Re-evaluating industrial panel SCCR with let-through current

[bhellmer]

A customer has select industrial panels that are labeled with an unacceptable SCCR of 5,000 amps. The label does not provide alternative ratings for when fed by current-limiting breakers or fuses.

Of course, I would prefer that different panels with an adequately labeled SCCR be selected. But there is resistance.

We have considered using transformers or inductors to reduce the available-fault current. But first, we want to consider feeding the panels with current-limiting fuses.

UL 508A permits the use of let-through currents when evaluating components on the load side of a current-limiting device protecting the incoming feeder.

I propose feeding these panels with current-limiting Type-J fuses. If re-evaluations according to UL 508A result in SCCRs that exceed the available fault currents, how do I label or otherwise document this re-evaluation and address the manufacturer’s conflicting label. I understand that this re-evaluated SCCR will be non-compliant and void manufactured NRTL listings.

 

[mayanees]

I would look at 240.86 and 240.86(A) which requires a PE to certify the installation.
Then utilize the published ampacity let-through charts for selection of a fuse that fits the requirements.

 

[bhellmer]

I would look at 240.86 and 240.86(A) which requires a PE to certify the installation.
Then utilize the published ampacity let-through charts for selection of a fuse that fits the requirements.

Fortunately, I am a P.E.

 

[Jraef]

That's the danger of not including a requirement for an appropriate SCCR in your bid specs. OEMs will take advantage of that to just apply the "courtesy" value of 5kA for untested or unlisted series combinations. It's going to take a while for the world to catch on to this concept and DEMAND that panels arrive that neet Article 409 of the NEC.

The concept of adding CL fuses to feed a panel with a low SCCR is very much misunderstood and may not work the way you are thinking of. As I interpret it, you have 2 choices.

1. The power components in the panel must be TESTED and LISTED with the fuses in series for this to apply.
2. You can use the tables in UL508A Supplement SB (Table SB4.2) as the peak Let-Through value. But you will see in there that even a Class J CL fuse, applied where the peak available current is between the threshold of where the fuse acts and 50kA, the best value of Let-Through is 6kA, and that's only if it's a 30A fuse! In essence it isn't going to get you there anyway.

 

[bhellmer]

That's the danger of not including a requirement for an appropriate SCCR in your bid specs. OEMs will take advantage of that to just apply the "courtesy" value of 5kA for untested or unlisted series combinations. It's going to take a while for the world to catch on to this concept and DEMAND that panels arrive that neet Article 409 of the NEC.

The concept of adding CL fuses to feed a panel with a low SCCR is very much misunderstood and may not work the way you are thinking of. As I interpret it, you have 2 choices.

1. The power components in the panel must be TESTED and LISTED with the fuses in series for this to apply.
2. You can use the tables in UL508A Supplement SB (Table SB4.2) as the peak Let-Through value. But you will see in there that even a Class J CL fuse, applied where the peak available current is between the threshold of where the fuse acts and 50kA, the best value of Let-Through is 6kA, and that's only if it's a 30A fuse! In essence it isn't going to get you there anyway.

Thank you for your helpful reply.

We did not specify this equipment. The owner selected it and told us to design around it.

Table SB4.2 only support 50kA, 100kA and 200kA as available fault currents. But this panel's feeder has only 16kA available. I was hoping that I could use the fuse manufacturer’s let-through values; they are more generous in general and especially at 16kA. But the following appears to require that only Table SB4.2 values be used:

SB4.3.3 For branch circuits supplied by a Class CC, G, J, L, RK1, RK5, or T fuse in the feeder circuit, the short circuit current rating on the line side of the fuse shall be one of the following:

a) The available short circuit current selected for the feeder fuse, any value up to the interrupting rating, when all of the individual components in the branch circuit have a short circuit current rating not less than the peak let-through current corresponding to the specific fuse class, ampacity and selected available short-circuit current employed from Table SB4.2, and the interrupting rating of all branch circuit protective devices or the short-circuit current ratings of any combination motor controller on the load side are not less than available short circuit current selected for the feeder fuse. For branch circuit protective devices not marked with an interrupting rating, or for combination motor controllers not marked with a short-circuit current rating, the values in Table SB4.1 shall be used;
​

For this project, we will probably be specifying line inductors or transformers to reduce the available fault current. We will model transformers in PowerTools or EasyPower because Table SB4.3 and SB4.4 are not very generous. We will also model line inductors in PowerTools or EasyPower because UL 508A does not address the use of line inductors for current-limiting. Line inductor cost less than transformers, but line inductors do not have taps to recover the voltage drop.

