Grounding a PV system to a Sub Panel in a detached structure

[JAZ140]

Hey guys and gals thanks for the help. Looking for the right answer in how the grounding should be accomplished to make the AHJ and the POCO happy. So here are the details...

2011 NEC
200 amp main panel feeding a 125 amp SUB in detached garage (150 feet away) via a 100 amp breaker
Sub breaker de-rated to 100 amps.
4 wire feed all CU 3 #3 and 1#8 GND.
Two ground rods driven out by the garage.

Panels, Inverter, production meter and safety disconnect are all located in or on the detached structure.

My AHJ has signed off but the POCO want the GEC to go from the inverter to the house. AHJ was ok with it going to the ground rods out at the detached structure.

 

[electrofelon]

Hey guys and gals thanks for the help. Looking for the right answer in how the grounding should be accomplished to make the AHJ and the POCO happy. So here are the details...

2011 NEC
200 amp main panel feeding a 125 amp SUB in detached garage (150 feet away) via a 100 amp breaker
Sub breaker de-rated to 100 amps.
4 wire feed all CU 3 #3 and 1#8 GND.
Two ground rods driven out by the garage.

Panels, Inverter, production meter and safety disconnect are all located in or on the detached structure.

My AHJ has signed off but the POCO want the GEC to go from the inverter to the house. AHJ was ok with it going to the ground rods out at the detached structure.

The AHJ is the correct one here, although note that the grounding electrode systems in each structure are bonded together by the feedee EGC so (if its done correctly) i would think the poco already has their wish

 

[PVfarmer]

My AHJ has signed off but the POCO want the GEC to go from the inverter to the house. AHJ was ok with it going to the ground rods out at the detached structure.

I'm not going to submit an opinion, but I will submit this link.
I'll submit a theory- if the AC service wires or xfmr get hit by lightning, you wouldn't want a ground path that went all the way to the detached structure?
I put a ? there so it's a question, not a statement!


The grounded conductor is a current-carrying conductor under normal operation—but the equipment grounds only carry current in case of a fault. For DC systems, the grounded conductor is usually the negative wire; in a SunPower system, it may be the positive wire. With some of the newer transformerless inverters, neither current-carrying conductor is grounded. For AC systems, it is the neutral wire. The grounded conductor should have only one connection to the GEC—at the system grounding point.

The system grounding point (aka “system bonding point”) is the place where the connection between the grounded conductor and the GEC is made. For a grid-tied PV system with multiple inverters, there will be one system grounding point for the AC system and one for the DC system of each inverter. In the case of a grid-tied inverter, the DC system grounding point is the internal ground-fault protection device (GFPD) fuse or GFPD system. This is what the NEC and the inverter manual are referring to when they say that there can only be one ground connection. To be more precise, it should say that only one DC system grounding point per inverter is allowed.
On the AC side of the system, there is also a system grounding point—usually inside of the main circuit breaker panel of the house, where a wire connects the grounding electrode conductor and the neutral, or grounded, conductor. This is why you’ll often see neutral and equipment ground wires sharing a busbar in a main AC panel—but it is not acceptable in a subpanel. You would have a second system grounding point on the same AC system if the equipment grounds and neutrals shared a busbar in the subpanel.
http://www.homepower.com/articles/solar-electricity/design-installation/ask-experts-pv-grounding

 

[PVfarmer]

JAZ-
It sounds like both your POCO and AHJ are "correct", but you have to use method #2 from this article.
Because your POCO wants you to.
It also sounds like you have right the wires in place already, which is cool for you.

Grounding Separate Structures
by John Wiles
Of the two grounding methods presented in this article, the first
method uses one less conductor and is therefore less expensive.

 

[jaggedben]

JAZ
The poco is way off base. Your rods at the detached garage are both AC and DC grounding for the solar system, regardless of which method your using under 690.47(C)(3).

If necessary, irreversibly crimp the #8 subpanel ground to some bonding jumpers and tell them that's your GEC.

 

[jaggedben]

JAZ-
...

Grounding Separate Structures
by John Wiles
...

Dude, 1998 was a long time ago for solar code. And it's not a stand allone system so the OP doesn't have an option for where the AC MBJ is. Let's be careful of citing articles that really don't apply.

