SCCR calculation

[touge]

Hi experts,

I am calculating the SCCR value and improving the rating but am not sure whether it is right or wrong.

I only need to meet the requirements for NEC and NFPA70/79, excluding UL508A.
However, NFPA and NEC require the calculation and marking of the panel SCCR.

The SCCR requirement from our facility is 65KA@480V

1. We have three 24VDC power suppliers supplied directly from the main branch at 480V.
I know there is no requirement for the 24VDC power supplier and its load.
How about the feeding breaker for these supplies?
For each 24VDC power supply, I am using a UL489 MCB with a breaking capacity of 10KA at 480V.
Will these three breakers affect the panel SCCR rating, or do I need to add a CC class fuse to enhance this branch's SCCR?

2. There is a cooling unit for the panel marked with a 5KA SCCR, powered from the 480V main branch without TR.
Do I need to add a CC class fuse to enhance the whole panel's SCCR rating?

3. There is a remote hydraulic system with a pump motor connected via an aviation plug,
and the aviation plug has no data about SCCR.
Do I need to add a CC class fuse to enhance the whole panel's SCCR rating?

4. For the SCCR rating calculation and label, is that a must to use UL listed components?
For example , The main breaker(infeed) for this panel is not a UL listed breaker,
and it doesn't mention the SCCR rating or interrupt rating by UL standard.
Could I just use the Ics value from maker for the SCCR calculation for this panel?
I am confusing for this part because the SCCR rating seems is not always as same as the Ics by maker's data.

Thank you!

 

[jim dungar]

Your panel needs to have a rating sufficient for the short circuit current available at its terminals. It sounds like someone chose 65kA as a default equipment value rather than calculating the actual available amps. This should be your first step.

 

[touge]

Your panel needs to have a rating sufficient for the short circuit current available at its terminals. It sounds like someone chose 65kA as a default equipment value rather than calculating the actual available amps. This should be your first step.

Hi , thank you for reply!


Yes, you are right,
based on the panel's power and possible length ,
I think the actual available amps might be much lower ,
however ,the facility team haven't calculate it yet.



I still hope that I could calculate my panel SCCR rating first and figure out how to increase it correctly

 

[petersonra]

Hi experts,

I am calculating the SCCR value and improving the rating but am not sure whether it is right or wrong.

I only need to meet the requirements for NEC and NFPA70/79, excluding UL508A.
However, NFPA and NEC require the calculation and marking of the panel SCCR.

There is no accepted means to determine the SCCR of an industrial control panel except for UL508a.

The SCCR requirement from our facility is 65KA@480V

1. We have three 24VDC power suppliers supplied directly from the main branch at 480V.
I know there is no requirement for the 24VDC power supplier and its load.
How about the feeding breaker for these supplies?
For each 24VDC power supply, I am using a UL489 MCB with a breaking capacity of 10KA at 480V.
Will these three breakers affect the panel SCCR rating, or do I need to add a CC class fuse to enhance this branch's SCCR?

UL508a covers this issue. You would likely be OK if you changed these MCBs to class CC fuses.

2. There is a cooling unit for the panel marked with a 5KA SCCR, powered from the 480V main branch without TR.
Do I need to add a CC class fuse to enhance the whole panel's SCCR rating?

UL508a covers this issue. For whatever reason, UL508a exempts most panel air conditioning units that are plug and cord connected from the SCCR determination.

3. There is a remote hydraulic system with a pump motor connected via an aviation plug,
and the aviation plug has no data about SCCR.

The SCCR determination in UL508a does not cover any loads found outside of the panel. If the plug has no SCCR, UL508a assigns a default rating to it of 5 kA (might be 10 kA). However, this is one case where current limiting fuses could be used to help lower the required branch circuit SCCR rating. HOWEVER, it is not a simple thing to do. You have to read VERY CAREFULLY the part of UL508a where it describes how this works.

Do I need to add a CC class fuse to enhance the whole panel's SCCR rating?

It may or may not help. You need to follow the rules in UL508a to figure that out. There are too many variables to tell you what may or not not be compliant.

4. For the SCCR rating calculation and label, is that a must to use UL listed components?
For example , The main breaker(infeed) for this panel is not a UL listed breaker,
and it doesn't mention the SCCR rating or interrupt rating by UL standard.
Could I just use the Ics value from maker for the SCCR calculation for this panel?
I am confusing for this part because the SCCR rating seems is not always as same as the Ics by maker's data.

Thank you!

You have to use components that UL508a tells you are allowed to make the UL508a calculation and all those parts are either UL listed or recognized. This is also covered in the UL508a spec. The Ics value found in IEC CB specs is not the same thing as the AIC rating for US style breakers, so those numbers are always different.

I can tell you this. MOST of the time, current limiting fuses can be introduced that will solve your problem. But, where to add them requires the designer to follow the rules in UL508a. And you have to do your homework.

Having been down this road many times, I can tell you that adding a current limiting fuse in the feeder circuit usually only works for feeder circuits < 30 Amps. If it is more than 30 Amps, chances are you will need to look at series rated combinations.