Combining 9 - Fronius Inverter to a single disconnec for a 100kW solar pv systemt

[ptevendale]

Hi. Though this is my first post, I have had so many questions answered though this forum.

I have a 3-phase, 208v, 100kW solar pv system. I am installing 2 - 10kW, 6 - 11.4kW and 1 - 15kW. I am planning on installing 2 - 200amp panel boards and use a screwed down back feed breaker for each inverter. Then have 3/0 conductors from each panel board to a 400 amp entrance rated disconnect. And from my solar disconnect run 3 pairs of 3/0 to the load side of the building main 400 amp disconnect using 3 -4 port taps (Where there is a pair of 3/0 for each phase leg). Does this interfere with the 6 disconnect rule?

Thanks for any help!

 

[jaggedben]

To your specific question...

... Does this interfere with the 6 disconnect rule?

You should check with your AHJ, but in my opinion, the answer is no, it does not run afoul of that rule. You have a single AC disconnect which disconnects all the inverters from the rest of the building. I'm going to presume that you also have no more than one DC disconnect for each inverter, and certainly no more than six for any single inverter. The code is a little unclear which disconnects this rule applies to, but if you can de-energize both all the solar and/or any single piece of equipment with less than six motions of the hand then I think you've met the intent.

Other comments...

... And from my solar disconnect run 3 pairs of 3/0 to the load side of the building main 400 amp disconnect using 3 -4 port taps (Where there is a pair of 3/0 for each phase leg). ...

This is probably not allowed, specifically the part about connecting on the load side. For it to be allowed under the 2011 code, the conductors you're tapping, and everything downstream to the next overcurrent device(s), would have to be rated close to 800A. Probably they are only rated for 400A. Under the 2014 code the tap may be allowed, but only if there are downstream overcurrent devices protecting the tapped conductors. Usually if the main building disco is 400A and your solar is also around 400A then you almost certainly need to do a supply side connection a.k.a. line-side-tap.

I am planning on installing 2 - 200amp panel boards and use a screwed down back feed breaker for each inverter. Then have 3/0 conductors from each panel board to a 400 amp entrance rated disconnect.

I believe you need to somehow have separate overcurrent protection for each panelboard.

705.12(D)(6) explicitly permits you to omit the 'screwed down' requirement.

I have not done any checking of your math on ampacities of conductors or panelboards.

 

[ptevendale]

To your specific question...



You should check with your AHJ, but in my opinion, the answer is no, it does not run afoul of that rule. You have a single AC disconnect which disconnects all the inverters from the rest of the building. I'm going to presume that you also have no more than one DC disconnect for each inverter, and certainly no more than six for any single inverter. The code is a little unclear which disconnects this rule applies to, but if you can de-energize both all the solar and/or any single piece of equipment with less than six motions of the hand then I think you've met the intent.

Other comments...



This is probably not allowed, specifically the part about connecting on the load side. For it to be allowed under the 2011 code, the conductors you're tapping, and everything downstream to the next overcurrent device(s), would have to be rated close to 800A. Probably they are only rated for 400A. Under the 2014 code the tap may be allowed, but only if there are downstream overcurrent devices protecting the tapped conductors. Usually if the main building disco is 400A and your solar is also around 400A then you almost certainly need to do a supply side connection a.k.a. line-side-tap.



I believe you need to somehow have separate overcurrent protection for each panelboard.

705.12(D)(6) explicitly permits you to omit the 'screwed down' requirement.

I have not done any checking of your math on ampacities of conductors or panelboards.

Thank you for great information.

In reference to the load side of the 400amp disconnect, yes I was going to tap the conductors on the load side of the disconnect but line side of the panel board. Would I still need a 800 amp disconnect? I will tap on the line side of the disconnect to avoid upgrading to 800 amp. I was thinking it would be a way to turn off the conductors going to the solar disconnect. Thanks again.

 

[jaggedben]

Thank you for great information.

In reference to the load side of the 400amp disconnect, yes I was going to tap the conductors on the load side of the disconnect but line side of the panel board. Would I still need a 800 amp disconnect? I will tap on the line side of the disconnect to avoid upgrading to 800 amp. I was thinking it would be a way to turn off the conductors going to the solar disconnect. Thanks again.

You wouldn't need the disconnect to be 800A, but you might all need the conductors involved to be rated so that the sum of the main and solar is not more than 120% of the conductors' rating. You should consult with your AHJ. The 2014 code is written is such a way that if the panelboard you mention has a main breaker, you could tap between the disconnect and panelboard (assuming the conductors are otherwise rated for the load or the solar, whichever is higher). If the panel board doesn't have a main breaker, your conductors need to be rated higher. Previous codes are much more vague on the matter and your AHJ could require higher rated conductors.

The relevant code sections are in 705.12(D).
Tapping on the line side avoids most of the issues and is a good way to go.