 

[ggunn]

Hey guys and gals thanks for the help. Looking for the right answer in how the grounding should be accomplished to make the AHJ and the POCO happy. So here are the details...

2011 NEC
200 amp main panel feeding a 125 amp SUB in detached garage (150 feet away) via a 100 amp breaker
Sub breaker de-rated to 100 amps.
4 wire feed all CU 3 #3 and 1#8 GND.
Two ground rods driven out by the garage.

Panels, Inverter, production meter and safety disconnect are all located in or on the detached structure.

My AHJ has signed off but the POCO want the GEC to go from the inverter to the house. AHJ was ok with it going to the ground rods out at the detached structure.

Are you interconnected through a backfed breaker or via a supply (line) side "tap"? It makes a difference to the grounding in many jurisdictions.

 

[JAZ140]

Drawing of Grounding

Drawing of Grounding

Here is how it stands now.

 

[JAZ140]

Are you interconnected through a backfed breaker or via a supply (line) side "tap"? It makes a difference to the grounding in many jurisdictions.

Breaker

 

[ggunn]

Breaker

Then you need an EGC all the way from the array hardware to the ground bar in the main service panel. If your inverter is DC ungrounded (most are these days), then you don't need a ground rod at the inverter. Whether you need a ground rod at the array is up to the AHJ, but if you have one it should be connected to the building ground.

 

[jaggedben]

Here is how it stands now.

That ought to be more than good enough for anyone.

As I suggested above, ask the POCO if they will accept irreversible crimping of bonding jumpers so that the #8 ground is continuous all the way from the subpanel to the UFER & water at the house. It is not required by code but might be the cheapest way to avoid a protracted argument. Point to 690.47(C)(3) says you can combine the EGC and GEC.

Meanwhile, you're already all good at the detached garage under 690.47(C)(2). Your rods connected to the subpanel are your AC grounding electrode that's required to be there for the subpanel even if no solar system is connected. You're using them as a common AC/DC electrode and you ran a separate DC grounding electrode conductor (the bare #8) to them from the inverter. As my neighbor's two-year-old would say, "All done."

BTW, you could have dispensed with the extra wire from the inverter to the rods, and just run a #8 through your meter and disconnect to the sub, as long as all the splices were irreversible. That's 690.47(C)(3) again.

 

[SolarPro]

Agreed. You should be good. Your AHJ may need a quick tutorial on non-isolated (transformerless) inverters.

This IAEI (International Association of Electrical Inspectors) Magazine article may be useful:

John Wiles' "Perspectives on PV"

Questions on Grounding

Question: Does the NEC require that a grounding electrode conductor (GEC) and a grounding electrode (ground rod) be connected to the new transformerless inverters? See photo 1. Section 690.47 in the 2011 National Electrical Code (NEC) does not exactly address this issue.

Answer: If the listed transformerless inverter (also called a non-isolated inverter) adheres to the requirements of Underwriters Laboratories Standard 1741 for PV inverters, the inverter will not even have a terminal for a grounding electrode conductor. These inverters are used with an ungrounded PV array. The UL standard requires a grounding electrode conductor terminal and the Code would require a grounding electrode conductor only when there is a bonding jumper in the direct current (dc) side of the inverter. In normal transformer-type of inverters (also called isolated inverters), this bonding jumper is part of the required 690.5 ground fault detection and interruption (GFDI) circuit. Transformerless inverters do not connect one of the dc circuit conductors in the PV array to ground (as allowed by NEC 690.35) and have no internal bonding jumper. Therefore, there will normally be no terminal to connect the GEC to and the NEC does not require a dc GEC.

In other words, there are no dc system grounding requirements with ungrounded PV systems. New language added to 690.47(B) in NEC 2014 clarifies this:

An ac equipment grounding system shall be permitted to be used for equipment grounding of inverters...for the ground-fault detection reference for ungrounded PV systems.

 

[ggunn]

In other words, there are no dc system grounding requirements with ungrounded PV systems.

That, of course, does not mean that you don't have to have an EGC bonded to the racking. I know that's not what you meant, but it does occasionally confuse people.

