AIC Rating

[anbm]

Calculated fault current at RTU (roof top unit) is 15,025A. RTU's controller data sheet shows AIC rating is 10,000A.
The RTU is protected by a fused disconnect at the unit, fuse's AIC rating is 200KA. Will this code comply even the unit's
controller AIC rating is less than calculated fault current?

 

[augie47]

Unless you have manufacturer documentation listing their equipment as being series rated with that fuse, you will need engineering documentation,

 

[kwired]

Unless you have manufacturer documentation listing their equipment as being series rated with that fuse, you will need engineering documentation,

I agree. Need to add some impedance to the supply circuit. Possibly make a longer run somehow, use aluminum instead of copper, use a line reactor....

 

[drktmplr12]

You used to be able to use the time current curve for a current limiting fuse to justify a lower AIC rating.

Now you need manufacturer publication stating a LISTED combination rating for the exact part numbers. Or else increase impedance as suggested.

Sent from my Pixel 2 using Tapatalk

 

[jim dungar]

You used to be able to use the time current curve for a current limiting fuse to justify a lower AIC rating.

Not in the past four decades, or so, if you are talking about molded case breakers.

Yes you used to be able to, kind of. 25 years ago I attended some Bussmann training, at their headquarters, and was taught that the old up-over-down method was pretty much no longer used for anything except bus bar bracing.
for the most part you need to go to manufacturers or UL tables for Short Circuit Current Ratings (SCCR). For breaker AIC selection you are pretty much limited to manufacturer's tables produced from actual testing.

 

[kwired]

You used to be able to use the time current curve for a current limiting fuse to justify a lower AIC rating.

Now you need manufacturer publication stating a LISTED combination rating for the exact part numbers. Or else increase impedance as suggested.

Sent from my Pixel 2 using Tapatalk

I think there is more understanding of the topic then there used to be which is why you see more enforcement then you used to. I don't think there has really been that much change in the rules. If anything we are seeing higher available fault current then we used to see in many situations - more desired power = larger sources are necessary = potentially higher available fault current.

 

[drktmplr12]

I think there is more understanding of the topic then there used to be which is why you see more enforcement then you used to. I don't think there has really been that much change in the rules. If anything we are seeing higher available fault current then we used to see in many situations - more desired power = larger sources are necessary = potentially higher available fault current.

And higher transformer efficiency requirements leading to lower impedances

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[augie47]

And higher transformer efficiency requirements leading to lower impedances

Sent from my Pixel 2 using Tapatalk

:thumbsup: