Isolation transformer to reduce AFC

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ShortCircuited140

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Hi,

I have a system where I have an available fault current of 12kA that feeds a 60kW pump system and a 30kW Vaporizer system that have an SCCR or 5kA.

I was thinking of adding a 150kVA 480D/480Y isolation transformer to bring down the AFC to below 5kA. Does anyone see any issues with doing this? Will the transformer have any limiting effects on the pump that I'm not thinking of?

Cheers
 
This is feasible provided the impedance of the 150 kVA transformer is high enough. With a 2% impedance, there will be about 9 kA of fault current.
A short circuit rating of 5000 A at 480 V is extremely low.
 
David Castor said:
This is feasible provided the impedance of the 150 kVA transformer is high enough. With a 2% impedance, there will be about 9 kA of fault current.
A short circuit rating of 5000 A at 480 V is extremely low.
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You may be thinking of infinite primary. He has 12kA available.

Using the Eaton website, and a standard impedance transformer, you will get 2500A on the secondary.
1697675125124.png
 
You've provided an oversized 150kVA transformer considering the load, so it should offer enough inrush current to start the 60kW pump system and a 30kW Vaporizer system. The isolation transformer is a good approach.

Another approach is if you could run the 480V feeder with 12,000AFC on the source end 200', you will get <5kAFC on the load end.
1697675540005.png
 
One thing to consider are the losses in that transformer, which will be 24/7/365 unless you power down the primary.

Another option is to find a local UL panel shop willing to take the panel in and rework it to apply a better SCCR to it. It might involve changing out a few components but it’s not at all difficult, your supplier was just lazy and cheap. 5kA is the “courtesy” default level of SCCR you can apply if you put zero effort into selecting your power components and just go with the cheapest thing you can find for every individual piece. Most reasonable component manufacturers now have simple charts or websites showing you which of their components are “series listed” at higher SCCR levels, but it means you must coordinate the selection process. Takes an extra hour to think about it…
 
Jraef said:
One thing to consider are the losses in that transformer, which will be 24/7/365 unless you power down the primary.

Another option is to find a local UL panel shop willing to take the panel in and rework it to apply a better SCCR to it. It might involve changing out a few components but it’s not at all difficult, your supplier was just lazy and cheap. 5kA is the “courtesy” default level of SCCR you can apply if you put zero effort into selecting your power components and just go with the cheapest thing you can find for every individual piece. Most reasonable component manufacturers now have simple charts or websites showing you which of their components are “series listed” at higher SCCR levels, but it means you must coordinate the selection process. Takes an extra hour to think about it…
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Yeah totally understood Jraef, power efficiency in this case is not a concern though. Agreed that the 5kA is a lazy approach and it could probably be improved, but will take additional time, redesign/build and that's current not on my side :)
 
I've actually seen some control panels manufactured that have the 5K rating on them but when I looked at them closely they could easily have been rated for much higher but for some reason the manufacturer chose not to do so.
 
petersonra said:
I've actually seen some control panels manufactured that have the 5K rating on them but when I looked at them closely they could easily have been rated for much higher but for some reason the manufacturer chose not to do so.
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Often the easiest way to do the SCCR is to just take the lowest component SCCR, slap that on the label and call it a day. Instead of looking for series rated improvements to the SCCR that requires a bit more paperwork. I chalk it up to not needing a higher SCCR or just plain laziness of the engineer.
 
ShortCircuited140 said:
Often the easiest way to do the SCCR is to just take the lowest component SCCR, slap that on the label and call it a day. Instead of looking for series rated improvements to the SCCR that requires a bit more paperwork. I chalk it up to not needing a higher SCCR or just plain laziness of the engineer.
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A lot of these places don't have people who really understand this stuff.
 
ShortCircuited140 said:
Hi,

I have a system where I have an available fault current of 12kA that feeds a 60kW pump system and a 30kW Vaporizer system that have an SCCR or 5kA.

I was thinking of adding a 150kVA 480D/480Y isolation transformer to bring down the AFC to below 5kA. Does anyone see any issues with doing this? Will the transformer have any limiting effects on the pump that I'm not thinking of?

Cheers
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Is the 12kA what is available at the equipment or is it what is available at the service or maybe the sub panel you are feeding said equipment from?

Remember your conductors do reduce available fault current The 30 kW load would only need 6 or 8 AWG conductors. If you had 12kA at the panel and ran 80 feet of 6 AWG to the 30 kW load, you would have right around 5kA available on the load end of the run. If you run 8 AWG it gets it closer to 3.7 kA
 
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