Question on NEC 2017 NEC 705.12(B)(2)(3)

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OK, you can call it an isolation switch if you prefer. And when the isolation switch is closed and the grid is up, everything is grid tied. So if the none of the inverters are outputting, the critical load panel would be drawing all its power from the grid.

[And obviously in lieu of a 500A 100% rated disconnect, you could use a 700A disconnect and 700A feeder (or 625A and use the next size up rule), but this seemed like a situation where the 100% rated disconnect might pay for itself, worth checking. You could also use a 600A disconnect and shrink one of your inverters slightly.]

Cheers, Wayne
 
milkyway said:
I think I follow somewhat. In this topology, would utility feed critical load panel during normal operation? That would be a requirement here.
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Yes, it would.

I don't like the word transfer switch because my brain thinks of a sitting diesel genset isolated from grid and not called upon until asked and transfer switched engaged.
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I'm with you. The code term is Micro-grid Interconnect Device. (MID) The heart of it is a normally open single throw relay.
 
jaggedben said:
Late to the thread, and I haven't read every word of it, but a couple clarifications...

First, the requirement to use 125% of inverter output is stated plainly near the beginning of 705.12. For example in the 2020 NEC it is 705.12(B) "The power source output circuit current multiplied by 125% shall be used in ampacity calculations for 705.12(B)(1) through (B)(3.)". Similar language can be found back to the 2014 NEC.

Second, I highly doubt that any AHJ (unless they are simply incompetent) will allow for less than the full output of both PV and storage, unless you have a listed Power Control System. Further, they may not even recognize Power Control Systems, since they aren't mentioned before the 2020 NEC and many places aren't on that yet. So if I were you and the customer does not have the budget for a 700A instead of 400A feeder to the micro-grid I would tackle that issue early on.
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First point, I agree 125% of inverter output is clearly stated to be used for calculations. The crux of my questioning was regarding upstream OCPD sizing which connects the switchgears. But that matter has been answered and made clear now.

Second point, I agree with you. Working with a vendor that has a sophisticated and listed Power Control System, thus my curiosity on how we can creative on this design by not needing to use full output power of all supply sources / inverters in calculations. But again as you mentioned, not all AHJ's will recognize this.
 
milkyway said:
First point, I agree 125% of inverter output is clearly stated to be used for calculations. The crux of my questioning was regarding upstream OCPD sizing which connects the switchgears. But that matter has been answered and made clear now.
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It may not be obvious, but everything that's in series between the power source and the service is part of the power source output circuit, regardless of it also being a normal feeder. So upstream OCPD can never be smaller than the system output. (Put another way, you can't guarantee there will always be some load.) Inverter output can only be bigger than power source output if it's a PCS.

Second point, I agree with you. Working with a vendor that has a sophisticated and listed Power Control System, thus my curiosity on how we can creative on this design by not needing to use full output power of all supply sources / inverters in calculations. But again as you mentioned, not all AHJ's will recognize this.
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2020 NEC is explicit that you can use 125% of the PCS output setting for upstream calculations required by 705. Hopefully your AHJ will see the reasonableness of following a code they may not be on for a couple years.
 
2020 NEC is explicit that you can use 125% of the PCS output setting for upstream calculations required by 705. Hopefully your AHJ will see the reasonableness of following a code they may not be on for a couple years.
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This is helpful. Would this verbiage be within section 705 in the 2020 NEC or in a different section? Need to get my handed on 2020 version.
 
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