SCCR of Equipment

[Charles_Evans]

Definitions:

SCCR - Short Circuit Current Rating
AIR - Amperage Interrupt Rating
Panel board - Enclosure, bus and lugs (NO OCPD).
OCPD - Over Current Protective Device

Assumptions:

1. All electrical equipment must have an SCCR or AIR as applicable.
2. UL Listed equipment must post the SCCR or AIR rating.
3. Panel boards and like must have an SCCR due to electromagnetic attractive forces due to the parrallel current carrying members of the equipment.

Background:

I regularly do design work for an existing facility where I try my best to provide them with accurate information to assist in proper operation and maintenance of their electrical systems. As part of that service, I have been working towards proper labeling of the equipment which includes the SCCR of the equipment - namely panelboards.

In an attempt to determine the SCCR I have consulted the factory for the posted rating of the panel board. I have only attempted this maybe 3 times so far and of those times I have not recieved a response, but recently I was able to speak with someone who informed me that their SCCR rating for their panel is determined by the lowest rated breaker per NEC. I understand that the OCPD AIR rating can de-rate the overall rating at that panel board, but it cannot change the panel board rating itself.

Questions:

1. Are UL listed panel boards to have an SCCR, independant of the OCPD. (I know the OCPD will finally determine the rating at the board per NEC).

2. What should be posted - the SCCR of the panel or the OCPD that limits the panel?

3. If the panel is posted with an SCCR based on the OCPD per UL listing and the panel is inadvertently derated by device(s) with a lower AIR rating then does the installation of that OCPD which derated void the UL listing for the board?

4. [Retorical] Assuming a LARGE facility (200+ panelboards, 6 service entrances, 3 levels of switchboard, on-site emergency generators, etc) with absolutely no records for available fault current, limited to no knowledge of motor loads and transformers on site contributing to fault current, partly complete and/or inaccurate one-lines, and old equipment most without ratings posted. How would a contractor correctly select an OCPD with the right AIR for an existing panel no knowing the available fault current and not having engineering calculations to indicate what the available fault current should be since this is an existing facility.

Ethically and professionally, would you feel that the engineer of record is responsible for calculating that fault current for a small project considering that the panels used are an existing condition?

Thank you for your time and consideration on this posting.

 

[charlie b]

Long Answer to Item 1:
All panels have been given, by their manufacturer, a rating for the maximum short circuit current that they can handle. They also have been given a maximum (let me call it) ?normal current rating,? or ?steady state current rating.? The OCPD must be selected so as to prevent a steady state current in excess of the panel?s rating. In addition, the amount of fault current available at the panel?s location must not exceed the SCCR. If you design a change to an existing facility, you are responsible for ensuring that your change does not cause the panel to experience currents in excess of either rating.

Most of the time, the short circuit current is not a problem. This is especially true if you are dealing with a 120/208 volt load, and if there is a step down 480V ? 120/208V transformer upstream of the panel in question. The amount of resistance internal to the transformer is generally enough to reduce the available fault current at the transformer secondary to below 10,000 amps. Most panels are rated for at least 10,000 amps. But if your design change affects the main switchboard or something directly connected to it, then perhaps it will be a bit of extra work to prove that the SCCR is within the panel?s rating.

Short Answer to Item 2:
Who says you have to post either?

Item 3:
Every breaker must be able to interrupt the amount of current that a fault could send through it. As discussed in item 1 above, this is usually not an issue. It becomes more likely to become an issue the closer you get to the utility transformer.

Item 4:
All I can offer is that if I were to be asked to place a bid for engineering services for such a facility, I would want to include a big chunk of hours to create a system model and to verify the as-build condition.

By the way, welcome to the Forum.

