Arc flash calculation for an existing building

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anbm

Senior Member
Location
TX
Occupation
Designer
If we install a new electrical panel
(fed out of an existing panel in existing building)
I know NEC requires new arc flash label for
it. We had note on plan for contractor
to carry this task (i.e hire 3rd to do arc
flash calculation and print label)...
my question is what is the limitation
for this arc flash calculation in order to
get the arc flash label for new panel?
Or how to perform arc flash calculation
for existing building?

It appears this will be huge cost issue if the calculation
will need total fault current calculation for entire electrical
distribution first in order to calculate arc flash level
at new panel level...

In future, if we want to add more new panels...
Will 3rd party start over the whole fault current
calculation again and then do arc flash cal.? Does NEC
have any exceptions for existing building in this case?

Thank you.
 
If the new panel is not fed from an adjacent transformer, there may be enough wire length (resistance) upstream of the new panel that the fault current can be given a practical limit based only on that impedance. The other part of the arc energy calculation will be the clearing time of whatever OCPD is expected to open first under various fault current levels.

If you are lucky, setting a useful upper limit on arc energy may be possible without examining the wiring all the way upstream to the service and adding in POCO fault current information.

Often several energy calculations have to be done assuming different fault current values and using the case that delivers the highest energy over the total time before the fault clears.

In any case, unless there is expected to be a large motor contribution to the fault current, it will not be necessary to examine the whole building, just the specific path upstream to the service.
 
You can comply with 110.16 with the generic sort of label that manufacturers provide that don't give any values or require calculations, just a warning. Refer to the Exhibit in the NEC Handbook.
 
dkidd said:
You can comply with 110.16 with the generic sort of label that manufacturers provide that don't give any values or require calculations, just a warning. Refer to the Exhibit in the NEC Handbook.
Click to expand...

The manufacturer still need to do the arc flash calculation before they can come up with the warning label right?
 
anbm said:
So the arc flash label does not need to have hazard risk category information on it?
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not by code. it is just a generic warning.

osha requires it indirectly by more or less enforcing NFPA70E which does require it.

I don't see how you can calculate the IE without a pretty decent and expensive survey of the electrical system if one has not been performed recently.

Just out of curiosity, how did you calculate the available short circuit current? You would need to have that to be able to properly specify the panel board.
 
petersonra said:
not by code. it is just a generic warning.

osha requires it indirectly by more or less enforcing NFPA70E which does require it.

I don't see how you can calculate the IE without a pretty decent and expensive survey of the electrical system if one has not been performed recently.

Just out of curiosity, how did you calculate the available short circuit current? You would need to have that to be able to properly specify the panel board.
Click to expand...

Got it now, we only need warning label not arc flash label at panels. Which facility does require arc flash label or this is up to the client?
 
anbm said:
Which facility does require arc flash label or this is up to the client?
Click to expand...

Everything is up to the guy paying the bills. How they chose to do it is also up to them.

I suspect what you really want to know is how much of it do you have to do. No one here can answer that because no one here is privy to the contract. The contract will specify what services your company are to supply.
 
Arc Flash Lablels

Arc Flash Lablels

NFPA 70E 2017 is only 110 pages. C'mon, that's less than our OLD 197x code books that we used to read for fun...you can handle it.

NFPA 70E 2017 is quite specific and is law under OSHA. In the Forward to the 2017 version, Item 10, you'll see "Field-marked equipment labeling requirements were revised to require the label to be updated where the arc flash hazard risk assessment identifies a change that renders the label inaccurate. Also, the documentation, installation, and maintenance of the field-marked label is the responsibility of the owner of the electrical equipment. [130.5(D)]" It would appear the "Owner" has hired you to do that documentation and installation of the field-marked label.

What should be on the label? Per 130.5(D), (D) Equipment Labeling.
Electrical equipment such as
switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in
other than dwelling units and that are likely to require examination, adjustment, servicing, or maintenance while energized
shall be field-marked with a label containing all the following information:
(1) Nominal system voltage
(2) Arc flash boundary
(3) At least one of the following:
a. Available incident energy and the corresponding working distance, or the arc flash PPE category in Table 130.7(C)(15)(A)(b) or Table 130.7(C)(15)(B)
for the equipment, but not both
b. Minimum arc rating of clothing
c. Site-specific level of PPE
Exception: Labels applied prior to September 30, 2011 are acceptable if they contain the available incident energy or required level of PPE.
 
110.24(A) Field marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved.

So an arc flash warning label is not all that is needed.

The available fault current marking addressed in 110.24 is related to required short circuit current ratings of equipment. NFPA 70E Provides assistance in determining the severity of potential exposure, planning safe work practices, and selecting personal protective equipment.
 
anbm said:
If we install a new electrical panel
(fed out of an existing panel in existing building)
I know NEC requires new arc flash label for
it. We had note on plan for contractor
to carry this task (i.e hire 3rd to do arc
flash calculation and print label)...
my question is what is the limitation
for this arc flash calculation in order to
get the arc flash label for new panel?
Or how to perform arc flash calculation
for existing building?

It appears this will be huge cost issue if the calculation
will need total fault current calculation for entire electrical
distribution first in order to calculate arc flash level
at new panel level...

In future, if we want to add more new panels...
Will 3rd party start over the whole fault current
calculation again and then do arc flash cal.? Does NEC
have any exceptions for existing building in this case?

Thank you.
Click to expand...

You dont need to know the entire electrical distribution system fault current to calculate the arc flash at that panel unless the panel you are adding has motors involved which will act as generators under fault conditions and inject the fault current into the system. If there are no motors involved you need to know what is the fault current at the upstream of the panel like a main panel or a disconnect feeding the panel..Once you know that you or who so ever is doing it will just model it from there and draw the oneline and calculate the arc flash.

