[RE-wrenches] GEC sizing for a high amp, low voltage PV system with microinverters

[Dave Click]

I think Gary may be overthinking this. If the micro-inverters have a GEC 
lug then you'll need to run a GEC.

If you run your GEC to a ground rod, then you don't need to use one 
larger than #6 per 250.166(C). (note also (D) and (E) but they're less 
useful to you here)
/
Commentary:/
It makes no sense to me why the NEC can require a 0.9in diameter, 
#600mcm GEC for some systems (not this one) per 250.166(B) and then say 
"oh wait, you installed a single ground rod? Then a #6 is all you need!" 
in 250.166(C). But it certainly makes our lives easier.

If the building GE is not a ground rod, you could still run your #6 GEC 
to that new ground rod, and then bond that rod to the existing electrode 
using whatever size jumper is required per 250.53 (which I believe would 
also be a #6). This lets you keep that 300' run as a #6 rather than a #4 
or #2 that may be required otherwise.

DKC

On 2013/9/17 23:56, Gary Willett wrote:
> Charlie:
>
> The answer depends on whether the "system" (modules and 
> micro-inverters) is "grounded" or "ungrounded". I think you are 
> describing a "grounded" system, but it's not totally clear based on 
> your issue description.
>
> If the micro-inverter is "transformer-less" the conductors are not 
> isolated between the DC-in and the AC-out. This requires the 
> micro-inverter to have no DC-side system ground. The micro-inverter 
> AC-side always requires an EGC.
>
> I am assuming that the PV modules are in close proximity to the 
> micro-inverters, and the 300' home-run is for the AC outputs of the 
> micro-inverters.
>
> Is either DC bus (PV+ or PV-) a grounded conductor, or are both busses 
> ungrounded? If the DC-side combiners and re-combiners have OCPDs and 
> disconnecting means for both PV+ and PV-, then your system is 
> considered to be "floating" or "ungrounded".
>
> If the system is "ungrounded", a GEC is not required on the DC-side of 
> the micro-inverter, and only an AC-EGC sized per NEC 690.45 is required.
>
> If the DC-side combiners and re-combiners have OCPDs and disconnecting 
> means for only one conductor (PV+ or PV-), then your PV system is 
> considered to be "grounded". If the system is "grounded", the combined 
> DC-GEC and AC-EGC sizing requirements in NEC 690.47(C)(3) apply. The 
> larger of the EGC (Table 250.122) and GEC [250.166(B)] sizes will be 
> required.
>
> More specifics on the PV module Vmpp and micro-inverter you're dealing 
> with would be helpful.
>
> Regards,
>
> Gary Willett
> Icarus Solar
>
>
> On 9/17/13 8:37 PM, Aladdin Solar wrote:
>> We will be installing a 40KW grid-tied PV system that uses PV modules 
>> wired in parallel as per the module manufacturer -- so it's a high 
>> amperage, low voltage system on the DC side. 2 branches of about 3KW 
>> of parallel wired modules each come together onto a DC bus. The DC 
>> branch conductors are sized as 2AWG.
>> The inverter system provided by the manufacturer consists of a number 
>> of UL1741 single phase microinverters that all are powered off of the 
>> low voltage DC bus. So there's a bunch of microinverters in parallel, 
>> combined output is single phase power.
>> Those microinverters have a ground lug labeled "GEC". You know where 
>> this is going...
>> Assuming we are following NEC 690.47(C)(3), we'll be treating this 
>> ground as GEC and following all the GEC rules. My understanding is 
>> that because the DC conductors within the array are 2AWG (NEC 
>> 250.166(B), we will need to connect all 12 branches and 6 DC buses 
>> with continuous 2AWG and bring it all the way back 300' to where the 
>> AC GEC bond is located. We're getting a lot of pushback from the 
>> electrical sub and even some disagreement from the module 
>> manufacturer on this--no one's ever seen a grounding conductor of 
>> this size being required for this application.
>> Please don't get too bogged down with understanding the low voltage, 
>> parallel module spec and the massive number of microinverters 
>> involved. What I'm looking for is confirmation that I am properly 
>> understanding how the NEC GEC requirements apply, especially to the 
>> GEC size.
>> Charlie Pickard
>> NABCEP Certified PV Installation Professional ^(TM)
>> Aladdin Solar, LLC
>>
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