 

[ron]

1. Are UL listed panel boards to have an SCCR, independent of the OCPD. (I know the OCPD will finally determine the rating at the board per NEC).
The panel bus does have an inherent SCCR, but as you mention, takes on the rating of the lowest listed device that can be put in that panel.
2. What should be posted - the SCCR of the panel or the OCPD that limits the panel?
The SCCR is posted by the manufacturer on new equipment nameplate per its UL listing. If it is not, I don't know of any National code that requires it.
3. If the panel is posted with an SCCR based on the OCPD per UL listing and the panel is inadvertently derated by device(s) with a lower AIR rating then does the installation of that OCPD which derated void the UL listing for the board?
The OCPD that is placed in the panel must be on the list of acceptable OCPD's from the manufacturer to indicate that it has been tested as an assembly.
4. [Rhetorical] Assuming a LARGE facility (200+ panelboards, 6 service entrances, 3 levels of switchboard, on-site emergency generators, etc) with absolutely no records for available fault current, limited to no knowledge of motor loads and transformers on site contributing to fault current, partly complete and/or inaccurate one-lines, and old equipment most without ratings posted. How would a contractor correctly select an OCPD with the right AIR for an existing panel no knowing the available fault current and not having engineering calculations to indicate what the available fault current should be since this is an existing facility.
Go to the nearest upstream transformer and perform an infinite primary calculation from that point downstream.
Ethically and professionally, would you feel that the engineer of record is responsible for calculating that fault current for a small project considering that the panels used are an existing condition?
If it is an existing panelboard and you are just using an existing spare breaker, I don't think a calculation is needed for SCCR.

 

[Jraef]

Just out of curiosity, where is the requirement for panelboards to have SCCR ratings on them? I'm not disputing this, I just don't know.

What I can offer however is that it is not necessarily true that the rating of any system is always the rating of the lowest rated device. That would be true only for untested systems, i.e. those cobbled together in the field using combinations of components never tested together by the manufacturer. If they are "series tested" you can indeed have a higher overall rating on a complete system than what the lowest device is rated (but not higher than the highest rated of course). Typically then, a panelboard manufacturer will series test all of the valid devices for that panelboard.

So if you cobbled something together using components not intended to be used in a particular panelboard, it also follows that you have voided the UL listing of said panelboard. the importance of that is up to you. Once installed and inspected, it is owned by the user. An inspector can however disallow connections to that system if non-listed devices are in use and he sees it. You may also want to discuss the issue of voiding UL listings with your Loss Department, i.e. some insurance companies will have riders saying that they will not pay out on losses attributable to systems using unlisted equipment.

 

[davidr43229]

Are UL listed panel boards to have an SCCR, independent of the OCPD. (I know the OCPD will finally determine the rating at the board per NEC).

The OCPD within the panelboard is the final determination of the SCCR rating, not the OCPD outside of the panel or switchboard.

Just out of curiosity, where is the requirement for panelboards to have SCCR ratings on them? I'm not disputing this, I just don't know

NEC 2005, article 409, Embedded within that is a FPN of UL508A, which is the guideline to determine the SCCR.

If the panel is posted with an SCCR based on the OCPD per UL listing and the panel is inadvertently derated by device(s) with a lower AIR rating then does the installation of that OCPD which derated void the UL listing for the board

Yes, due to the fact that the SCCR rating was not determined with that newly component installed within .

How would a contractor correctly select an OCPD with the right AIR for an existing panel no knowing the available fault current

I agree with Ron, you have to make some assumptions here.

Ethically and professionally, would you feel that the engineer of record is responsible for calculating that fault current for a small project considering that the panels used are an existing condition

This code section, only pertains to panels designed and installed after the adoption of NEC 2005 and not existing installations, published with a "Withstand rating". Now if this is a new install, UL and the AHJ should not differentiate between a small or large installation
just my $.02

 

[dinkelja]

I don't see any point in marking anything yourself; unless you want to go all the way and calculate bolted fault kA and the arc flash hazzard. Are they really wanting you to do an Arc Flash Analysis too? With any current limiting devices in a PDP, the the limiting factor is most likely the bus SCCR. Usually current limiting CB's 65ka @ 480V, Class J, RK1, RK5 Fuses at 200kA. (my observations)

 

[jim dungar]

I believe we have gone off on a small tangent.

The OP was asking about article 408 panelboards not 409 industrial control panels.

Article 408 does not require short circuit labeling on a panelboard.

