available fault current and 2011 NEC

mjmike

Senior Member
Under the 2011 NEC, there was a change (new section) at 110.24(B) that requires the service entrance maximum available fault current to be checked if something changes that effects this. So lets say a system was designed for 39kA available at the service with twenty 10kA branch panels. Down the road, there is a service upgrade and the new available fault current is around 68kA. At this new kA rating, the 20 branch panels now may no longer be acceptable.

Based on the new code section, it appears only the service panel needs to be checked and updated. It does not say anything about downstream panels. Is my understanding correct in that distribution equipment other than the service panel does not need checked?

Secondly, if under the 2008 NEC, there was no requirement for the AIC to be checked. So lets say there was a building addition and the utility changed their transformer and a service entrance main breaker was changed. The service panel bussing was still adequate. The AIC would not need checked, is this correct?

There are service changes all the time and it does seem impractical to have a service upgrade require all the branch panels to be checked and possibly changed out so it looks like the code is addressing this by just requiring the service panel to be checked.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
There is nothing in 110.24 that requires you to do anything more than provide a label stating the maximum available fault current.
I don't see any real purpose for that code section.
110.9 and 110.10 require that you install equipment suitable for the available fault current and those sections have been in the code for sometime.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
There is nothing in 110.24 that requires you to do anything more than provide a label stating the maximum available fault current.
I don't see any real purpose for that code section.
110.9 and 110.10 require that you install equipment suitable for the available fault current and those sections have been in the code for sometime.
Posting the available fault current, per 110.24, makes it easier for a visual inspection that 110.9 and 110.10 have been, and will be, complied with.
Many times the utility fault level is not shown on the plans sent in for review and permitting, so there is no official record.
Yes, the value supplied is most likely the utilities 'design' or minimum required level and probably drastically exceeds the actual available amount.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Posting the available fault current, per 110.24, makes it easier for a visual inspection that 110.9 and 110.10 have been, and will be, complied with.
...
At that time it is really too late...the wrong equipment has already been installed. This needs to be addressed at the design and or plan review stage, not at the inspection stage.

It may be helpful for future work, assuming the posted information is still correct.

Yes, I am aware that the information is required to be updated, but given that it is really utility changes that will change the available fault current at the service equipment, it is not really likely that the label will be updated when it should be. The utilities do provide that information to their customers, but how often does that information get to someone who has any idea what it means or what to do with it?
 

mjmike

Senior Member
Let me simplify my question, if the AIC rating into the building changes say from 42kA to 65kA, 110.24(B) appears to require the service panel to be changed. Noting appears to require switchgear served by a feeder to require replacement. Am I understanding this correctly?
 

kwired

Electron manager
Location
NE Nebraska
Let me simplify my question, if the AIC rating into the building changes say from 42kA to 65kA, 110.24(B) appears to require the service panel to be changed. Noting appears to require switchgear served by a feeder to require replacement. Am I understanding this correctly?

You were required to install equipment according to its ratings before 110.24 ever existed and still are.
110.24 merely makes us mark that equipment with the available fault current, and has nothing to do with whether or not the fault current and equipment actually are compatible with each other.

When changing equipment and resulting in higher fault current levels, there is still a possibility you can leave existing equipment if it is series rated for whatever you have upstream from it. Also if you feed something existing from a newer larger capacity supply, you can possibly still have reduced current levels at the existing equipment depending on size and length of conductors supplying it.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
At that time it is really too late...the wrong equipment has already been installed...
It might prevent it from being energized.

It may be helpful for future work, assuming the posted information is still correct.

Yes, I am aware that the information is required to be updated, but given that it is really utility changes that will change the available fault current at the service equipment, it is not really likely that the label will be updated when it should be. The utilities do provide that information to their customers, but how often does that information get to someone who has any idea what it means or what to do with it?
Major utilities in this area, IL, MI, MN, and WI, often supply 'design level' fault currents to be used for equipment selection. These values will almost never ever be exceeded, without a change to the customer's equipment.

For example, Xcel Energy says:"...show the available RMS symmetrical fault currents that may be expected at the secondary terminals of distribution transformers. Each fault current value listed in the tables is based on the lowest percent impedance transformer that might be set initially or as a replacement. No primary source or secondary line impedance has been included since it is generally relatively small, may change, and cannot be accurately forecasted."
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Jim,
I just don't see a use for this rule. If they are not going to look at what the fault current is before the project is built, I don't see them posting a number that exceeds the rating of the installed equipment.

Typically we are talking larger installations where there is some type of plan review. I know in a lot of areas, you do not get approved plans without showing the available fault current from the utility.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
Typically we are talking larger installations where there is some type of plan review. I know in a lot of areas, you do not get approved plans without showing the available fault current from the utility.
We normally need to contact the utility for fault current info for our 'systems study'.
Most of the plans, I have seen lately, do not show the utility fault current. While there may be a note that says to provide xxxAIC rated breakers, there is a note that says to confirm fault current with local utility. In many cases it appear the 'designer' doesn't even know the name of the POCO.

If the fault current is known at plan review time, then posting it on the service entrance equipment is not onerous. As-built drawings are almost never available. It seems every 'old' one-line I have have looked at is stamped 'not for construction'.

No, I would not trust this label as gospel. I would only use it as one of my evaluation tools.

Personally, I think it makes a just little more sense than the NEC arc flash warning one.
 
