Commercial Rooftop NEC 2014 - 690.47(C) &(D)

[pcanning87]

Hey Guys,

First time designing a commercial rooftop with 2014 Code, I have rapid shutdown covered through string inverters on the roof and contactor disconnects for DC circuits > 10 feet from the array.

I'm planning on meeting 690.47(D) by running a new DC GEC from building steel to a ground lug on each separate, UL2703 listed, racking array. My thought is to meet 690.47(C) by connecting this DC GEC to each inverter, and therefore avoid using 690.47(C)(3). So each inverter would have a separate EGC and GEC grounding connection.

Assuming the building steel is part of the existing premises grounding electrode system, my GEC is sized per 250.166 and the GEC is installed in accordance with all of 250, am I missing anything? Is there any room for simplification?

Thanks in advance.

 

[jaggedben]

On first consideration it seems to me sufficient to meet the code. Whether it's the most advisable safety-wise might be debatable (see https://www.youtube.com/watch?v=YuDqXFvRv94). But if the building is primarily steel construction then I suppose it probably doesn't matter so much.

 

[pcanning87]

On first consideration it seems to me sufficient to meet the code. Whether it's the most advisable safety-wise might be debatable (see https://www.youtube.com/watch?v=YuDqXFvRv94). But if the building is primarily steel construction then I suppose it probably doesn't matter so much.

The building frame is entirely steel construction. I definitely understand the concerns with 690.47(D), Do you think the system would be safer with either of these changes:
1. Connect the supplementary GEC only to the racking and use 690.47(C)(3) so the inverters are not directly connected to the supplementary GEC/GE.
2. Pull the supplementary GEC from a ground rod instead of building steel.

Thanks again.

 

[jaggedben]

If the concern is lightning I'm really no expert. However to the best of my understanding neither of those things would make anything safer. You would still have EGCs from arrays to inverters to AC tie-in locations, and thus still have loops that lightning or other stuff could follow. The best thing in my opinion would be to run grounding for all arrays to a single point on the grounding electrode system (the conductors could be EGCs), without creating multiple connections to the ground. But if you're concerned about satisfying your AHJ that you've followed 690.47(D), that may not be sufficient. There's a reason that section is being changed again in 2017 to no longer be a requirement.

You may want to look at http://forums.mikeholt.com/showthread.php?t=181119 and what has been said about another electrode not being required if the structure already has one. Your situation is a little different though.

 

[pv_n00b]

First determine if the building steel is bonded to the grounding electrode system, it’s not automatic. If it is then connect a GEC from the array mounting to building steel.

But if you are installing a roof mounted array for a PV system that is supplying loads in the same building then exception 1 applies. If you can show that you would install the aux grounding electrode within 6’ of the existing grounding electrode then exception 2 applies. About the only time 690.47(D) is really required would be if the array were mounted on the roof of a building that did not contain the served loads and the service entrance panel.