110.3(B) says if there is a manufacturer's label, it must be followed.
110.9 requires equipment to be rated for the current it will interrupt.
110.10 covers SCCR in a general manner
240.60(C) and 240.83 respectively require fuses and breakers to be marked with their interrupting ratings.
240.86 requires series-rated panels to be marked

 

[Jraef]

I believe we have gone off on a small tangent.

The OP was asking about article 408 panelboards not 409 industrial control panels.

Article 408 does not require short circuit labeling on a panelboard.

110.3(B) says if there is a manufacturer's label, it must be followed.
110.9 requires equipment to be rated for the current it will interrupt.
110.10 covers SCCR in a general manner
240.60(C) and 240.83 respectively require fuses and breakers to be marked with their interrupting ratings.
240.86 requires series-rated panels to be marked

That's what I was driving at too, I just wanted to hear it from the OP. He never mentioned article 408, but he did mention panelboards. Too many people are applying Article 409 requirements to things that it does not cover. I just went through this with someone on a packaged air handler that had a single local disconnect switch on it; he assumed he had to have an SCCR rating label on that disconnect switch.

SCCR labeling under Article 409 applies ONLY to "Industrial Control Panels", with a fairly good description right in the beginning of what is covered by it.

 

[Charles_Evans]

OP - Response

OP - Response

Thank you all for the insightful and well executed responses. Yes, I was speaking of article 408 and UL labeling.

Article 408 does not require short circuit labeling on a panelboard.

I agree, I did not find specific language that indicated that the NEC requires this listing be posted. However, UL does require that the maximum listed fault current be posted (for listed equipment).

(The following is just rambling and holds no real Code or Calculation related content.)

In discussing this with some other PE's I know, I raised the ultimate question. When are we liable for the mistakes of others?

For example:
What if there is an existing distribution panel board with no SCCR posted, board is fully populated with 10k breakers and the project cannot afford an exhaustive SCCR calc to determine the rating.

We do a down and dirty calc from the service entrance to the board and find the the calculated fault current at 20k.

1. Do we install a breaker with proper AIR rating based on calculation knowing full well that the board has been derated and can no longer effectively handle that calculated value. <-- What would be the point since the board is derated and would not carry the fault current. Are we at fault for a past mistake or is this on the owner's shoulder and by us putting in the correctly rated AIR breaker we are clean?

2. Do we tell the owner they need to change out all the breakers due to a fault current issue? This could mean extensive work considering this is a distribution board, multiple down panels and did I mention this is on a critical branch so outages would not be tolerated without extensive coordination in multiple departments. <-- Without more information I would have too since this could be dissasterous in a real fault condition. But should this hold up the project? Should I not just go ahead and put in the right AIR breaker and wash my hands of it?

3. Do I bag the job and only do new work? <--I don't believe this to be the right answer, we are in the business of safety and good engineering. We should always strive towards safety and proper design work no matter the cost. How much is a human life worth? Besides I love existing buildings, that is where the real fun is and the real engineering comes into play.

4. Ron brings up a fourth option, which really begs the question is it right of us to do this?

If it is an existing panelboard and you are just using an existing spare breaker, I don't think a calculation is needed for SCCR.

What if we use an existing breaker and don't do a SCCR calc. Who is at fault for that if a condition arises? <-- I would think the engineer should know better than to just use an existing breaker without verifying its SCCR with the calculated conditions, you never know what might have changed since the breakers installation. In addition, what about its functionality (How many people ensure the old existing breaker will actually trip under an overcurrent condition, I have seen many such cases where it doesn't). When is the last time you saw a contractor test an existing breaker or verify the fault current at that panel before making modifications?

 

[davidr43229]

I just went through this with someone on a packaged air handler that had a single local disconnect switch on it; he assumed he had to have an SCCR rating label on that disconnect switch.

Article 440.4 (B) includes SCCR on the air handler, not the disconnect. And I agree we have strayed (once again) from the OP post, sorry...
Just my $.02

 

[jim dungar]

As a PE, I am duty bound to advise and warn. If the panel does not meet the available SCA the customer must be advised.

OSHA (via their use of NFPA 70E) requires that existing equipment be able to handle the available fault current.