Not another SCCR question... oh no.

[Rotato]

Sorry guys. Still trying to figure out calculating SCCR Ratings. I am trying to follow what guides I can find from Bussmann and Allen Bradley, which reference UL508 SB4. I have a very basic and straight forward circuit, I will attach a picture to this post.

My question is, what is the rating of the Feeder/Branch that controls the motor. No, I didn't design this circuit. But am still curious as to what it is. The Feeder fused disconnect is a current limiting fuse that the mfg. states the let-thru current is 3kA for my Available Fault (unkown at this time just doing worst case for now). I am assuming that since the let thru current is 3kA and the contactor and overload are rated for 5kA SCCR then I can bring up the SCCR rating of this Feeder/Branch to the fused disconnect of 100kA SCCR. Am I correct on this?

Just for my understanding, would they consider this a Feeder or a Branch Circuit? Its actually the first time I seen a circuit laid out like this. Usually the Feeder goes to a bunch of branch OCPDs then to the power components. But in this case its right from the Feeder to the power components and also has a branch involved. Would this be considered a Feeder. And if so, can I even do the current let-thru modification for the motor controls?

Thank you, in advance.

 

[petersonra]

This is a branch circuit as drawn. UL508a does not allow current limiting fuses in branch circuits to use the let thru current as part of the SCCR determination.

It's possible the contactor/overload combination has some kind of series rating with a class J fuse of that size.

Incidentally, whoever did the circuit design did not properly protect the transformer primary.

 

[Jraef]

It is POSSIBLE that the Eaton starter is series listed with an LPJ fuse. But that information, and the SCCR that it says the series combo is listed as, MUST come from Eaton, you cannot “roll your own” in the field. Then assuming that starter is not part of a UL508a listed control panel, the Eaton starter label would need to SAY on it, something like “SCCR is 100kA when protected by Bussman LPJ fuse no larger than xx amps.” If you cannot show that information, you cannot connect it to a system capable of more than 5kA.

Fuse mfrs like Bussman publish these papers on let-through and series listings, without adequately explaining that the information is not to be used in the field. The intended audience for those papers is equipment manufacturers who are looking to attain a series SCCR listing on their devices and want to know, in advance of the expensive required testing, if it is going to pass. But the fact remains, manufacturers can ONLY attain a series listing for an SCCR at higher than 5kA by testing with specific components. Panel builders can list an assembly at a higher SCCR if ALL of the power devices are listed in series combinations by the manufacturers.

 

[Rotato]

So If we can't use the let-thru current of a CL fuse. What is the point.

If all your components have a SCCR or Interrupt Rating over the current let-thru of the Main Feeder CL Fuse. Then everything depends on the CL fuse at that point for safety reasons.

If a OCPD down stream tripped, but its Interrupt Rating was over the current let-thru of the main feeder CL fuse. Then there is no harm no foul because worst case scenario the Feeder CL Fuse would have tripped if it got above the let-thru. Now I get that you should coordinate your circuit protection so your not tripping your main instead of your branch. That to me is a separate topic and shouldn't affect what your cabinet SCCR is.

Why wouldn't your cabinet be rated at the CL fuse, but instead the SCCR of the lowest component or series rating? Just doesn't make logical sense to me at this moment. Kind defies the point of the CL fuse in my opinion.

Not that I am defending the circuit design of the image I posted. Its rough to look at. But, just not understanding the logic behind not being able to rate your cabinet to your main Feeder CL Fuse if all you downstream components have a Interrupt Rating or SCCR above the current let-thru of the main feeder CL Fuse.

Can someone shine some light on to what I am missing here? Thank you in advance

 

[Jraef]

It’s not that CL fuses can’t be used, they can. They just don’t change the SCCR unless TESTED AND LISTED that way.

When we go to INSTALL something, the NEC requires that the device be rated at or above the available fault current of the system. That rating is the SCCR, which is established by TESTING and confirmed for the field people by LISTING. So the devices must have the listing information on them when you install them. No listing info., no passing inspection.

 

[Rotato]

SO in the scenario given in the first post. If I was to install a BCPD (that has an Interrupt Rating above the let through current) in front of the motor contactor (making it a branch circuit). Could I use the Feeder CL fuse then according to UL508A SB4 to bring the installation up to the SCCR of the CL Fuse? Or am I still off base here? It would help if I owned a copy of UL508A, might have to pick that one up.

 

[jim dungar]

You might want to look at more fuse literature than simply the stuff from Bussmann.
Try Ferraz and Littelfuse.

 

[petersonra]

SO in the scenario given in the first post. If I was to install a BCPD (that has an Interrupt Rating above the let through current) in front of the motor contactor (making it a branch circuit). Could I use the Feeder CL fuse then according to UL508A SB4 to bring the installation up to the SCCR of the CL Fuse? Or am I still off base here? It would help if I owned a copy of UL508A, might have to pick that one up.

Yes and no. It only matters if you are listing to the UL508a standard. You can use the allowances found in that standard if you are under the supervision of a NRTL such as UL or ETL and they authorize it. You still can't do it in the field.