Existing AIC Rating

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Dreameng

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I have a question to all of you. for Example:

If you have an existing service that has a 30KAIC rated switchboard and you are adding a new circuit breaker to that DSBN section, I would normally add a new 30KAIC rated circuit breaker. It has been recently brought to my attention that I cannot do that, that I would need to run a fault study to determine whether or not the system was designed correctly, due to the fact that it could have been originally done wrong. So what I have been told is that I would need to do this to all the existing systems we work on. Is this practical? Is there a section in the code that says I dont have to do this under the assumption that the existing service was designed correctly?
 
It might be helpfull if you mentioned thye jurisdiction that you are working in, for example in my area , an argument can be made that you need only be concerned with the OCPD that you are adding. (not to say you can add) It is adviseable to to the fault study. You generally are responsible for the work you are engaged in.
 
I don't think the NEC is going to have an answer to this one. But I don't think you are responsible for verifying that an original design was not done incorrectly. If, by chance, the present installation has an available fault current in excess of the rating of the existing board, then adding a breaker to that board, in the manner you described, is not going to make the situation worse.

Things would be vastly different if the breaker you are adding will serve one or more large motors. That could materially impact the present value of short circuit current available to the gear. In that case, a new fault study would be needed.

Welcome to the forum.
 
Many people may be surprised to find that the original AIC calculations for a building are no longer good, simply because the POCO changed a transformer, for whatever reason.

You may have an existing building where 10K was fine and now everything should be 22K, but how are you to know?
 
cowboyjwc said:
Many people may be surprised to find that the original AIC calculations for a building are no longer good, simply because the POCO changed a transformer, for whatever reason.

You may have an existing building where 10K was fine and now everything should be 22K, but how are you to know?
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Time to be nit picky, we make calculations for short circuit amps (SCA) not amps interrupting capacity (AIC).:roll:

Seriously though, John makes a good point. This is one reason that most of the large POCOs in my area provide a "design" level fault current for equipment selection purposes rather than a realistic level. Their design level usually takes into account possible transformer replacements and system upgrades.
 
It's an engineer's task. We would create an electronic model of the facility. That would start with a one-line diagram, and would require such information as transformer ratings, large motor loads, and conductor sizes and lengths. It is a complicated math problem, but there are several commercially available software packages that do the math for us. The results would be displayed either as a list or as an image of the one-line diagram with the calculated short circuit currents shown at each bus.
 
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