transformer feeding it was 2000 KVA at 480 volts. This selection was made bearing in mind that in addition to the available short circuit current from the transformer, there would be current supplied in phase on at least one of the three phases from the chiller motor itself acting as a generator using the inertial energy of rotation and therefore the total current available on one phase could exceed 50,000 amps momentarily. Would a 50,000 AIC breaker have been sufficient to clear the fault? Fortunately we will never know.


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## BBQ (Nov 16, 2010)

Shorty Circuit said:


> BBQ I don't think you quite understand the distinction between a short circuit study and a coordination study.



You would be wrong in that assumption.


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## Shorty Circuit (Jun 26, 2010)

BBQ said:


> You would be wrong in that assumption.


It's getting tiresome for me belaboring this point but it's clear to me that you don't understand it. One last try and that's it for me on this one.

"I know what you are trying to say and I agree with it .... kind of."

"A 'coordination study' as you put it is in fact a "Short circuit coordination study""

Overloading a circuit, say connecting too much load on a convenience outlet circuit is not a short circuit fault. The trip may occur from too much current in the intermediate or long time region of the time current curve of the OC devices, not due to instantaneous short circuits. What is more, the ability of a circuit breaker to interrupt a serious short circuit fault is only one point at the extreme of the time current curve. A short circuit study looks only at that point and usually only for the breaker directly downstream of the power source such as the utility transformer. It is only concerned with the ability to interrupt (clear) the maximum available current in a short circuit, not which breaker opens first. A coordination study looks at the entire time current curve range for all of the OCs in the part of the network under study to determine if the curves cross at any point in time and to select and adjust them so that they don't. If that isn't clear to you, I suggest you do further reading on your own to get a better understanding of the difference.


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## BBQ (Nov 16, 2010)

Shorty Circuit said:


> It's getting tiresome for me belaboring this point but it's clear to me that you don't understand it.


It is becoming clear to me that you are more than a bit pompous. 

I am not an EE, I do however deal with breaker ratings and coordination studies very often.

I have 25 years in large commercial work and run the service dept for a very large EC. When a 277 volt lighting circuit short takes out the main of a 80,000 sq ft supermarket my phone rings. When an RTU starts up and takes out the main on a large shopping center I hear about it. 

In countless calls over the years these issues almost always trace back to the breaker settings still being on the factory low and fast settings. The only fix is to get an EE on board and provide the settings. 



> One last try and that's it for me on this one.


Oh bless you for your help, my day just got so much better. :laughing:



BBQ said:


> "I know what you are trying to say and I agree with it .... kind of."
> 
> "A 'coordination study' as you put it is in fact a "Short circuit coordination study""





> Overloading a circuit, say connecting too much load on a convenience outlet circuit is not a short circuit fault.


Ah thanks for the obvious. :laughing:




> The trip may occur from too much current in the intermediate or long time region of the time current curve of the OC devices,


It _may_, but in a modern building of proper design it would be very rare for a service or feeder breaker to be overloaded. The load calculations of the NEC are very conservative and so are most EEs.



> What is more, the ability of a circuit breaker to interrupt a serious short circuit fault is only one point at the extreme of the time current curve. A short circuit study looks only at that point and usually only for the breaker directly downstream of the power source such as the utility transformer. It is only concerned with the ability to interrupt (clear) the maximum available current in a short circuit, not which breaker opens first.
> 
> 
> A coordination study looks at the entire time current curve range for all of the OCs in the part of the network under study to determine if the curves cross at any point in time and to select and adjust them so that they don't.


The only real difference we have here is what we are calling the studies, I know the purpose of both. If you do some Googling there is support for both our positions. Chalk it up to local terminology.



> If that isn't clear to you, I suggest you do further reading on your own to get a better understanding of the difference.


But drop the talking down to me as you do not know me enough to talk to me like I am your kid.


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## Shorty Circuit (Jun 26, 2010)

BBQ said:


> It is becoming clear to me that you are more than a bit pompous.
> 
> I am not an EE, I do however deal with breaker ratings and coordination studies very often.
> 
> ...


"I am not an EE"

I am.

"When a 277 volt lighting circuit short takes out the main of a 80,000 sq ft supermarket my phone rings. When an RTU starts up and takes out the main on a large shopping center I hear about it. "

When the phone rings and they tell you the fire department is there because the building is burning down, you'll know it was not due to a coordination problem.

"It is becoming clear to me that you are more than a bit pompous." 

Pompous? Maybe. A dope? Definintely for having wasted this much time with you. One nice thing about hittng your head against a brick wall is that it feels soooooo good when you stop.


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## acrwc10 (Jan 28, 2007)

:laughing::laughing::laughing:

Ok ladies, one of you is going to need to let the other get the last word in. :whistling2:


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## BBQ (Nov 16, 2010)

acrwc10 said:


> :laughing::laughing::laughing:
> 
> Ok ladies, one of you is going to need to let the other get the last word in. :whistling2:


I am no lady and I doubt he is either but he can have the last word. :thumbsup:


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