 

[JAZ140]

Revised Grounding

Revised Grounding

Re read the SMA book for this and it does specify EGC for the array grounding terminal. SO boom no GEC needed! Thanks guys.

Also should I add some crimps to the EGC from the house ufer to the detached structure ground rods just for the hell if it?? Help the POCO feel warm and fuzzy!

Trying A JPEG file this time.

I did run a #8 for the inverter EGC. Just miss labeled the drawing. One thing I have not done is bond the EMT to that ECG..... I did bond the DC EMT as it is over 250 Volts but not the A/C conduit.

 

[SolarPro]

That, of course, does not mean that you don't have to have an EGC bonded to the racking. I know that's not what you meant, but it does occasionally confuse people.

Correct. Equipment bonding and system grounding are different. You can save a lot of time and materials by having a good understanding of this difference.

 

[PVfarmer]

I did run a #8 for the inverter EGC.

This is still all on paper (or a drawing), right?
From the 7700TL installation manual- looks like your 6 AWG is ok, but not the 8 AWG for EGC.

edit: I don't see in the manual where the 7700TL has that G/green line as an AC output grouped with the others- on pg 34 they're in different spots, the EGC comes from a spot such as the one the green line you deleted came from.

Page 36
6.3.1 Conditions for the AC Connection
Cable requirements:
☐ Conductor cross-section L1, L2, N: 12 AWG to 6 AWG (3.3 mm² to 13.3 mm²)
☐ Conductor cross-section equipment grounding conductor: 12 AWG (3.3 mm²)

 

[jaggedben]

This is still all on paper (or a drawing), right?
From the 7700TL installation manual- looks like your 6 AWG is ok, but not the 8 AWG for EGC.

8awg EGC is fine for a 7700TL. In fact, 10awg is okay.

I don't see in the manual where the 7700TL has that G/green line as an AC output grouped with the others- on pg 34 they're in different spots, the EGC comes from a spot such as the one the green line you deleted came from.

There is a lug grouped with the AC output conductors. It takes a 10 awg or 8 awg wire just fine. Take it from someone who has actually installed these inverters. And this was not the question.

 

[ggunn]

Re read the SMA book for this and it does specify EGC for the array grounding terminal. SO boom no GEC needed! Thanks guys.

Also should I add some crimps to the EGC from the house ufer to the detached structure ground rods just for the hell if it?? Help the POCO feel warm and fuzzy!

Trying A JPEG file this time.

I did run a #8 for the inverter EGC. Just miss labeled the drawing. One thing I have not done is bond the EMT to that ECG..... I did bond the DC EMT as it is over 250 Volts but not the A/C conduit.

I don't see whay you are showing irreversible crimps right next to blocks where the two ground wires are connected to each other. The crimps are not necessary.

 

[jaggedben]

Trying A JPEG file this time.

I did run a #8 for the inverter EGC. Just miss labeled the drawing. One thing I have not done is bond the EMT to that ECG..... I did bond the DC EMT as it is over 250 Volts but not the A/C conduit.

Arguably you should bond the #8 from the inverter to the EMT per the rules for a GEC. Technically the 2011 NEC requires it. But as has been said, under UL standards an EGC is all that's required. (And probably that will be true under the 2017 NEC, when you get there in six years).

Again, the irreversible splices going back to the house aren't necessary by code but if it makes the POCO happy and gets the job interconnected for the tax credit this year, small price to pay.

 

[PVfarmer]

the POCO want the GEC to go from the inverter to the house.

If the listed transformerless inverter (also called a non-isolated inverter) adheres to the requirements of Underwriters Laboratories Standard 1741 for PV inverters, the inverter will not even have a terminal for a grounding electrode conductor.

Then you need an EGC all the way from the array hardware to the ground bar in the main service panel.

Isn't the POCO going to say instead of the GEC that they want the EGC to go straight from the inverter to house panel?
Shouldn't the N from the inverter also go straight to the house? Is it possible to run the inverter into a 40A breaker in the main panel instead of the sub?

Is there maybe a inverter voltage drop issue with the inverter N and G both connected to the subpanel 150 away from the main?