250.97 and ungrounded inverters

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Was just discussing this section of code with a coworker and we noticed it says "for circuits over 250v to ground"

Since the PV source circuits and output circuits are ungrounded their voltage to ground is 0.

We are still installing bonding bushings, but this got us thinking about it and I wanted to see what others thought about this and are doing.

Anyone omit a bonding bushing because of this verbiage and end up having a discussion with AHJ about it?
 

GoldDigger

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The code separately says that the voltage to ground of an ungrounded system shall be taken to be the highest line to line voltage present.
 

pv_n00b

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I think you could make an argument that this does not apply to truly ungrounded PV systems. But we have built ourselves into a situation where we are calling systems grounded that really are not solidly grounded, just sometimes grounded, and systems that are ungrounded that are really high resistance grounded through the circuits that test for ground fault current flow.

The code separately says that the voltage to ground of an ungrounded system shall be taken to be the highest line to line voltage present.

Isn't that in 690 and related to determining the system voltage rating of ungrounded systems? I don't see a reference to this in 250.
 
I think you could make an argument that this does not apply to truly ungrounded PV systems. But we have built ourselves into a situation where we are calling systems grounded that really are not solidly grounded, just sometimes grounded, and systems that are ungrounded that are really high resistance grounded through the circuits that test for ground fault current flow.

Further, note that we call transformerless inverters "ungrounded" when in fact they are a grounded system (assuming the AC side is grounded which essentially always is). I think I recall some chatter about the 2017 introducing a new term for transformerless inverter systems: "reference grounded."
 
The code separately says that the voltage to ground of an ungrounded system shall be taken to be the highest line to line voltage present.

Do you know where that is at?

I thought 690.35 or .41 would have addressed this but I don't see it there.

The highest line to line makes sense to me, I'd just like to know where it is at so I can share it with the rest of our team.
 

Carultch

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Was just discussing this section of code with a coworker and we noticed it says "for circuits over 250v to ground"

Since the PV source circuits and output circuits are ungrounded their voltage to ground is 0.

We are still installing bonding bushings, but this got us thinking about it and I wanted to see what others thought about this and are doing.

Anyone omit a bonding bushing because of this verbiage and end up having a discussion with AHJ about it?

In general, a ungrounded transformerless system will "float" with each polarity at approximately half of the operating voltage on nearly equal and opposite sides of ground. Bi-polar systems work on a similar principle, except the difference is that they are center-tapped with a grounded conductor rigidly constraining the system to be half above and half below ground. Ungrounded/transformerless/non-isolated systems will not be as rigidly linked to this trend.

Because there isn't a firm guarantee of where ground will be, you need to treat it just as you would treat any ungrounded AC system regarding the "voltage to ground" figure, whether it is for work space or for bonding practices. So suppose one polarity is unintentionally grounded somehow, what will the voltage be on the other? Also, you need to use either the cold VOC, or maximum system voltage, rather than VMP or operating voltage.

I see this as a situation in excess of 250V to ground, where you probably should be installing more substantial bonding means when required by 250.97. Remember, if you use up all the ring knockouts, or if you don't have them in the first place, you can still use standard locknuts for electrical continuity when this rule applies.

Mike Holt does a good job teaching the logic of 250.97. He gives the simple example of a fault through a 1 ohm of resistance, and shows how a 277V fault gives over 5 times as much heat generation as a 120V fault, straight from P=V^2/R. So to keep excessive heat from building up in a faulted ring knockout that isn't meant for a 277V fault, you simply provide an alternative path through the bonding bushing.

Then he gave an example specific to PV systems on the DC side, and realized that this is uncharted territory. This isn't a conventional situation where the source can provide kiloamps of fault current, and where voltage directly drives the current. This is a situation where the irradiance & cell temperature define an intimate curve relating current and voltage. With a short circuit, your voltage diminishes to zero. And in any case, short circuit current is limited by the configuration of modules itself, usually single digit amps per string. He admitted that applying this requirement to PV systems is stupid, but it is what we have to do anyway.
 
