jneilson
Good morning.

I have a 13kw set with a 100A limited circuit transfer switch.  In the transfer switch installation manual, there's a paragraph about Fault Current, and a label that needs to be placed on the cover of the transfer switch for the AHJ.  I take it that this has to do with the generator appliance which has a 60A double pole breaker on it and not the 100A breaker feeding the transfer switch from the utility.

So, my interpretation is that the Fault Current Rating is 60 amps and that the Available Fault Current would also be 60 amps. Am I correct?

Thanks for your help!
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Brian Baughman
The fault current label applies to the available fault current from the normal power source and the actual short circuit fault current rating of the ATS.   Generators actually do not produce high amounts of fault current, but you 13 kW produces more than 60A of fault current.  Using the transient and subtransient values from the alternator the actual fault current can be calculated. This is why NEC Article 445.11 and UL 2200 requires this information to be on the generator dataplate or installation instructions.

As the label applies to the ATS in the 2017 NEC:

In one- and two-family dwellings, only the fault current rating of the ATS has to be labeled on the ATS.  A RXSW200A3 ATS is rated at 22,000 A.
In commercial installations, you are required to perform an available fault current calculation in accordance with acceptable industry practice for non-service rated ATS's.  For service rated, your utility provider can provide you with the actual number.  The available fault current and the date the calculation was performed, and fault current rating of the ATS has to be field labeled on the ATS.

As the label applies in the 2020 NEC,  it's same as the 2017 NEC except the label is no longer required for installations on one- and two-family dwelling units.

The reason why the short circuit rating of the ATS based on the specific overcurrent device protecting the ATS has to be field labeled on the ATS is that many large amperage ATS's are listed with multiple short circuit current ratings.  Generac has some ATS's where this range is between 22,000 to 200,000 A based on the upstream overcurrent protection device.  The label is required so personnel can equip the proper PPE based on the available fault current and the rating of the equipment.
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jneilson
Brian,

I apologize for the late response, and I'm finally getting to the point where I can use it.  This is very helpful and I appreciate it!

Now, something else has come up.  I have followed the transfer switch wiring diagram to a tee (my opinion).  However, a supervisor for the electrical inspector told me over the phone that circuit grounds entering my main panel need to be landed on the shared neutral/ground bus in my 1980 Murray panel, and cannot pass through.  This makes no sense as I have a #6 ground running from the sub panel ground bus to the neutral/ground bus in the main panel.  Please see attached photo.  Do you know of any code that supports this?  I'm making up all wires in the j-box, and he says that only neutrals and hots can pass through.

I appreciate your help.

Thanks,
John
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Brian Baughman
The equipment grounding conductors from the branch circuit should of remained connected to the equipment grounding terminals in your main panel, so your inspector is 100% correct.  Your NEC code reference is 250.148.
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jneilson
Brian,

Thanks for getting back on this matter; I appreciate it very much.  Oh boy, more rework.  The main panel is so cluttered that I thinking of adding a 14 conductor grounding bus where I have some space and tie that into the bus in the main panel.  I've got (1) 10 ga., (5) 12 ga., and (8) 14 ga. grounds, so would a #4 between the bus and the main neutral/ground bus be large enough?

The frustrating part is that the Generac sub panel has a separate ground bus which won't be needed.  And the sub-panel wiring diagram shows exactly what I did.  So, what good is the ground bar in the sub panel if ground wires cannot pass through?

Thanks again,
John
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jneilson
Here's the Generac wiring diagram that I was referring to.
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Brian Baughman
The separate ground bar is there in case an installer needs to install new branch circuits from that panelboard.  The junction box in the wiring diagram that you attached is in between the generator and ATS.
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jneilson
Brian,
Good evening.
I installed a continuous multi conductor feed between the generator and ATS, and installed a 12X12 j box between the main panel and ATS, pictured herein, to make up the wiring.  The Generac wiring diagram calls for ground ties between the sub panel bus and the bus in the main panel.  So, I'm not understanding the need to land circuit grounds in the main panel when both are tied together.  The code reference provided primarily refers to j boxes and not passing through a main panel in a situation where the sub panel is grounded back to it.
Thanks for your help!
John
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jneilson
Brian,
Please see highlighted paragraph in the attachment.
Thanks,
John
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