UL508A SCCR

[Contech]

New to the forum and not sure if this is in the right spot.

I have been looking for an explanation of why the current limiting in branch circuits cannot be factored into panel short circuit ratings. What I have is a panel with a 200A MCB and 20 zones of heaters powered through SSR's. All the branch circuits use CC fuses and the Ip values are less than the 5kA rating for the SSR's.

Also wondering if anyone knows of a manufacturer who provides a combination rating for SSR's. Crydom did at one point but has discontinued. The 5kA default is a huge problem (as I see it) for this type of application.

 

[Jraef]

... I have been looking for an explanation of why the current limiting in branch circuits cannot be factored into panel short circuit ratings. ...

Who told you that?

One of the approved methods of complying with NEC Article 409 is to have a "Listed" panel that has an SCCR rating applied. UL panel shops can do that. If you are a UL panel shop, do you have a copy of UL 508A Supplement SB? It's laid out in there. If not, I strongly suggest you have a talk with your UL representative.

If you are NOT a UL 508 panel shop, then yes, you cannot do it willy-nilly in the field. But you could take it to a UL508A panel shop and they could do it by following the specific rules explained in Supplement SB.

I dounbt anyone has a series combination listing on SSRs, but if anyone would, it would be Siemens, because they make SSRs and they make Circuit Protective Devices.

 

[Contech]

Let me clarify why I am asking. No we are not a UL 508 Panel shop. We are a machinery manufacturer in Canada trying to avoid problems with equipment we ship to the US.

I have spent some time reading the standard as well as numerous application examples available (primarily from fuse manufacturers). Cooper-Bussman has some excellent information as well as an on-line circuit builder and SCCR calculator.

What I gather from the information are the following definitions.
FEEDER CIRCUIT ? The conductors and circuitry on the supply side of the branch circuit overcurrent protective device.
BRANCH CIRCUIT ? The conductors and components following the last overcurrent protective device protecting a load.

Following the procedure laid out we determine the component SCCR values per SB4.2. When it comes to "modifying the available short circuit current within a portion of a circuit in the panel due to the presence of current limiting components as specified in SB4.3" SB4.3 refers to current limiting components in the feeder circuit that limit available short circuit current. It was this information that led me to my original question. If I am misinterpreting something I would love someone to set me straight.

Thanks

 

[BKearney]

Contech,

Take a look at this link:
http://ep-us.mersen.com/oem/resources/sccr_select_guide.cfm

There is a powerpoint file at the bottom of that page (CR Codes & Standards PowerPoint) that walks thru some examples of how to read 508a pertaining to SCCR. Perhaps you have seen it but it is one of the best breakdown examples I have seen so far. The PPT file shows how to interpret SB4 and gives examples of how at first pass you may think a branch is only 5kA but with the right fusing, distribution block and main box fused disconnect, you can actually rate the branch at a higher kA using tables SB4.1 and SB4.2. It has some good examples...

Lacking someone with knowledge on 508a to talk to directly (like a 508a panel shop guy), it is about as clear as mud, and takes some staring at 508a and some examples before it will click into place.

Hope that helps some...