Does my panel's AC unit limit the panels SCCR rating to 5kA?

[BN32]

I need to achieve a 65kA overall panel rating but am starting to think that may not be possible if I'm understanding the 508A standard correctly.

Is my panel SCCR rating limited to the 5kA rating of the AC unit since the peak let through current for a 70A class J fuse @ 100kA (11.5kA) is greater than 5kA?

All the other branches are 65kA or greater this was my weak link.

See details below.

Main panel disconnect fusing: LPJ-70SP - 300kA Vac RMS Sym.

AC Unit branch circuit protection: 140M-C2E-C10 - 65kA @ 480Y/277V

AC Unit: Rittal 5kA

 

[drktmplr12]

my take is yes.

have you considered class cc fuses or an isolation transformer?

 

[BN32]

my take is yes.

have you considered class cc fuses or an isolation transformer?

I believe class cc fuses only go up 30A so that wouldn't be an option.

I haven't considered and isolation transformer. I may have to.

 

[Jraef]

It doesn't work that way. This has been discussed several times in here, but the Reader's Digest version is that if you read the UL 508A Supplement SB section on how to determine SCCR, your choices are to submit everything in the panel to UL for series testing ($$$$), accept the "untested" default value of 5kA, or use EXISTING Series Listings of devices with the fuses (or breakers) they are series listed with, because the component mfrs have ALREADY done the Series Listings for you. So you would look up every component in the power stream and find out what they are listed for by themselves, and the lowest listed device is the limit of your SCCR. But if those components have been SERIES listed with other devices and you use those exact devices, you can use the higher series listing they have. Almost every power component mfr now offers a comprehensive list of their devices in series with fuses or breakers. If you use fuses, you must use the SPECIFIC fuses they used, or ones with the same characteristics. If you use breakers, you must use ONLY the exact breaker it was listed with, which will mean (generally) the mfr of the component will end up being the same as the breaker, because few, if any, mfrs are willing to spend the money it takes to series list something with a competitor's product.

For example let's say it is a motor starter. The contactor alone has an SCCR of 10kA, the Overload Relay has an SCCR of 5kA and the breaker is rated for 65kA. Untested and uninvestigated, you are stuck with 5kA max, because the OL is rated for 5kA. But if you get the Series Listing info from the manufacturer, you find out that with that breaker, contactor and OL, the Series Listing is for 65kA,so now your panel can be listed at 65kA SCCR.

 

[petersonra]

I suggest that you look closely at the supplement that describes how to calculate the short-circuit current rating of a control panel. Especially the part about air conditioning units that are cord and plug connected.

 

[BN32]

I suggest that you look closely at the supplement that describes how to calculate the short-circuit current rating of a control panel. Especially the part about air conditioning units that are cord and plug connected.

The AC unit is not cord and plug connected.

 

[BN32]

It doesn't work that way. This has been discussed several times in here, but the Reader's Digest version is that if you read the UL 508A Supplement SB section on how to determine SCCR, your choices are to submit everything in the panel to UL for series testing ($$$$), accept the "untested" default value of 5kA, or use EXISTING Series Listings of devices with the fuses (or breakers) they are series listed with, because the component mfrs have ALREADY done the Series Listings for you. So you would look up every component in the power stream and find out what they are listed for by themselves, and the lowest listed device is the limit of your SCCR. But if those components have been SERIES listed with other devices and you use those exact devices, you can use the higher series listing they have. Almost every power component mfr now offers a comprehensive list of their devices in series with fuses or breakers. If you use fuses, you must use the SPECIFIC fuses they used, or ones with the same characteristics. If you use breakers, you must use ONLY the exact breaker it was listed with, which will mean (generally) the mfr of the component will end up being the same as the breaker, because few, if any, mfrs are willing to spend the money it takes to series list something with a competitor's product.

For example let's say it is a motor starter. The contactor alone has an SCCR of 10kA, the Overload Relay has an SCCR of 5kA and the breaker is rated for 65kA. Untested and uninvestigated, you are stuck with 5kA max, because the OL is rated for 5kA. But if you get the Series Listing info from the manufacturer, you find out that with that breaker, contactor and OL, the Series Listing is for 65kA,so now your panel can be listed at 65kA SCCR.

Thanks Jraef. While true, supplement (SB4.3.3) allows you to increase the SCCR rating of a branch circuit with the use of current limiting fuses in the feeder circuit just as long as the components in the branch circuit have a short circuit current rating not less than the peak let thru current of the fuse. That doesn't really help me here though considering the I'm using 70A class J fuses in my feeder.

 

[petersonra]

The AC unit is not cord and plug connected.

You might want to look at that part anyway. I don't have the standard handy but I seem to recall some special rules for cabinet air conditioners.

Btw, two smaller air conditioners will work better than one larger one anyway.

 

[BN32]

You might want to look at that part anyway. I don't have the standard handy but I seem to recall some special rules for cabinet air conditioners.

Btw, two smaller air conditioners will work better than one larger one anyway.

There used to be an exception for AC Units on branch circuits rated 60A or less but that has since been removed and only applies to cord and plug connected units. Other than that, I haven't found anything else specific to AC units.