Available Fault Current - 110.24 (B) Exception

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solarmatt

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Location
TN
Occupation
PV Engineer
TL;DR: Do most commercial/industrial electrical services use the exemption in NEC 110.24(B), or is this rule just rarely enforced?

Hi everyone,

I’ve had several inspectors ask for available fault current labeling on our PV installations, for both load-side and line-side points of interconnection. From what I understand, only two NEC rules directly address fault current labeling: 110.16(B) and 110.24(B).

  • Load-side systems:
    Our installations don’t involve service equipment, so 110.16(B) doesn’t apply.
  • Line-side systems:
    Our AC disconnect is considered service equipment. If rated at 1200A or more, the labeling in 110.16(B) applies.
However, 110.24(B) is less clear. It requires updating the fault current labeling mandated by 110.24(A) when modifications affect the maximum available fault current at the service. But in most cases, the existing service equipment doesn’t have a fault current label per 110.24(A).

So my questions are:

  1. Is this rule often unenforced, or are these facilities relying on the 110.24(B) exception (industrial facilities exempt when only qualified persons service the equipment)?
  2. If the exception is commonly used, can I argue that it should apply to our PV installations?
  3. If this is a case of inconsistent enforcement, how do I handle being asked to update a nonexistent label on equipment we didn’t install?
For context, I’m trying to avoid getting into debates about the negligible fault current contribution from most PV installations, and I suspect some inspectors might also be pushing for NFPA 70E compliance, which isn’t enforceable.

Most of our work is under the 2017 NEC.

Thanks in advance for your insights!
 
Most of the time when I cite 110.24 for service equipment the electrical contractors don't understand the requirement or claim no one has ever made them do this before. Often they mistakenly feel arc flash hazard labels on the equipment meets the requirement.

I've had one PV contractor call for the exception for 110.24(B) on a 4mw project. My rebuttal was that it is not an industrial occupancy as defined by NFPA

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It took them a few weeks, but they were able to label the service equipment correctly.

I try to tell contractors upon permit submission that this is a requirement, but most don't pay attention.
 
MyCleveland said:
May I ask what is shown to you that indicates the number was calculated or just a bunch of lotto numbers stringed together?
Click to expand...

It was a signed/stamped engineered document that they provided from the utilities fault current calculation.

Usually on smaller projects it tends to just be a document from the utility showing their calculations.
 
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If the use of the facility matches the the industrial occupancy definition in 3.3.205.8 of NFPA 101, what sorts of documentation would qualify the installation for the exemption? Many of our projects could fit this definition, if we could verify that "conditions of maintenance and supervision ensure that only qualified persons service the equipment."

And in the cases where the exemption doesn't apply, are there acceptable methods of documenting the AFC calculation that do not require a PE stamp? This is especially important for PV systems, since the fault current contribution by the PV system is not a part of most fault current calculation tools like Bussman FC2 or Mike Holt's spreadsheet.

I'm trying to avoid adding additional engineering fees to our projects, if possible.
 
solarmatt said:
...

And in the cases where the exemption doesn't apply, are there acceptable methods of documenting the AFC calculation that do not require a PE stamp? This is especially important for PV systems, since the fault current contribution by the PV system is not a part of most fault current calculation tools like Bussman FC2 or Mike Holt's spreadsheet.

I'm trying to avoid adding additional engineering fees to our projects, if possible.
Click to expand...

Why not just do one of those calculations and then simply add the max fault current from the inverter(s) datasheet?
 
Unless there is other contract language, I would simply provide a letter with the amount of fault current the PV system produces.
It is then up to the customer to use this information however they want/need to.

Your contribution will be minimal. For the most part inverters only produce about 110% of their full load output as fault current.
 
jim dungar said:
Unless there is other contract language, I would simply provide a letter with the amount of fault current the PV system produces.
It is then up to the customer to use this information however they want/need to.

Your contribution will be minimal. For the most part inverters only produce about 110% of their full load output as fault current.
Click to expand...
Reversing the fault direction...what type of rating values (SCCR) do the inverters carry typically?
Vary by manufacturer? output rating? or other?
 
Thanks guys. I'll do that on our projects moving forward.
For the record, not all inverter datasheets include maximum fault current, but the CPS string inverters do.
Standardizing on this will be easier than asking to apply an exception anyway.


MyCleveland said:
Reversing the fault direction...what type of rating values (SCCR) do the inverters carry typically?
Vary by manufacturer? output rating? or other?
Click to expand...
I wouldn't assume an inverter would have an SCCR, but I could be wrong.
 
MyCleveland said:
Reversing the fault direction...what type of rating values (SCCR) do the inverters carry typically?
Vary by manufacturer? output rating? or other?
Click to expand...
If a PV inverter allows short circuit current to pass through it (like, from the AC side to the DC side) then basically the inverter has failed already. So denoting an SCCR would make little sense. The short circuit would happen because the inverter failed, rather than vice versa.
 
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