690.47 (C) (3) combined DC GEC/ AC EGC sizing

[tmkelly2]

Is it just me or did the "Ground the hell out of it" group totally screw up the 2011 code?

I've got an array field with (4) combiners (2-12 string and 2- 13 string) going to a recombiner. output ampacity required 694a.
So I'm running parallel 500kcm conductors to the inverter from the recombiner.

225kw 480v inverter puts out 271a 3?, requires 500kcm conductors (all copper)..

Per the code the combo ground needs to be "no smaller than the largest DC conductor", so the ground is parallel 500's and the 480v inverter output is single 500's???

Is there any way to put sanity into the equasion? pretty hefty ground since the GEC isn't really for fault current anyway?

Is there something I missed?

 

[SolarPro]

Sounds like you're confusing system grounding requirements with equipment grounding requirements.

On the one hand, equipment grounding conductors (EGCs) are sized according to 690.45, which refers you to Table 250.122. Per this table, the circuit OCPD rating will determine the size of your EGC.

On the other hand, the dc grounding electrode conductor (GEC) is sized according to any one of the three options described 690.47(C). While these options generally refer you to 260.166, subsections (C) through (E) will likely limit the maximum size of your GEC.

 

[tmkelly2]

Sounds like you're confusing system grounding requirements with equipment grounding requirements.

On the one hand, equipment grounding conductors (EGCs) are sized according to 690.45, which refers you to Table 250.122. Per this table, the circuit OCPD rating will determine the size of your EGC.

On the other hand, the dc grounding electrode conductor (GEC) is sized according to any one of the three options described 690.47(C). While these options generally refer you to 260.166, subsections (C) through (E) will likely limit the maximum size of your GEC.

690.47 (C)(3) is the combined GEC for the DC and the EGC for the AC of the inverter. One single wire to handle both situations. sized per 250.166 (B) wouldn't the GEC for the DC be parallel 500's since that is the size of the DC from the recombiner to the inverter? If that is the case, my conductors from the inverter to the point of connection (building service) would be (3) 500kcm conductors at 480v (single conductors per phase) and parallel 500's for the GEC/EGC combined ground. But if I eliminated the recombiner and ran the 4 combiner circuits directly to an internal recombiner in the inverter my largest DC conductor would be a 3/0 and is still larger than the #3 required for the AC EGC with a 350a ocpd.
Where do I have it confused? Is there any way to ignore the DC size from the recombiner?
That extremely oversized GEC doesn't make sense to me. Especially since any ground faults in the dc won't go in that direction.

 

[jaggedben]

As SolarPro stated, 250.166(C) thru (E) will usually place a practical limit on how large the GEC actually needs to be.

 

[tmkelly2]

Practical? come on guys, look at my example.
largest combiner circuit conductor is a 3/0, but when combined in a recombiner and routed to the inverter it is parallel 500's.
My ac inverter conductors are single 500's
if I don't use the separate recombiner my combined ground wire is a 3/0, but if I use the recombiner that goes to parallel 500's per 250.166 (B), .....

250.166 (C), (D), and (E) don't apply, I am running the DC GEC in the same conduit back to the service as the ac conductors to tie DC and AC systems together and using the 690.47 (C) (3)
Can anyone suggest how the code will allow smaller than the parallel 500's?
These vague "go look in the code" answers aren't much help.

 

[SolarPro]

You don't have to use a combined dc GEC and ac EGC.

For example, if you were able to install a ground rod or a ground ring to accomplish the dc system ground per 690.47(C)(1), then 250.166(C) or (E) would apply and the dc GEC would not be required to be larger than 8 AWG or 2 AWG, which would allow you to size the ac EGC per 250.122; note that you would still need to bond the dc GEC to the existing grounding electrode, and the size of ac GEC will likely guide the placement of the ground rod.

While I am confident that there's a way to avoid using parallel 500 kcmil conductors, I'm not sure what else to tell you based on the minimal amount of information that you have provided. Perhaps this is a better question for the PE who produced and stamped the electrical engineering plan set?

 

[jaggedben]

Practical? come on guys, look at my example.
largest combiner circuit conductor is a 3/0, but when combined in a recombiner and routed to the inverter it is parallel 500's.
My ac inverter conductors are single 500's
if I don't use the separate recombiner my combined ground wire is a 3/0, but if I use the recombiner that goes to parallel 500's per 250.166 (B), .....

Repeating the question won't really change the answer...:happyno:

250.166 (C), (D), and (E) don't apply, ...

Why not? And if not, can you make any of them apply? What is the AC grounding electrode?

I am running the DC GEC in the same conduit back to the service as the ac conductors to tie DC and AC systems together and using the 690.47 (C) (3)

Is that your only option? Could you run a parallel raceway for either the DC GEC or a bonding jumper?

Can anyone suggest how the code will allow smaller than the parallel 500's?
These vague "go look in the code" answers aren't much help.

If all else fails, consult an electrical engineer and invoke 90.4. ("By special pennission, the authority having jurisdiction may waive specific requirements...")

 

[tmkelly2]

You don't have to use a combined dc GEC and ac EGC.

For example, if you were able to install a ground rod or a ground ring to accomplish the dc system ground per 690.47(C)(1), then 250.166(C) or (E) would apply and the dc GEC would not be required to be larger than 8 AWG or 2 AWG, which would allow you to size the ac EGC per 250.122; note that you would still need to bond the dc GEC to the existing grounding electrode, and the size of ac GEC will likely guide the placement of the ground rod.

While I am confident that there's a way to avoid using parallel 500 kcmil conductors, I'm not sure what else to tell you based on the minimal amount of information that you have provided. Perhaps this is a better question for the PE who produced and stamped the electrical engineering plan set?

Putting in grounding electrodes on the dc serves no purpose in reducing the parallel 500's required to bond the dc to the ac system.

 

[SolarPro]

Why not? Is the existing ac GEC parallel 500s?

Per 690.47(C)(1) the bonding jumper between the dc GE and the ac GE is determined by the size of the dc GEC or the ac GEC, whichever is larger. If you install a ground rod or plate or ring at the inverter, the the dc GEC will be considerable smaller than parallel 500s. If the existing ac GEC is smaller than parallel 500s, then the bonding jumper just got smaller than the combined conductor required under (C)(3).

If you still think parallel 500s are required, you can probably save money by hiring a PE to review your proposed solution and suggest alternative design options.

It seems to me that you basically want to design the GE system such that you will use the smallest conductor size, presumably the dc GEC, for your longest run. If the existing ac GEC is quite large, you will want to locate the dc GEC as close as possible or practicable to the ac GEC so as to minimize the bonding jumper length. The dc GE type will set the maximum size of the dc GEC, and will be much smaller than parallel 500s.

 

[SolarPro]

Remember that 690.47(C) gives you three system grounding options.

We've talked about options (1) and (3). When I was overseeing projects in the field, we probably used option (2) most often.

 

[SolarPro]

FWIW: the back half of this article may be helpful to you:

Grounding Compendium for PV Systems