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

Wed Sep 18 07:49:58 PDT 2013

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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