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Old 02-25-2015, 09:15 PM   #1
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Default EGC sizing of Ungrounded (floating) PV system

Hey everyone, just joining these boards, hoping to lean on the collective knowledge here, and fill a couple holes in my understanding of PV systems.

I recently joined up with a solar design firm, and have seen notes on older sets of plans which did not make sense to my current Code understanding.

Does this NOTE make sense to put in with the collected electrical calculations?:

DC EQUIPMENT GROUNDING CONDUCTOR TO AND FROM INVERTERS IS SAME SIZE AS UNGROUNDED CONDUCTORS OR LARGER, PER NEC 690.45, 250.122

The way I read this, is that when dealing with the DC wiring, the EGC shall be as large or larger than the ungrounded DC conductors. The references don't make sense to me though. I read through 250.122 and 690.45 and didnt see that requirement myself.

We almost always use #10 PV wire for the DC runs, but this is saying if the PV wire were #8 (for instance a very hot roof or full conduit), then my EGC would need to be upsized to a #8 as well?

What am I missing?
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Old 02-25-2015, 09:20 PM   #2
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I always size the egc for pv systems per 250.66. I drive 2 ground rods, and then loop back to the inverter. Yes you can size per 250.122, but I chose not to. Metal arrays on a roof need to have a good path to earth.
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Old 02-25-2015, 10:58 PM   #3
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Having the equipment grounding conductor the same size or larger would make sense if the system was installed in a non-dwelling location, and not supplied with ground fault protection.

Quote:
690.5 exception #2.
Exception No. 2: Photovoltaic arrays installed at other
than dwelling units shall be permitted without ground-fault
protection if each equipment grounding conductor is sized
in accordance with 690.45.
Quote:
690.45 Size of Equipment Grounding
Conductors
(B) Ground-Fault Protection Not Provided. For other
than dwelling units where ground-fault protection is not provided
in accordance with 690.5(A) through (C), each equipment
grounding conductor shall have an ampacity of at least
two (2) times the temperature and conduit fill corrected circuit
conductor ampacity.
Here is the commentary for 690.45(B)

Quote:
Equipment-grounding conductors on PV systems without
ground-fault protection may have ground-fault currents that
circulate continuously until the ground fault is corrected.
These conductors must have their ampacity corrected for the
conditions of use just like the circuit conductors. Fault currents
from parallel source circuits may allow currents in
equipment-grounding conductors up to twice the normal circuit
current under ground-fault conditions.
And the informational note.

Quote:
Informational Note: The short-circuit current of photovoltaic
modules and photovoltaic sources is just slightly above
the full-load normal output rating. In ground-fault conditions,
these sources are not able to supply the high levels of
short-circuit or ground-fault currents necessary to quickly
activate overcurrent devices as in typical ac systems. Protec-
tion for equipment grounding conductors in photovoltaic
systems that are not provided with ground-fault protection is
related to size and withstand capability of the equipment
grounding conductor, rather than overcurrent device operation
I would guess that the note on the drawings you are referring too did not come from a residential PV array. It has been a while since I had to give article 690 a lot of thought. I haven't done any solar in a while.
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Old 02-26-2015, 10:26 AM   #4
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Thanks for your comments cabletie.

As I was going home last night I had time to work it through in my head.

Your comments support what I came up with, that if there was an issue where one of the conductors was shorted to the frame, and current was circulating through the EGC, you'd want to size it to carry the full ampacity of at least one string.
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