I have found that UL 508A SB5 satisfactorily addresses labeling. I can leave the manufacturer’s label with the “Standard SCCR” and provide an additional label with the “High Fault SCCR”.

 

[petersonra]

I don't see any need at all to put a new short circuit current rating label on the panel if you have reduced the available short circuit current. Just put a label on the panel that reflects the actual available short circuit current. If you use Transformers or inductors to reduce it below five Ka you are good to go. No need to play with the labels that came with the panel.

 

[bhellmer]

I don't see any need at all to put a new short circuit current rating label on the panel if you have reduced the available short circuit current. Just put a label on the panel that reflects the actual available short circuit current. If you use Transformers or inductors to reduce it below five Ka you are good to go. No need to play with the labels that came with the panel.

I concur. I included the sentence without proper context. The intended context was when re-evaluating a panel using current-limiting fuses and their let-through current.

 

[Sahib]

The implication of point2 in post4 is a higher size CL fuse loses its current limiting function for a lower fault current.

 

[wbdvt]

How are you determining the short circuit at these panels? Did you model to the panel using the available fault current from the POCO or was it done using infinite bus?

 

[bhellmer]

How are you determining the short circuit at these panels? Did you model to the panel using the available fault current from the POCO or was it done using infinite bus?

We have modeled the building in PowerTools back to the service using available fault current data from the utility company.

 

[petersonra]

I wonder if anybody has ever been injured because an industrial control panel has a lower short circuit current rating nameplate then what the actual short circuit current is

 

[Sahib]

I wonder if anybody has ever been injured because an industrial control panel has a lower short circuit current rating nameplate then what the actual short circuit current is

More generally, Any accident due to inadequate CB interuptimg capacity? However absence of such statistical data should not be construed to mean permission for inappropriate CB /panel design.

 

[Jraef]

How far do you want to take it? No standards whatsoever? Everybody gets to do or use whatever the heck they want to and the heck with rules and everyone just accepts whatever consequences befall them?

 

[jim dungar]

How far do you want to take it?

I have always viewed 110.9 as being the requirement for Protective Devices to be applied within their AIC rating and 110.10 as corresponding requirement that equipment, including control panels, be applied based on their SCCR. There really isn't much grey area with these two sections, but I know many electricians that appear never to have read them.

 

[petersonra]

I did not suggest ignoring the rules but like many rules in the National electrical code I kind of wonder how the requirement in any practical manner actually relates to whether the installation is actually safe or not. Kind of like with afci's

 

[ron]

We have modeled the building in PowerTools back to the service using available fault current data from the utility company.

Depending on the circuit size, I have found iterating with PowerTools with feeder / circuit length until I get an acceptable calculated fault current results in good ROI.

I find inductors are a hassle, especially because they are the stepmother (no harm meant to stepmothers on this forum) of the transformer industry and are often misunderstood by most.

 

[petersonra]

Depending on the circuit size, I have found iterating with PowerTools with feeder / circuit length until I get an acceptable calculated fault current results in good ROI.

I find inductors are a hassle, especially because they are the stepmother (no harm meant to stepmothers on this forum) of the transformer industry and are often misunderstood by most.

wire is pretty cheap. even if you have to add an extra 100 feet in the run somewhere it can be a pretty cost effective solution.

 

[ron]

wire is pretty cheap. even if you have to add an extra 100 feet in the run somewhere it can be a pretty cost effective solution.

Agreed. The Electrical Contractors look at me weird, but I tell them to "run it long" and go around the room or there and back ....