# Aic ratings of downstream equipment



## Tim Bullard (Jul 28, 2020)

I have a 4000 amp Eaton main distribution panel rated at 65kAIC. Do I need a current limiting fuse between that and an off brand MCC rated at 10kAIC.


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## micromind (Aug 11, 2007)

The AIC rating of any device is easy to figure but also difficult.....

The easy part is it needs to be rated higher than the available fault current. 

The hard part is figuring how much fault current is actually available. 

If the 4000 amp board in question is supplied by a source that's capable of only 9500 amps, then 10K AIC rated devices would be ok anywhere. 

If the supply is capable of 15,000 amps and the MCC is a long way from the board and the wire feeding it is small, there could be enough impedance to cause the available fault current at the MCC to be less than 10K. 

Depending on the load calcs, the PUCO around here would most likely feed a 4000 amp board with a 2500KVA transformer. At 277/480, this transformer would have a full-load current of 3008 amps and if the impedance was 5%, and the primary source was infinite (with that size, it wouldn't be infinite.......lol), the available fault current would be 60,168 amps at the transformer. 

Due to the impedance of breakers, wire and such, the fault current would be lower the further away from the transformer you get.


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## JRaef (Mar 23, 2009)

And as to the 2nd part of your question, putting a current limiting fuse ahead of thr 10kA rated MCC is not going to help you, officially. That is, unless the MCC has been tested and listed in Series Combination with the fuses (and trust me, it hasn't). Your only hope is, as micromind said, that the total circuit impedance drops your Available Fault Current at the MCC terminals to under 10kA. Just because the gear is RATED for 65kA doesn't mean you actually have that much available. You have to start with find that out.

Side note:
Where did you find an MCC with only a 10kA rating though? The lowest I've ever seen is 35kA.


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## Tim Bullard (Jul 28, 2020)

*AIC Rating*



JRaef said:


> And as to the 2nd part of your question, putting a current limiting fuse ahead of thr 10kA rated MCC is not going to help you, officially. That is, unless the MCC has been tested and listed in Series Combination with the fuses (and trust me, it hasn't). Your only hope is, as micromind said, that the total circuit impedance drops your Available Fault Current at the MCC terminals to under 10kA. Just because the gear is RATED for 65kA doesn't mean you actually have that much available. You have to start with find that out.
> 
> Thanks, The MCC is coming from Italy and UL approved at 10k.
> I did the AIC calks based on conductor size and distance and of course I can't get close to that with power company serving 42k at the transformer.


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## Tim Bullard (Jul 28, 2020)

Thanks, The MCC is coming from Italy and UL US approved at 10k.
I did the AIC calks based on conductor size and distance and of course I can't get close to that with power company serving 42k at the transformer.


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## paulengr (Oct 8, 2017)

Tim Bullard said:


> Thanks, The MCC is coming from Italy and UL US approved at 10k.
> I did the AIC calks based on conductor size and distance and of course I can't get close to that with power company serving 42k at the transformer.



There you go. Only way you are going to reasonably get that low is add impedance. As in feed from a smaller isolation transformer. As a stab without cabling (won’t make much difference here) figure kva / 1.732 / voltage / %Z/100 = kA which is the infinite bus assumption. If you want something more exact the Cooper Bussmann point by point method is pretty simple. So if you know desired current and system voltage that gives you kVA minimum. There are ANSI numbers for %Z but standard is around 2-4% depending on size. I’ve seen customs up to 12%. It’s easy to do...they just add more turns if you can afford it. Be careful...it also affects the voltage drop dramatically so starting big motors can be an issue.

But honestly. It’s not worth the hassle. MCC is cheap as long as you order it at least MLO. If you just order bare sections you get the 300% mark up.


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## kb1jb1 (Nov 11, 2017)

Just curious. Why would they buy something from Italy? Is the MCC proprietary to other machines or equipment?


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## varmit (Apr 19, 2009)

The only sure code compliant method would be to replace or modify the existing MCC. If the MCC is modified or the breakers are replaced, a new UL inspection is required. Unless this is a really large and special MCC, replacement can be the most cost effective.( But may take time to obtain the new gear.)

I deal with this often on Asian made equipment panels. This equipment usually arrives with 5k to 10k AIC breakers. The plant electrical system fault current usually is 40k to 50k. The proper rated replacement breakers take about 30% to 50% more physical space, so it takes some creativity to fit everything. 

There is a local, to me, company that recertifies everything.


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## micromind (Aug 11, 2007)

This is ways out in left field but I wonder if a current limiting reactor could be placed between the gear and the MCC.

I've never seen one for 480 but I've installed more than a few on medium-voltage (usually 12.5 - 25KV) systems. A lot of them are at power plants mainly because both the grid and the gens will contribute to a fault resulting in a substantially larger flash........


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