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

Jay Peltz jay at asis.com
Wed Sep 18 08:24:11 PDT 2013 

I believe this is the tenK solar product being talked about. 

Jay
Peltz power

Sent from my iPhone

On Sep 18, 2013, at 8:49 AM, August Goers <august at luminalt.com> wrote:

> Hi Charlie,
>  
> I can't help but be intrigued by your setup here. It seems like the complete opposite of where our commercial systems are going with transformerless 1000 V inverters entering the market. Anyway, I'm sure it's an interesting project and there may very well be advantages to the system you're building.
>  
> I agree with Dave below that you should be okay with a #6 run to a ground rod. Otherwise, your logic for sizing the GEC seems correct. We deal with this similar issue when designing battery backup systems and the GEC can get unwieldy very quickly. I would just point out your sizing logic per 250.166(C) on your plans for the inspector to see.
>  
> Best,
>  
> August
>  
> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave Click
> Sent: Wednesday, September 18, 2013 5:38 AM
> To: re-wrenches at lists.re-wrenches.org
> Subject: Re: [RE-wrenches] GEC sizing for a high amp, low voltage PV system with microinverters
>  
> 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 ™
> Aladdin Solar, LLC
> 
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