110.24 Infinite Bus

jwfrsch

Member
Location
Virginia
Would it be considered a violation if due to reasons that could include not being comfortable with it or being uncertain about conductor sizes that the power company may choose to use, if someone marked a main panel board per 110.24, based on the infinite bus rating of the transformer Instead of calculating the resistance of the service wires?

If one were to do this, would they separately be required to do the calculation for the resistance of the wires to meet 110.16 V and ensure that the actual fault current is sufficient to trip the breaker in the clearing time on the label?
 
Would it be considered a violation if due to reasons that could include not being comfortable with it or being uncertain about conductor sizes that the power company may choose to use, if someone marked a main panel board per 110.24, based on the infinite bus rating of the transformer Instead of calculating the resistance of the service wires?

If one were to do this, would they separately be required to do the calculation for the resistance of the wires to meet 110.16 V and ensure that the actual fault current is sufficient to trip the breaker in the clearing time on the label?
The way I read 110.24 is the value does NOT have to be the actual value, just a maximum which could be interpreted to mean almost anything. That requirement is pretty much worthless.
 

jwfrsch

Member
Location
Virginia
The way I read 110.24 is the value does NOT have to be the actual value, just a maximum which could be interpreted to mean almost anything. That requirement is pretty much worthless.

I think the purpose of it is to ensure that a future service person who is adding to or otherwise modifying the equipment is aware of the available fault current so that they can select the appropriate interrupting ratings (someone doing electrical should know this anyway but we have to put a label there regardless). My question is this: a number of people as a matter of risk management may not want to calculate things like the impedance of the conductors from the utility and whether or not that reduces the available fault current at the service. Because if you print that there you could be accountable for it. Doing an infinite bus calculation based on the transformer’s impedance and requiring all equipment to have an interrupting rating that is corresponding to that word be safe to do although it might be overkill. The question is would this violate the NEC. Would the NEC require that you do the calculation on the conductors when in reality that calculation isn’t always that reliable because the impedance is caused by things like the power factor of the load, the type of raceway you’re using whether metal or plastic, and more. So some may prefer just to put the label that has the higher value instead of playing engineer and making a study to justify all your assumptions in impedances of the wiring.

Doing this may be fine for 110.24 but could cause problems with 110.16 B.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I agree that 110-24 requires only a maximum value. The reality is that the numbers people are calculating in most cases probably exceed by a lot the real values of the available SCC. This is unavoidable. For example, suppose the real available SCC from the utility is 10 kA. But the utility is telling everyone it is 20 kA to account for possible network changes, different transformers that might be used, etc. If you are basing whether the CB will open on the 20 kA number and it is really 10 kA, you could get a big surprise.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
I think available fault current is just that so that would be preferred but 110.24 seems more focused on interrupting ratings, so if the Utility wasn't forthcoming with the information I think infinite bus method would suffice. And I wouldn't see the need to include the impedance of the feeder.
But 110.16 B is arc-flash related and for that calculation I don't think infinite bus is appropriate. However, in some cases the Utility doesn't have the info so I would use different scenarios of Utility source strength ranging from infinite bus to maybe 1-2 MVA to see what's appropriate for the arc-flash label at the service equipment.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
But 110.16 B is arc-flash related and for that calculation I don't think infinite bus is appropriate. However, in some cases the Utility doesn't have the info so I would use different scenarios of Utility source strength ranging from infinite bus to maybe 1-2 MVA to see what's appropriate for the arc-flash label at the service equipment.
If I understand it correctly, the way that the software works to calculate arc flash IE values is to calculate for the worst case based on iterating from some minimal value of SCC to the maximum value of SCC.
 
I don't see how skipping conductor impedance in the calculation would be an issue or code violation. In my experience, when I have asked utility for the AFC value and could see the actual transformer and calculate it myself, their number is often two to three times what it actually is (I say actual, but I mean still assuming infinite bus, but using the actual transformer impedance). Of course often we have to go with the POCO figure because you may not have access to the transformer or it may not even exist yet. So the point is, these figures are generally not very accurate and you just need a maximum for 110.24.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
If I understand it correctly, the way that the software works to calculate arc flash IE values is to calculate for the worst case based on iterating from some minimal value of SCC to the maximum value of SCC.
This is a manual operation for most older software versions.
 

jwfrsch

Member
Location
Virginia
110.24 doesn’t so much deal with arc flash but it is more there to deal with making sure that the correct interrupting rating is selected. My point is that if someone is overly cautious in applying the label, might this be fine for 110.24 but create a problem with 110.16 because they will have labeled with a higher value and then might have also marked the clearing time on the breaker with the assumption of that very high value? If that higher marked value doesn’t actually exist, then the breaker may not open within the clearing time on the label. Or are people just putting two labels on the equipment? Or are they just not thinking about this?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
110.24 doesn’t so much deal with arc flash but it is more there to deal with making sure that the correct interrupting rating is selected. My point is that if someone is overly cautious in applying the label, might this be fine for 110.24 but create a problem with 110.16 because they will have labeled with a higher value and then might have also marked the clearing time on the breaker with the assumption of that very high value? If that higher marked value doesn’t actually exist, then the breaker may not open within the clearing time on the label. Or are people just putting two labels on the equipment? Or are they just not thinking about this?
You raise a good question. I had the same thought when this was added to the 2017 edition. I raised this issue at an IAEI section meeting right after this was put in the 2017 edition. It fell on deaf ears. 110.24 and 110.16(B) are an area of the code that is being wildly overlooked and many electricians and AHJs not even understanding the basics of these concepts. And POCOs that won't give accurate AFC is not helping.
 

