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

Wed Sep 18 10:19:27 PDT 2013

Hi Bill and wrenches,

To clarify, our new Enphase M250 microinverter does not have a grounding
terminal.  The grounding of the M250 will be provided through the Engage
cable.  We only have a hole in the lid, in the rare case that a lug will be
desired or required by the AHJ, but we do not provide the grounding clip
with the microinverter.  A grounding terminal can be added to the
microinverter.  We have an application note on alternate grounding methods
for the M250, if needed.

On Wed, Sep 18, 2013 at 9:37 AM, Bill Brooks <billbrooks7 at yahoo.com> wrote:

> Dave, Gary, and Charlie,****
>
> ** **
>
> First, the directions from the manufacturer should be the guide as to
> whether a GEC is required or not. Just because a GEC terminal is located on
> the equipment, does not mean that one is required. Case in point—Enphase’s
> new IG product does not require a GEC, yet it has a grounding terminal for
> AHJs that might insist on one. This terminal  is not intended for a GEC,
> but it is intended for an extra EGC if the AHJ requires it (LOL on all the
> abbreviations). ****
>
> ** **
>
> The 2014 NEC makes it very clear that ungrounded PV systems do NOT require
> an GEC on the dc side and that the ac EGC is sufficient to provide a ground
> reference for the required dc EGC.****
>
> ** **
>
> “690.47(B) *Direct-Current Systems.* …*An ac equipment grounding system
> shall be permitted to be used for equipment grounding of inverters and
> other equipment and for the ground-fault detection reference for ungrounded
> PV systems.*****
>
> ** **
>
> “690.47(C)(3) *Combined Direct-Current Grounding Electrode Conductor and
> Alternating-Current Equipment*
>
> *Grounding Conductor.** An unspliced, or irreversibly spliced, combined
> grounding conductor shall be run from*
>
> *the marked dc grounding electrode conductor connection point along with
> the ac circuit conductors to the grounding*
>
> *busbar in the associated ac equipment. This combined grounding conductor
> shall be the larger of the sizes specified*
>
> *by 250.122 or 250.166 and shall be installed in accordance with
> 250.64(E).* *For ungrounded systems, this conductor shall be sized in
> accordance with 250.122 and shall not be required to be larger than the
> largest ungrounded phase conductor.**”*
>
> * *
>
> Red letters not Jesus’ words. This is the new language of the 2014 NEC. It
> is now in print.****
>
> ** **
>
> The very practical reason that a 6AWG is only required with a single
> ground rod has to do with the current carrying capacity of a 5/8”, 8’
> ground rod in soils of low resistivity—nothing more.****
>
> ** **
>
> Bill.****
>
> ** **
>
> ** **
>
> *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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-- 

Cordially,

*Nick Soleil*

*Field Applications Engineer
*

*Enphase Energy*

Mobile: (707) 321-2937

**

*Enphase Commercial Solar.*
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