Utility AIC exceeds MSB Rating

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woods95662

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I have a situation on a project. The engineer missed the AIC from the utility is 68200 and the MSB is rated for 65k. The gear can't be fixed without being rebuilt. A few questions. Does the utility use the length of the secondaries for their calculation? If so then couldn't we increase the length of the secondaries to reduce the AIC?
 
You would have to ask that question of the utility. Many give the fault current at the secondary terminals of their transformer.
If you have motor loads, the fault contribution from the motors may have to be added to the current from the utility.
 
I'm being told we can't do a series rating by the gear manufacturer. Would this exclude me from putting a set of fuses before the MSB to reduce the AIC?
 
woods95662 said:
Would this exclude me from putting a set of fuses before the MSB to reduce the AIC?
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Absolutely.

It is a common 'old wives tale' that fuses fix every fault current problem. The NEC clearly states, and has for decades, that series ratings need to be tested or engineered.

Did the project specifications require you to verify the utility fault current, as part of the approval process, before you ordered the equipment?
I see this in almost 100% of specifications, but I bet it happens on fewer than 40% of the projects sent to me.
 
jim dungar said:
Absolutely.

It is a common 'old wives tale' that fuses fix every fault current problem. The NEC clearly states, and has for decades, that series ratings need to be tested or engineered.

Did the project specifications require you to verify the utility fault current, as part of the approval process, before you ordered the equipment?
I see this in almost 100% of specifications, but I bet it happens on fewer than 40% of the projects sent to me.
Click to expand...

Jim,
Is it allowable to use a fuse/breaker combination to correct the problem if the combination exists and he can verify it under engineering supervision ?
 
What about reducing the trip unit amperage? The utility states if we get down to 3500A well be under the threshold. It's a two transformer setup each 1500kVA for 480V service.
 
augie47 said:
Jim,
Is it allowable to use a fuse/breaker combination to correct the problem if the combination exists and he can verify it under engineering supervision ?
Click to expand...
The only provision for an "engineered" series combination is for an existing installation.
If there is a listed combination for the main breaker in series with an upstream fuse you should be good to go.
 
That was the utilities suggestion. I just talked to my IEM rep and he stated that even though that is the fault current at the transformer that is not the actual AIC at the gear with the secondaries taken into account to the main breaker. He suggested having an engineering study done with the feeder length and if need be lengthen the feeders to reduce the AIC per the engineering.
 
woods95662 said:
That was the utilities suggestion. I just talked to my IEM rep and he stated that even though that is the fault current at the transformer that is not the actual AIC at the gear with the secondaries taken into account to the main breaker. He suggested having an engineering study done with the feeder length and if need be lengthen the feeders to reduce the AIC per the engineering.
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Yes, you hire some skilled in perfoming Short Circuit studies. They will be able to take into account the sources of fault currents and the impedance of the conductors.

Actually, the AIC is a function of the how the breaker is manufactured.
You are talking about changing the amount of available Short Circuit Amps.

It may seem like I am making big deal about proper terminology, but eventually it does become important.
AIC is kind of like a conductor's ampacity (for the most part it is a function of how it is built) versus the available Short Circuit Amps which is like the amount of current that flows due to a load.
 
jim dungar said:
Yes, you hire some skilled in perfoming Short Circuit studies. They will be able to take into account the sources of fault currents and the impedance of the conductors.

Actually, the AIC is a function of the how the breaker is manufactured.
You are talking about changing the amount of available Short Circuit Amps.

It may seem like I am making big deal about proper terminology, but eventually it does become important.
AIC is kind of like a conductor's ampacity (for the most part it is a function of how it is built) versus the available Short Circuit Amps which is like the amount of current that flows due to a load.
Click to expand...


Ok Jim so am I on the right track? I just want to be clear.
 
I re read your statement and I believe I am. If i prove the short circuit current available at the main breaker due to the impedance of the conductors is below the AIC of the breaker then problem solved. It's 10 runs of 600 CU 60' long right now. We have a pull section it is going through we could loop the conductors to lengthen them if need be.
 
woods95662 said:
...If i prove the short circuit current available at the main breaker due to the impedance of the conductors is below the AIC of the breaker then problem solved...
Click to expand...
As long as there is nothing within your facility (like a large motor, generator, or parallel service etc.) that could contribute additional fault current, yes.
....We have a pull section it is going through we could loop the conductors to lengthen them if need be.
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I've never personally seen it done, but have heard of that being used as a solution to reduce the available fault current.
 
I worked for "the utility" for many years. The answer to your first question is probably not. The utility engineer might be aware of the number, size, and lengths of the secondary conductors but typically he wouldn't know and wouldn't care. His responsibility ends at the the transformer secondary terminals in most (not all) cases.

The bad news is that if you asked three different engineers from this utility for the fault current you might get three different answers. Did they calculate using actual system impedance or did they assume an infinite bus? I did both but usually the latter, which will produce a slightly larger number. Did they have the transformer nameplate data or did they just use 5.75% or some other "good guess"? I always made an earnest effort to get the nameplate number, but had a good feel for what to use when I couldn't. You might go back to the utility with these details and ask if a recalculation is appropriate. And whatever their final answer is, be sure to get it in writing.
 
woods95662 said:
I re read your statement and I believe I am. If i prove the short circuit current available at the main breaker due to the impedance of the conductors is below the AIC of the breaker then problem solved. It's 10 runs of 600 CU 60' long right now. We have a pull section it is going through we could loop the conductors to lengthen them if need be.
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Yes you are correct.

Although if these are service entrance conductors you may need to put the extra length outside of the building.
 
jim dungar said:
Yes you are correct.

Although if these are service entrance conductors you may need to put the extra length outside of the building.
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They are service entrance conductors and outside is where the pull section is we used to get into the building. The MSB is on a raised dock indoors so we set a pull section outside to get up into the the dock. Thanks a lot for everyone's help I've got a gamelan now.
 
Iron_Ben said:
I worked for "the utility" for many years. The answer to your first question is probably not. The utility engineer might be aware of the number, size, and lengths of the secondary conductors but typically he wouldn't know and wouldn't care. His responsibility ends at the the transformer secondary terminals in most (not all) cases.

The bad news is that if you asked three different engineers from this utility for the fault current you might get three different answers. Did they calculate using actual system impedance or did they assume an infinite bus? I did both but usually the latter, which will produce a slightly larger number. Did they have the transformer nameplate data or did they just use 5.75% or some other "good guess"? I always made an earnest effort to get the nameplate number, but had a good feel for what to use when I couldn't. You might go back to the utility with these details and ask if a recalculation is appropriate. And whatever their final answer is, be sure to get it in writing.
Click to expand...

Great points Ben I'm going to ask those questions.
 
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