kAIC question

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I'm designing a commercial PV system that is interconnected through a 200A backfed breaker. The building electrical drawings specify 65kAIC ratings on their switchgear. Do I need to have 65kAIC on the PV equipment as well even though our gear is on the other side of a 1000A main breaker and a 200A interconnection breaker from the service?
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I'm designing a commercial PV system that is interconnected through a 200A backfed breaker. The building electrical drawings specify 65kAIC ratings on their switchgear. Do I need to have 65kAIC on the PV equipment as well even though our gear is on the other side of a 1000A main breaker and a 200A interconnection breaker from the service?
You might.

A fault on the incoming feeder to your equipment will see the fault current from the utility, less any reduction from the feeder impedance.
 

Carultch

Senior Member
Location
Massachusetts
65 KAIC could very well be sized in excess of what is really needed. Such as standardizing on parts between related jobs, rules of thumb that are conservative for the size of the service, or rounding up to what was most readily available at the time it was built.

For any given device, the KAIC rating needs to meet one of the following in order to comply.
1. Meet or exceed the available fault current at that particular point in the circuit.
2. Be series-rated with an upstream overcurrent device on its line side (i.e. closer to the source of most of the fault current, which is usually the utility service). Breaker manufacturers have series rating charts that show a part number pattern of line side devices and a part number pattern of load side devices. There are also series ratings with given classes of fuses as the line side device and given part number patterns of load side breakers. It is a specific combination of devices that are tested together. Don't expect series ratings across multiple breaker brands.
3. Take credit for the impedance of the feeders between the service point and the point of the device in question, to determine how available fault current diminishes along the circuits. You'll need to know the wire size, conduit type (i.e. metal vs non-metal), length, number of parallel sets, conduit type, voltage and single vs 3-phase.

There is a fault current calculator spreadsheet available on this website that enables you to input the specs of a transformer and the specs of the service conductors, feeder, and branch circuit to determine the fault current at any given point. This does not include motor contribution fault current.
 
I'm designing a commercial PV system that is interconnected through a 200A backfed breaker. The building electrical drawings specify 65kAIC ratings on their switchgear. Do I need to have 65kAIC on the PV equipment as well even though our gear is on the other side of a 1000A main breaker and a 200A interconnection breaker from the service?
I agree with Jim, yes you need that breaker to be rated 65k MINUS conductor impedance if you want to see what that does for you.

This is a good example of when it could pay to use the same brand of equipment to try to get a series rating. That said, these days a fully rated (say 65k) 200A breaker is not going to be that much more that a 10 or 22k. For example, a quick and dirty search showed a siemens QR23B200 (10k) being essentially the same price as a HQR23B200 (65K). Those are 240V breakers, not sure if this is 480 or not.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
You might.

A fault on the incoming feeder to your equipment will see the fault current from the utility, less any reduction from the feeder impedance.
So the fact the the panel is behind the 200A backfed breaker makes no significant difference to the available fault current at the panel? If so, is that because of the time constant on the breaker?
 
So the fact the the panel is behind the 200A backfed breaker makes no significant difference to the available fault current at the panel? If so, is that because of the time constant on the breaker?
Short answer no, that breaker does nothing for you unless it's a tested combination. Longer answer is it comes down to the phenomenon of dynamic impedance, and essentially no one has been able to model the interaction and behavior of two circuit breakers in series without actually testing them.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Short answer no, that breaker does nothing for you unless it's a tested combination. Longer answer is it comes down to the phenomenon of dynamic impedance, and essentially no one has been able to model the interaction and behavior of two circuit breakers in series without actually testing them.
Thanks.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
So the fact the the panel is behind the 200A backfed breaker makes no significant difference to the available fault current at the panel? If so, is that because of the time constant on the breaker?
The specifications call for 65kA rated equipment. Do you need to meet that spec?
Per NEC 110.09 and 110.10 your equipment needs to be rated for the fault current available at it's terminals. The specs list an AIC rating they do not give you an actual fault current, so it looks like you should make your own calculations.
 
The specifications call for 65kA rated equipment. Do you need to meet that spec?
Per NEC 110.09 and 110.10 your equipment needs to be rated for the fault current available at it's terminals. The specs list an AIC rating they do not give you an actual fault current, so it looks like you should make your own calculations.
Yeah that is a good point. Sometimes someone does not want series ratings used. I would confirm that 65k requirement whether you need to meet it.

As I said previously, I am guessing it's negligible cost difference and may not even be worth the time investigating whether you can use a 22k or 10K breaker.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The specifications call for 65kA rated equipment. Do you need to meet that spec?
Per NEC 110.09 and 110.10 your equipment needs to be rated for the fault current available at it's terminals. The specs list an AIC rating they do not give you an actual fault current, so it looks like you should make your own calculations.
The specs I have are for the service to and in the building; we were not given any for our equipment. I'm not trying to circumvent any directives from the customer; if we had been given kAIC specs to conform to I wouid not have asked the question.
Yeah that is a good point. Sometimes someone does not want series ratings used. I would confirm that 65k requirement whether you need to meet it.

As I said previously, I am guessing it's negligible cost difference and may not even be worth the time investigating whether you can use a 22k or 10K breaker.
Your point is well taken, though what I was asking about was the kAIC rating of our AC combiner panel on the other side of the 200A interconnection breaker in the 65kAIC MDP.
 

pv_n00b

Senior Member
Location
CA, USA
I'm designing a commercial PV system that is interconnected through a 200A backfed breaker. The building electrical drawings specify 65kAIC ratings on their switchgear. Do I need to have 65kAIC on the PV equipment as well even though our gear is on the other side of a 1000A main breaker and a 200A interconnection breaker from the service?
You have choices. The quick and easy choice is to just match the AIC rating of the service equipment and assume it is still rated correctly and the utility has not done anything to increase the available fault current. Another choice is to ask the utility for the available fault current and calculate what the available fault current will be at the PV equipment. Then choose equipment with the appropriate AIC rating.
OCPDs do not do anything to lower the available fault current at any downstream point in the system. When doing power studies the short circuit study assumes the OCPDs are all closed and have no effect on the available fault current. What an upstream OCPD can do is if correctly series rated with a downstream OCPD it can raise the AIC rating of the downstream OCPD.
 
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