30k AIC > 10k

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augie47

augie47

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Thsi project will be turned over to an engineer, but I'm looking for a clue.
Inspecting a job with a 500 kva transformer, AIC 30,100. 1200 amp MDP with 800 branch breaker feeding a MCC./ MCC is marked with maximum withstand of 10k. Any ideas as to how this can be designed to meet Code ?
 
augie47 said:
Thsi project will be turned over to an engineer, but I'm looking for a clue.
Inspecting a job with a 500 kva transformer, AIC 30,100. 1200 amp MDP with 800 branch breaker feeding a MCC./ MCC is marked with maximum withstand of 10k. Any ideas as to how this can be designed to meet Code ?
Click to expand...

I assume that is the infinite bus fault. I recalculated the fault estimating the primary fault and could not get to 10 ka.
 
I've seen two jobs where a reactor was installed to reduce AIC, but I am totally in the dark concerning them. Is that a possibility ?
 
augie47 said:
I've seen two jobs where a reactor was installed to reduce AIC, but I am totally in the dark concerning them. Is that a possibility ?
Click to expand...
That's one way to engineer an installation to lower the available fault current by increasing the system impedance. Another is to use Current Limiting fuses or circuit breakers which have a let-through of under 10kA at the maximum available fault current from the transformer. Either way, those sorts of coordinated systems typically need to be stamped by a PE.
 
I have advised to consult a P.E. I just like to have some idea of possible acceptable plans. I could not find a 800 amp fuse that had a let-thru as low as 10 with a 30k available. Do you know of one ?
 
I have seen where they add length of conductors as well. Obviously that is not always a practicle method.
Adding a disconnect with the proper rating and the added length of conductors may work, I have seen that as well.

How large is the system?
 
unfortunately short runs. 1200 amp service approx 50 ft to transformer.
Feeder only about 50 ft also.
 
I'm also a little surprised at an MCC having only a 10kA rating!? There must be something in it that was not evaluated. Can that / they be relocated? Typically it's something like a VFD of a brand not of the same mfr as the MCC, a common problem that engineers who don't care about things like this end up causing when they stick to insisting on a specific manufacturer. What I have done in those cases is put a feeder CB in the MCC and move the VFD to a nearby wall in its own enclosure. Most of the time the VFD as a stand-alone unit has high enough ratings when protected by fuses, they just lose that when mounted inside a competitor's MCC because they have never been tested together. The "courtesy" untested rating the MCC mfr could put on it used to be 10kA. Now it's even lower: 5kA untested.
 
Jraef said:
That's one way to engineer an installation to lower the available fault current by increasing the system impedance. Another is to use Current Limiting fuses or circuit breakers which have a let-through of under 10kA at the maximum available fault current from the transformer. Either way, those sorts of coordinated systems typically need to be stamped by a PE.
Click to expand...
You can't just design a system using the let-through of the OCPD unless you are doing a reto-fit of an existing building. If you are working on a new installation, you would have to use a listed and tested "series rated" system. 240.86
 
I agree with Don's comment.
As a side note, I've used a reactor before to reduce fault current, but the key items to consider for me was, the air core type had lots of EMI/EMF all around it and had to be located far from ferrous material. The Iron core type reactor had poor voltage drop characteristics.

Try to be sure the calculation is correct first, which may make everything okay.
 
I agree with Don also, but 240.86 refers to circuit breakers not fuses. Systems can be series rated by engineering supervision or manufacturer tested systems. All downstream devices can be protected using fuse let thru ratings with proper coordination. Circuit breaker series ratings can only be taken as single tier (as tested) and listed by the manufacturer, so I don't think you will find a circuit breaker to motor starter series rating (though I don't know this for a fact, maybe someone knows better). Some areas of the country make you take the available fault current of the utility transfomer at the main disconnect without any reduction (which I don't understand, but that's the way it is). If you are series rating using fuses (or circuit breakers) to protect downstream devices, you still need to provide proper labeling at the interrupting device. See 110.9, 110.10, 110.22(B) and 110.22(C). Also remember that the future is a long time, I have seen many instances when the utility will change transformers for many reasons and the existing systems are no longer protected for the new available fault currents.
 
ronmath said:
I agree with Don also, but 240.86 refers to circuit breakers not fuses. Systems can be series rated by engineering supervision or manufacturer tested systems. All downstream devices can be protected using fuse let thru ratings with proper coordination. ...
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Ron,
You can't protect downstream circuit breakers using engineering design and fues. The only code permitted way to protect downsteam breakers, other than in an existing installation, is the use of listed and tested series combination ratings. The listed and tested system may use fuses, but an engineer is not permitted to design a system using the time current and let through curves.
 
Thanks Don, yes, I see that FPN in 110.22(B) now thanks. The main point I was trying to make is that there is specific published data by the fuse manufacturers for series ratings that may be used to protect downstream breakers. Personally, I fully rate all the systems I design for now and possible future conditions.
 
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