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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