Ground wire sizing for off-grid system

[real246]

My first post so I'll introduce myself first. I'm in the middle of an off-grid house install (house is under construction), it's my own house by the way. 7.32kw PV, 28.8kw LiFePO4 batteries, Schneider Electric inverter (6848) and 2x 600-80 Charge controllers from Schneider Electric. I opted to only do the solar electrical portion and let a contractor take care of the A/C panel and beyond.

I have yet to buy my grounding wire and that's always been my weak point, grounding, in terms of code compliance. I'm running 6 batteries, each 100AH, 48V. Each battery is using #2 copper for 3 feet and fed into a combiner (midnite model: MNBCB 1000/50), then that has a 7 foot copper run of 4/0 cable. All this is run in a 6x6 steel wireway.

Note: This is an ungrounded/floating system, no current carrying conductor is connected to ground.

So here's the meat of the question/confusion:

Code (NEC 2014) says in 690.45 to reference 250.122 and it's easy(ish) to understand, just size the wire appropriately to the chart based on the amperage. BUT.........

690.47 references 250.166 which states:

(B)"Not Smaller Than the Largest Conductor. Where the
dc system is other than as in 250.166(A), the grounding
electrode conductor shall not be smaller than the largest
conductor supplied by the system, and not smaller than
8 AWG copper or 6 AWG aluminum. "

This one makes me think I need 4/0 grounding cable to my concrete encased grounding rod (house foundation rebar) because I have a 4/0 cable in the system. So what gauge grounding wire do I run? I was prepared to buy #6 bare copper soft drawn but now I'm perplexed.

Hopefully I provided enough details to get an answer or spark a discussion, if not let me know what else is needed.

Ryan

[Mike90250]

That's a pickle. 4/O is going to melt rebar !

[real246]

That's a pickle. 4/O is going to melt rebar !

Indeed that would be a pickle.

Can I use the language of 250.66(D) for my justification or am I stuck on (B)? My gut tells me (D) is appropriate after reading it for the 100th time in the past few days.

"(D) Connected to a Concrete-Encased Electrode. Where connected to a concrete-encased electrode as in 250.52(A)(3), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than 4 A WG copper wire."

[Mike90250]

So, I don't know the particulars of the code, just the intent and what I had to do 10 years ago. I had to use 1/2" rebar, an unbroken 20' stick, and well connected to the rest of the rebar mesh in the foundation. I think we used #6 or #4 copper.

Talk to your electrician buddy or the inspection department, lay out your thoughts, and ask what they would like to see you do. The inspector is going to see it eventually