Ground wire AWG

[cdsolar]

In getting ready to wire up my ground mount (finally), I know NEC says that ground wire subject to damage needs to be AWG 6. Ok. So I wire up the exposed panel/array grounding wire as AWG 6.


So say I connect said ground wire to a busbar in my combiner box and want to run said ground wire along with my + and - PV wires underground in RMC back to the side of the barn where my required disconnect is, so as to connect this ground wire to the ground rod beneath the disconnect where the rest of the system will also be grounded. Does said underground wire also need to be AWG 6?


My thought is that it does not. Only the exposed ground wires, such as on the array itself, and from the disconnect box down to the ground rod need to be AWG 6. I can probably do AWG 10 for the underground portion of the ground wire. (I will do AWG 8 for the PV +/- as I am concerned about voltage drop based on various calculators online).


For reference I am doing an off-grid solution to provide power for my barn. And yes I am subject to inspection.

Am I correct in my thoughts as to underground ground wire size? Thanks

 

[pvgirl]

The RMC, or other metal conduit can be used as the ground conductor, you may need grounding bushing if there plastic boxes on either end, to bond the conduit.

 

[timselectric]

Use this table to determine the EGC size, based on your PV circuit amperage.

 

[PhilIA]

Use this table to determine the EGC size, based on your PV circuit amperage.

If I have two strings in the same conduit do I size a single ground wire per string or the combined value of both strings?

 

[timselectric]

If I have two strings in the same conduit do I size a single ground wire per string or the combined value of both strings?

Size per the largest single circuit amperage.

 

[cdsolar]

So it appears I have two answers:

@pvgirl: use the RMC conduit as the grounding conductor from the array to the grounding rod, with my PV +/- in the conduit

@timselectric: suggests to use the NEC chart to select the AWG for the ground wire (for me that would be 10awg) and run it in the RMC back to the ground rod.

Both appear to be to code. Any reason for one over the other? I know the ultimate answer is 'ask your ahj' but I'd like to understand the reasons for both. @pvgirl does save me close to 90 dollars, but I'd still like to know the reasons for one or the other beyond $$.

 

[timselectric]

Personally I wouldn't use metal conduit.
But if you are, it's an acceptable grounding conductor.

 

[zanydroid]

Possibly fewer points of failure with a wire as ground vs the conduit.

Bonding to conduit saves money and conduit fill volume.

Accessibility of properly installing bonding lug, nuts, or threading into a box is another consideration.