Calculating SCCR for panels

UL508A Supplement B is what most in the US will use to calculate SCCR. I don't think you can buy just SB but you really should get a copy. In the end it is what your calculation will be evaluated against.

Any company that makes overcurrent protection devices for industrial use will likely have a document similar to what scott.lawrence linked to. They are, as expected, all similar in that they are a mix of good info and reasons why you should use their product. Check out a few different version and you should get a good feel for which way to go.

Generally speaking, fuses will result in a higher SCCR than circuit breakers but that is by no means a guarantee; just a likelyhood.

Keith
 
Calculation is kind of pointless unless you are a registered PE willing to stake your livelihood on the results. The SCCR you can apply to a panel is basically going to be determined by the lowest rated device OR SERIES COMBINATION of devices used in the power circuit of your panel. So you can either use all components that have series listings at higher values (attained via testing that is too expensive to do for anyone other than the manufacturers), accept the lowest level of any device in the panel, or accept the "courtesy" 5kA SCCR for untested components. Doing so of course puts a major burden on the installer to limit their available fault current to 5kA, which is very very difficult in an industrial facility.
 
Unless you have a very complicated panel - basically find the SCCR ratings (listed by manufacturer) of the protective devices on your power feed.

For example - if I had a panel with 2 motors and a 24VDC power supply and a disconnect, I would find the SCCR value of the disconnect circuit breaker, each motor's circuit breaker, and the power supply circuit breaker. Whatever is lowest would be my SCCR rating.

If you are using breakers or protective devices that are not UL listed, you will be stuck with the default rating - which in many cases is unacceptable for industrial applications.

For UL listing, make sure you have a page in your drawings that list the UL files and SCCR ratings for each device (or combination of devices). For the example above, I would probably just include a chart on the 480 distribution page, but a separate page is also a good idea.

Bussman makes a good publication that I use for most UL jobs. Search for 'Bussman SPD' or 'Bussman Selective Protective Devices'. It has charts for motors, references for calculations, etc.
 
Originally posted by Rson:

Unless you have a very complicated panel - basically find the SCCR ratings (listed by manufacturer) of the protective devices on your power feed.


That's kind of the way it was BEFORE SCCR came into the picture. We don't design particularly complicated panels and it is very seldom the OCPDs that are the limiting devices for SCCR.

Do you have a power distribution block? Those are often no better than 100kA even if you have Class J or better fuses in front of them. Depending on the size of conductors you connect to them the SCCR can be significantly lower.

Is your contactor actually a listed combination with the OCPD in front of it? That will drop the branch to the contactor's default SCCR.

You are unconditionally correct with the power supply branch in your example but only because power supplies are consider exempt from SCCR consideration.

You really need to go through the whole exercise using an accepted evaluation method to know where you sit.

Keith
 
I’m a recently retired controls engineer who took several SCCR classes and have the examples put out by Siemens and Bussman but I could never get the hang of doing it to my satisfaction.

All of our panels were quite different with transformers, PLC’s, 480V components, etc. I could never get a straight answer to my questions. Glad that I’m out of it now.
 
That's kind of the way it was BEFORE SCCR came into the picture. We don't design particularly complicated panels and it is very seldom the OCPDs that are the limiting devices for SCCR.

Do you have a power distribution block? Those are often no better than 100kA even if you have Class J or better fuses in front of them. Depending on the size of conductors you connect to them the SCCR can be significantly lower.

Is your contactor actually a listed combination with the OCPD in front of it? That will drop the branch to the contactor's default SCCR.

You are unconditionally correct with the power supply branch in your example but only because power supplies are consider exempt from SCCR consideration.

You really need to go through the whole exercise using an accepted evaluation method to know where you sit.

Keith

Yes, for motors we typically use combination starters, which are no better than 65k usually. As another example, a fused disconnect is usually rated to a certain SCCR with a certain type of fuse - so you need to use that fuse.

If you follow what is in the manufacturer's data sheets, you at least have a piece of paper to back you up.
 
I’m a recently retired controls engineer who took several SCCR classes and have the examples put out by Siemens and Bussman but I could never get the hang of doing it to my satisfaction.

All of our panels were quite different with transformers, PLC’s, 480V components, etc. I could never get a straight answer to my questions. Glad that I’m out of it now.
Transformers, PLCs etc don’t count. Only devices on the POWER circuit that could open or operate during a fault will count. So the protective device FOR the transformer would count, but not the transformer itself nor anything down stream of it, like a power supply, control components etc. 🤾
 
Calculation is kind of pointless unless you are a registered PE willing to stake your livelihood on the results. The SCCR you can apply to a panel is basically going to be determined by the lowest rated device OR SERIES COMBINATION of devices used in the power circuit of your panel. So you can either use all components that have series listings at higher values (attained via testing that is too expensive to do for anyone other than the manufacturers), accept the lowest level of any device in the panel, or accept the "courtesy" 5kA SCCR for untested components. Doing so of course puts a major burden on the installer to limit their available fault current to 5kA, which is very very difficult in an industrial facility.

So are you saying only PE's can figure out the SCCR or just only they can calculate the rating for the device itself?

I guess I should reword the question then... How do you go about finding the SCCR for a control panel, so I can put the proper label on the front of the cabinet?

Obviously I'm not a PE, so if I'm not allowed to put the rating on it, I won't.



Thanks!
 
I suspect jraef is speaking philosophically. There is a level of defined accountability that exists for licensed individuals that does not exist for practitioners at large. Licensed individuals can lose their license as a result of gross or repeated errors. In addition a minimum level of knowledge is required to obtain and keep that license and that knowledge is tested and confirmed.

The inferences for calculation by a non-licenses practitioner are:
1) if there are no ramifications for an incorrect calculation, how do you know the required care was taken in the calculation
2) if the required care was taken, how do you know the level of knowledge of the practitioner was sufficient to make a correct interpretation of the calculation.

In reality I don't see anything in either NFPA 70 or UL508A that would indicate that the determination of an SCCR using Sup B is only valid if calculated by a PE.

Keith
 
That's kind of the way it was BEFORE SCCR came into the picture. We don't design particularly complicated panels and it is very seldom the OCPDs that are the limiting devices for SCCR.

Do you have a power distribution block? Those are often no better than 100kA even if you have Class J or better fuses in front of them. Depending on the size of conductors you connect to them the SCCR can be significantly lower.

Is your contactor actually a listed combination with the OCPD in front of it? That will drop the branch to the contactor's default SCCR.

You are unconditionally correct with the power supply branch in your example but only because power supplies are consider exempt from SCCR consideration.

You really need to go through the whole exercise using an accepted evaluation method to know where you sit.

Keith

Keith,
You bring up a lot of good points and point out the many layers os SSCR calc’s.
It’s like an onion where you need to keep peeling back the layers. My bosses just assumed I was making things too complicated.
 

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