EGC sizing DC vs AC

[Dbarrett]

I had a quick question on the sizing of EGC in a PV system. We use primarily SMA sunny boy inverters or SolarEdge. When sizing the EGC, my understanding is the conductor on the roof needs to be a 6awg bare copper due to being exposed to physical damage. From the soladeck or jbox/ combiner, what code or table is supposed to be used to determine the size? One more question, after the power is inverted, can you just use 250.122 as your sizing requirements there after? A lot of confusion with this topic in all aspects of the trade. Thanks.


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[ggunn]

I had a quick question on the sizing of EGC in a PV system. We use primarily SMA sunny boy inverters or SolarEdge. When sizing the EGC, my understanding is the conductor on the roof needs to be a 6awg bare copper due to being exposed to physical damage. From the soladeck or jbox/ combiner, what code or table is supposed to be used to determine the size? One more question, after the power is inverted, can you just use 250.122 as your sizing requirements there after? A lot of confusion with this topic in all aspects of the trade. Thanks.


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I don't think there's any difference between AC and DC; use 250.122.

 

[Dbarrett]

So the EGC only has to be sized using 250.122? Meaning a 10awg is to 60a throughout the system? Our plans call for a 6awg throughout. From my research, I believe it should be a 8awg throughout. Our plans also call for the EGC and GEC to be combined.


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[Dennis Alwon]

So the EGC only has to be sized using 250.122? Meaning a 10awg is to 60a throughout the system? Our plans call for a 6awg throughout. From my research, I believe it should be a 8awg throughout. Our plans also call for the EGC and GEC to be combined.


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Since they are combined that may be why the engineer has stated 6 awg. The problem is it doesn't matter what the nec states if the engineer plans show a larger conductor then you need to use it.

 

[Dennis Alwon]

In some cases the grounding electrode conductor cannot be smaller than the neutral conductor so if your neutral is a #6 then the grounding electrode conductor must also be. I assume 250. 166 would be applicable to pv systems also unless it is modified in article 690

 

[jaggedben]

First you need to decide if it's just an EGC or if someone is calling the same conductor a GEC for some purpose. Such a purpose might be to satisfy 690.47(D) if you are on the 2014 code, or to satisfy an AHJ who just doesn't get it. (I've met one or two like that.) If you are on the 2011 or 2017 NEC there should be no need for a GEC.

If it's not an GEC, either because one isn't required or because you are running the GEC elsewhere, then an EGC that is not exposed on the array only needs to meet 250.122. See 690.45 for more details; you can use an assumed OCPD rating if there isn't actually one, and you don't have to upsize for voltage drop.

A bare EGC on the array is minimum 6awg if your AHJ is enforcing that. (690.46) Most AHJs I work with don't enforce that. The requirement does not mean that you have to run 6awg throughout the installation, only where exposed. FWIW, on our residential systems, everything is 10wg except for two or three AHJs.

 

[Dbarrett]

So what does the code say to size the GEC as the EGC? Forget about what the plans or AHJ say.


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[jaggedben]

So what does the code say to size the GEC as the EGC? Forget about what the plans or AHJ say.

What exactly is your question? I can't quite make sense of your wording there.

I will guess the exact answer your looking for depends highly on the code cycle. For 2017 NEC there are no GEC requirements whatsoever. For earlier codes it's more complicated. But unless a conductor is a combined EGC/GEC - and there was never a code requirement to combine them for a given installation - then the EGC is just sized to 250.122 (or 6awg minimum where exposed).

 

[Dbarrett]

Sorry for the confusion. Bottom line is the PV EGC is sized in accordance with 250.122? I have no idea why my company combines the two. Doesn’t seem necessary or practical. Thanks for your help


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[tortuga]

So what does the code say to size the GEC as the EGC? Forget about what the plans or AHJ say.


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Dbarrett if your in NV your probably on the 2017 NEC See 250.121 and 690.47.
hope this helps.

 

[jaggedben]

Sorry for the confusion. Bottom line is the PV EGC is sized in accordance with 250.122? I have no idea why my company combines the two. Doesn’t seem necessary or practical. Thanks for your help

You didn't confirm the code cycle, but here's a couple more comments:

For a transformerless inverter (which they all are nowadays, pretty much) the 2014 NEC makes reference to a GEC but allows it to be sized like an EGC (250.122). This is different from the array electrode requirement in 690.47(D).

If the AHJ is enforcing 690.47(D) then it makes sense to combine that GEC with the EGC if the AHJ allows it either in general or for that given installation. But 690.47(D) is such a hairbrained requirement that having a standard approach in designs to try to cover all AHJ's and installs is, in my experience, a fools errand. We have one AHJ which allows us to meet it by running an 8awg green to the existing premises electrode. We have another where I have to ask exactly what they want every time. The rest, thank goodness, don't enforce it.

Your designers may just not be caught up on the latest code and/or may be engaging in a perhaps ill-advised strategy of specifying the 'worst case scenario' requirements for every job.

 

[ggunn]

Sorry for the confusion. Bottom line is the PV EGC is sized in accordance with 250.122? I have no idea why my company combines the two. Doesn’t seem necessary or practical. Thanks for your help.

Most of the time, yes, but in the instances where the grounding conductor is indeed a combined EGC/GEC, it must be sized to whichever is the larger directed by 250.122 and 250.66.

But whatever the plans call for trumps (small t) either table if you are being inspected for conformation to the plans, as do the AHJ's requirements if they are different from what the tables call for. For example, an AHJ we deal with a lot requires a minimum #8 copper ground for all PV systems, so to avoid confusion by our installers, we use a minimum #8 copper ground in all our PV installations.

 

[Dbarrett]

Do you switch neutrals or not? Haha
Do I need a new thread for this...


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