Short Circuit and AIC Rating Requirement

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Samardas

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Los Angeles, CA, United States
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Electrical Engineer
I have calculated Short Circuit ratings at various points down from the Main Switchboard to the Sub-panel. At the entry of the sub-panel the calculation came out to be about 12,400 Amps. The existing breakers inside the sub-panel are rated 10,000 Amps AIC. Do I have to change all the breakers to more that 12,500A AIC or can I use them as is? I want to use the breakers as "Fully Rated" not series rated since the existing sub-panel is very old (May be, Cutler-Hammer). The drawing is due this Friday (07/25/14). Can anyone help me please?:? Thanks a LOT!
Samar
 
It might be permissible if you could demonstrate the equipment was series-rated as installed, see 240.86.

Otherwise, you've got a code violation leaving equipment in that has an AIC less than the available fault current. We see this problem a lot. Transformers get changed and nobody thinks to check and see what the downstream gear ratings are before they do it.
110.9 Interrupting Rating.
Equipment intended to interrupt current at fault levels shall have an interrupting rating not less than the nominal circuit voltage and the current that is available at the line terminals of the equipment.

Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage not less than the current that must be interrupted.
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You can't just replace the breakers with a higher rated model, unless you know the bus of the panelboard itself can support that higher rating. The internal panelboard bus has a withstand rating that goes into the overall panelboard short circuit rating.
 
This is a question better asked of the engineer supervising you and not random people on the Internet. There are a lot of variables that can affect the answer.
 
Not bidding with correct fault current just cost us $10,000.00. Estimator bid project with 65KAIC and it is actually 100KAIC. Gear went from $159k to $170K. Guess which PM has to eat this crap!!
 
Replacing all the individual breakers in an old/obsolete panel can be expensive, especially if you are directing the contractor to use breakers with a high interrupting rating. It may be more economical to just replace the entire panel. If the panel is downstream of a fuse, it is likely that you could use 10K breakers that series rate with the class of fuse installed as long as there is documentation that they are a tested combination.
 
Short Circuit and AIC Rating Requirement

Thank you all for your valuable comments. Yes, I am trying to break ice in this design field to make my living. I am a PE with some past design experience - for some of the well-wishers who want to know about me. I can also see some of you have licenses and some with tons of experience who answer questions that are tremendously helpful. Hence my effort in getting answers. Again Thanks for your Help! Have a Great Day Everyone.
 
I would have another look at the fault current calculation. It will have many pieces of information that you have to provide to the software. Many of them are mere estimates. For example, what values are you using for cable lengths, where did you get those values, and can you be certain they are as precise as you could wish? If you take the time to reevaluate the cable lengths, it may be possible that you could use higher values in the software model. That would have a tendency to reduce the calculated fault current. There are other variables within the software's calculation methodology that might have some room for reevaluation.

Another approach, one that might or might not work but might be cheaper than replacing the panel and all the breakers, is to look into using a smaller conductor size for the panel feeder. If a smaller feeder has sufficient ampacity for the load, and if you can tolerate the impact on voltage drop, this could also bring the calculated fault current down to a lower value. The really easy part is to make the change within the software model and see if it gives you a value below 10,000.

The bottom line here is that the software is a tool for use by the engineer. It is not the expert, you are.
 
Short Circuit and AIC Rating Requirement

Thanks a lot Charles. Although I got the length of the conductors from the drawing, but I will take another closer look at the length I had considered in my calc. Also, I will look into other possibilities you have suggested. The main problem is the owner is reluctant to change the breakers to 22K (from 10K AIC in old panel), that I suggested. However, if the recalc does come out to be so, they will have no other choice but to do it. Thanks again.
 
charlie b said:

The bottom line here is that the software is a tool for use by the engineer. It is not the expert, you are.
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This is very good advice.

Samardas said:
Although I got the length of the conductors from the drawing, but I will take another closer look at the length I had considered in my calc. Also, I will look into other possibilities you have suggested. The main problem is the owner is reluctant to change the breakers to 22K (from 10K AIC in old panel), that I suggested. However, if the recalc does come out to be so, they will have no other choice but to do it. Thanks again.
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If the correct answer is to change out the panel, change out the panel. Don't let anyone pressure you into doing something that will result in an unsafe system. The right solution may not be the most popular, but it still is the right solution.
 
You could either replace transformer ahead of panel with unit with higher impedance, or add a current limiting reactor in front of the panel. Would have to do a trade off on cost.
 
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