I have a customer that was required to make significant changes to downstream panels and equipment due to upgraded service with a higher available fault current. This was in Lakeland, FL but the customer was faced with complying or having their CO revoked. They subsequently took on an internally driven review of all of their plants and planned to increase the minimum acceptable SCCR of all puchased panels to keep from being caught again. Many existing panels have SCCRs of only 5 or 10kA and they were looking at making the minimum 65kA. I am fairly certain that arc flash is the main driver of all of that.
 

kwired

Electron manager
Location
NE Nebraska
I have a customer that was required to make significant changes to downstream panels and equipment due to upgraded service with a higher available fault current. This was in Lakeland, FL but the customer was faced with complying or having their CO revoked. They subsequently took on an internally driven review of all of their plants and planned to increase the minimum acceptable SCCR of all puchased panels to keep from being caught again. Many existing panels have SCCRs of only 5 or 10kA and they were looking at making the minimum 65kA. I am fairly certain that arc flash is the main driver of all of that.
Increasing the withstand rating of a piece of equipment does not change the potential arc flash hazard. When there is a fault the energy is still released in the blast. The ability of equipment to withstand a particular fault current simply is how much energy can the equipment handle during such an incident without self destruction. If you short the bus bars of a panelboard with 65kA short circuit rating you still get a big arc flash, but the breaker has a much better chance of surviving 65kA than a breaker with a lower rating. Any current limiting features may help reduce effects but the full potential energy is still available on the supply side of the current limiting device should the fault happen there.
 

kwired

Electron manager
Location
NE Nebraska
SCCR is not the right term for the equipment. It is AIC. SCCR is more for HVAC, motor controllers, elevators, and control panels.
SCCR would apply to the panel bus and how well it is secured and able to withstand high currents, but AIC would apply to the breakers or any other interrupting devices. The SCCR of a panelboard bus is likely much higher than the AIC of the breakers attached to it though.
 

mjmike

Senior Member
Increasing the withstand rating of a piece of equipment does not change the potential arc flash hazard. When there is a fault the energy is still released in the blast. The ability of equipment to withstand a particular fault current simply is how much energy can the equipment handle during such an incident without self destruction. If you short the bus bars of a panelboard with 65kA short circuit rating you still get a big arc flash, but the breaker has a much better chance of surviving 65kA than a breaker with a lower rating. Any current limiting features may help reduce effects but the full potential energy is still available on the supply side of the current limiting device should the fault happen there.
I think what the post is getting at, is by increasing to 65kA they are planning for an increase in kA in the future. For example, instead of placing a 22kA panel they could end up seeing say 30kA someday in the future, they will put in 65kA now to avoid changing the panel in the future.

However, assuming fully rated, I would like to see them find 65kA branch circuit breakers for regular load centers such as Square D NQ panels. you are limited in kA ratings for basic breakers like 10A, 15A, 20A, etc.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
However, assuming fully rated, I would like to see them find 65kA branch circuit breakers for regular load centers such as Square D NQ panels. you are limited in kA ratings for basic breakers like 10A, 15A, 20A, etc.
They have ratings up to 65kAIC when you use their QO-VH and QH breakers. Rarely would you find these breaker stocked locally, they are almost always special order.

Manufacturers have been successfully implementing Series ratings, for these type of panels (e.g. 200A branch circuit panels), for almost 30 years.
 

kwired

Electron manager
Location
NE Nebraska
I think what the post is getting at, is by increasing to 65kA they are planning for an increase in kA in the future. For example, instead of placing a 22kA panel they could end up seeing say 30kA someday in the future, they will put in 65kA now to avoid changing the panel in the future.

However, assuming fully rated, I would like to see them find 65kA branch circuit breakers for regular load centers such as Square D NQ panels. you are limited in kA ratings for basic breakers like 10A, 15A, 20A, etc.
They have ratings up to 65kAIC when you use their QO-VH and QH breakers. Rarely would you find these breaker stocked locally, they are almost always special order.

Manufacturers have been successfully implementing Series ratings, for these type of panels (e.g. 200A branch circuit panels), for almost 30 years.
I agree with Jim, series ratings may mean you only need a main with the higher rating and others are rated for in series with that main. This basic concept happens on your basic QO and Homeline loadcenters all the time. A factory installed main breaker in a 225A or less panel is typically a 22KAIC rated breaker, but can have 10KA branch breakers installed even if the available current is more than 10K.

Problems with series ratings is most manufacturers only test and list their own equipment, so if you have a main in GE switchgear and a QO panelboard being supplied, there is likely no published information that this combination of equipment has been tested/listed, and to have required action done for approval may cost more than other alternatives, but if the main gear were Square D chances are much greater this information is easier to obtain just from published material.
 
The panels in question were almost all control panels. I believe the NEC requires the SCCR of an industrial control panel to meet or exceed the available fault current of the feeder or branch supplying it. Don't have the code handy to look up the articles but there are lots of papers and what not out there from the circuit protection manufacturers about compliance. Some sources I look at say this DOES affect arc flash calculations but I am not expert on arc flash at all.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
The panels in question were almost all control panels. I believe the NEC requires the SCCR of an industrial control panel to meet or exceed the available fault current of the feeder or branch supplying it. Don't have the code handy to look up the articles but there are lots of papers and what not out there from the circuit protection manufacturers about compliance. Some sources I look at say this DOES affect arc flash calculations but I am not expert on arc flash at all.
One, NEC reference is 110.10.

The SCCR does not affect the calculations for arc flash incident energy. However, it may affect the risk assessment you perform when developing your safe work practices and make your PPE choices.
 
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