jwfrsch

Member
Location
Virginia
Using caution in applying to labels per 110.24 might mean printing a high available fault current value while using caution for the requirement of 110.16B might actually be the opposite because if you’re going to put it on a label you better make sure that the circuit breaker will actually clear at the fault current level that’s there within the time that you state that it will. Because people are going to be making decisions about PPE based on that. If you put a higher fault current value in your calculation you run the risk of not having verified that the circuit breaker will actually operate in the time listed. It wasn’t such a big deal before because you could just take the cautious route but now you have to do a more complex calculation unless you want to have multiple labels on the equipment for multiple purposes which would just cause confusion. The problem is that AC impedance for things like the service conductors in between the transformer and service disconnect is actually not that simple of a calculation. And the NEC doesn’t specifically say that it’s acceptable to use just the DC resistance, so in addition to the problems you have with getting information from the utility, you now have the burden of making a decision on how to do this calculation and bearing with it the liability.

Adding to this is the new 240.87 requirement that you document that the circuit breaker clearing time when put in maintenance mode is below the available arcing current, which is actually different than the fault current and even more of a calculation involving many variables in order to achieve.

I think they should do a number of things:

1. There should be a proposal to the national electrical safety code that they add a rule requiring utilities to provide when a customer requests and whenever they change their system, what the available fault current is at the point of transition of ownership.

2. I think the NEC should specify what tables to use for calculating this impedance.
3. For 240.87 I think they should invest in doing some research in work to standardize products and come up with some kind of simpler system involving tables that’s more prescriptive when it comes to calculating available or concurrent because it has way too many variables to be something required by an installation standard without any details. They could work to standardize clearing times on circuit breakers. They could develop a table that may be requires certain clearing times based on the supplying transformer kVA and impedance percentages. Requiring a calculation of available arcing current is something that involves so many variables and it’s a topic of various IEEE papers. Those papers are often written by engineers with doctorate degrees and unlike things like Ohm’s law, they’re still changing in theory of how it should be calculated.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
Using caution in applying to labels per 110.24 might mean printing a high available fault current value while using caution for the requirement of 110.16B might actually be the opposite because if you’re going to put it on a label you better make sure that the circuit breaker will actually clear at the fault current level that’s there within the time that you state that it will. Because people are going to be making decisions about PPE based on that. If you put a higher fault current value in your calculation you run the risk of not having verified that the circuit breaker will actually operate in the time listed. It wasn’t such a big deal before because you could just take the cautious route but now you have to do a more complex calculation unless you want to have multiple labels on the equipment for multiple purposes which would just cause confusion. The problem is that AC impedance for things like the service conductors in between the transformer and service disconnect is actually not that simple of a calculation. And the NEC doesn’t specifically say that it’s acceptable to use just the DC resistance, so in addition to the problems you have with getting information from the utility, you now have the burden of making a decision on how to do this calculation and bearing with it the liability.

Adding to this is the new 240.87 requirement that you document that the circuit breaker clearing time when put in maintenance mode is below the available arcing current, which is actually different than the fault current and even more of a calculation involving many variables in order to achieve.

I think they should do a number of things:

1. There should be a proposal to the national electrical safety code that they add a rule requiring utilities to provide when a customer requests and whenever they change their system, what the available fault current is at the point of transition of ownership.

2. I think the NEC should specify what tables to use for calculating this impedance.
3. For 240.87 I think they should invest in doing some research in work to standardize products and come up with some kind of simpler system involving tables that’s more prescriptive when it comes to calculating available or concurrent because it has way too many variables to be something required by an installation standard without any details. They could work to standardize clearing times on circuit breakers. They could develop a table that may be requires certain clearing times based on the supplying transformer kVA and impedance percentages. Requiring a calculation of available arcing current is something that involves so many variables and it’s a topic of various IEEE papers. Those papers are often written by engineers with doctorate degrees and unlike things like Ohm’s law, they’re still changing in theory of how it should be calculated.
I couldn't agree more.
I think the electrical industry has over-reacted to arc flash with 240.87 and the new 240.67 for fuses. Not to mention NFPA 70E where the only permissible energized work is on systems where it's more dangerous to turn it off.
But as an electrical power systems engineer who makes a living doing arc flash studies I appreciate it because there's lots of opportunities for me and my SKM power system analysis software package. :)
 
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