Now how do you get the fault current ahead of the panel that you are gonna put in? I am pretty sure there must have been some recent short circuit studies done on that building..grab that and then you are all set...
 
shortcircuit1 said:
You dont need to know the entire electrical distribution system fault current to calculate the arc flash at that panel unless the panel you are adding has motors involved which will act as generators under fault conditions and inject the fault current into the system. If there are no motors involved you need to know what is the fault current at the upstream of the panel like a main panel or a disconnect feeding the panel..Once you know that you or who so ever is doing it will just model it from there and draw the oneline and calculate the arc flash.

Now how do you get the fault current ahead of the panel that you are gonna put in? I am pretty sure there must have been some recent short circuit studies done on that building..grab that and then you are all set...
Click to expand...

But the fault current used for an short circuit study is not normally the same fault current used for an arc flash study.

Short circuit study: Use a conservative value for fault current. The highest fault current the service could ever see. It might even account for a transformer replacement where the new transformer might have a lower impedance.

Arc flash study: Use a realistic value. The most likely value or range for actual fault current at the service given the current installation.
 
cowboyjwc said:
110.24(A) Field marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current.
Click to expand...
That applies only to the service equipment. This thread is discussing a panel added to the distribution system.

 
azhapp said:
It would appear the "Owner" has hired you to do that documentation and installation of the field-marked label.
Click to expand...

I don't think we have any idea what the owner has hired anyone to do.

NFPA 70E is a maintenance guide. Even if it does require a panel specific label (& I'm not convinced it does) I don't believe all the requirements necessarily fall on the design engineer, or the installing contractor.

The NEC very clearly requires a generic label. I think that meets the intent for an installation.

From a practical side, it seems like the type of installation should also be considered. For example, there are thousands of small panels in small commercial businesses that should simply be shut off before any hot work is performed. And since these are frequently small services, the arc flash energy is normally low enough that a calculation seems like overkill.

On the other hand, there are also a lot of factories, where its almost impossible to shut down a panel. And these tend to have larger services, more 480V, and probably higher arc flash energy. Seem like these panels should probably have specific labels, although I think that responsibility falls back on the owner.
 
steve66 said:
I don't think we have any idea what the owner has hired anyone to do.

NFPA 70E is a maintenance guide. Even if it does require a panel specific label (& I'm not convinced it does) I don't believe all the requirements necessarily fall on the design engineer, or the installing contractor.

The NEC very clearly requires a generic label. I think that meets the intent for an installation.

From a practical side, it seems like the type of installation should also be considered. For example, there are thousands of small panels in small commercial businesses that should simply be shut off before any hot work is performed. And since these are frequently small services, the arc flash energy is normally low enough that a calculation seems like overkill.

On the other hand, there are also a lot of factories, where its almost impossible to shut down a panel. And these tend to have larger services, more 480V, and probably higher arc flash energy. Seem like these panels should probably have specific labels, although I think that responsibility falls back on the owner.
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I think a specific label is required with both the available short circuit current and a label that says what the IE is and what PPE is required to deal with the arc flash risk.

It is the owner's responsibility to see that these labels are applied. He can pay a third party to do the site survey and calculations and apply the labels but he is the one responsible to see it is done.

The electrical code does require a generic type label and I think it is reasonable to suggest the installing contractor is responsible for that under the generic heading of "meeting NEC requirements". But the NEC does not require anything more than the generic label as far as arc flash goes.
 
steve66 said:
But the fault current used for an short circuit study is not normally the same fault current used for an arc flash study.

Short circuit study: Use a conservative value for fault current. The highest fault current the service could ever see. It might even account for a transformer replacement where the new transformer might have a lower impedance.

Arc flash study: Use a realistic value. The most likely value or range for actual fault current at the service given the current installation.
Click to expand...

Yes short circuit study uses bolted fault current to determine withstand ratings of the devices and arc flash uses arcing fault current to determine the incident energy and arc flash boundary..Software takes care of this you dont have to specify different fault currents.Once you specify the available short circuit at the panel it will calculate arc flash and available short circuit on the new panel.
 
shortcircuit1 said:
Yes short circuit study uses bolted fault current to determine withstand ratings of the devices and arc flash uses arcing fault current to determine the incident energy and arc flash boundary..Software takes care of this you dont have to specify different fault currents.Once you specify the available short circuit at the panel it will calculate arc flash and available short circuit on the new panel.
Click to expand...

I always use different numbers, and I think you will find that's the case with most people doing arc flash studies.

For short circuit studies, I usually use the transformer impedance (or maybe even the utilities specification for the lowest impedance transformer they will ever install), and I usually assume infinite available current on the primary. That's pretty standard for short circuit studies.

For arc flash, I always use the actual xformer impedance, and they utility company always does a study of their upstream network to give me a realistic estimate of the available current on the xformer primary.
 
steve66 said:
I always use different numbers, and I think you will find that's the case with most people doing arc flash studies.

For short circuit studies, I usually use the transformer impedance (or maybe even the utilities specification for the lowest impedance transformer they will ever install), and I usually assume infinite available current on the primary. That's pretty standard for short circuit studies.

For arc flash, I always use the actual xformer impedance, and they utility company always does a study of their upstream network to give me a realistic estimate of the available current on the xformer primary.
Click to expand...

In most of the cases utility generally gives the available short circuit on the xfmr's secondary.Then you have the cable length and size to consider before it gets to the building or sometimes they give the short circuit where the service enters the building...
 
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