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

Wed Sep 18 09:37:09 PDT 2013

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 T
Aladdin Solar, LLC

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