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Carultch

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Location
Massachusetts
Further, note that we call transformerless inverters "ungrounded" when in fact they are a grounded system (assuming the AC side is grounded which essentially always is). I think I recall some chatter about the 2017 introducing a new term for transformerless inverter systems: "reference grounded."

Good lord! How many different terms can we have for the exact same PV system topology?

Ungrounded/non-isolated/transformerless/type-TL/floating / it never ends....
 
My question is because 250.97 specifically says "250v to ground".

Dwellings units are allowed a max of 150v to ground. However we have and use 240v across lines everyday without violating the 150v to ground rule.

Also since there is no mention of it in 690.35 or .41 (and I haven't been able to find it anywhere else in 690).

This is why I brought this question up.

It's more of a technical question of is the bonding bushing necessary on the DC conduit for an ungrounded inverter.

According to 250.97 I see it technically as its not necessary because the voltage of the DC conductors to ground is 0. However logically I think it should be used.

For a $5 part and 4 minutes of labor we will still install them. I just want to know is there somewhere else in the code that makes us interperate 250.97 as line to line and not line to ground.
 
Good lord! How many different terms can we have for the exact same PV system topology?

Ungrounded/non-isolated/transformerless/type-TL/floating / it never ends....

Exactly! Ungrounded inverters are called such because none of the PV circuit lines are bonded to ground. This has nothing to do with equipment grounding nor the AC side. The industry and manufacturers call them ungrounded inverters and that's what we need to stick with.
 

Carultch

Senior Member
Location
Massachusetts
Was just discussing this section of code with a coworker and we noticed it says "for circuits over 250v to ground"

Since the PV source circuits and output circuits are ungrounded their voltage to ground is 0.

We are still installing bonding bushings, but this got us thinking about it and I wanted to see what others thought about this and are doing.

Anyone omit a bonding bushing because of this verbiage and end up having a discussion with AHJ about it?

I've also had discussions about 250.97, regarding another questionable example.

Enclosure 1 = steel with no KO's
Enclosure 2 = steel with ring KO's remaining
Enclosure 3 = fiberglass
Run an EGC through all conduits in all cases, and bond to each enclosure where possible.
>250V-ground

Connect enclosure 3 to enclosure 1: standard locknuts alone are sufficient. All that is needed, is standard locknut continuity with enclosure 1.

Now connect enclosure 2 to enclosure 1. The conduit has the same continuity to ground as it had in the previous example, if you use standard locknuts alone. But the other side no longer has zero continuity, instead it has insufficient continuity. So would you, or would you not need a bonding bushing on enclosure 2?
 

ActionDave

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Do you know where that is at?

I thought 690.35 or .41 would have addressed this but I don't see it there.

The highest line to line makes sense to me, I'd just like to know where it is at so I can share it with the rest of our team.
250.4(B) has the rules for ungrounded systems. 250.4(B)(2) has the wording about bonding in a manner capable of max voltage to ground.
 

Carultch

Senior Member
Location
Massachusetts
My question is because 250.97 specifically says "250v to ground".

Dwellings units are allowed a max of 150v to ground. However we have and use 240v across lines everyday without violating the 150v to ground rule.

For a $5 part and 4 minutes of labor we will still install them. I just want to know is there somewhere else in the code that makes us interperate 250.97 as line to line and not line to ground.

I'm trying to find the reference that specifies it for the general sense of ungrounded systems, but no such luck yet. I don't have the book in front of me.

Bear in mind, it is probably written in a manner that is intended for systems such as ungrounded delta three phase, as opposed to DC. If I recall correctly, it states that when a system is ungrounded, then the greatest voltage between any conductors shall be used where the NEC specifies voltage-to-ground. If there is a single phase/polarity to ground fault on an ungrounded system of any kind, then the remaining lines will be at the full line-to-line voltage when measured to ground.
 

ActionDave

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Now connect enclosure 2 to enclosure 1. The conduit has the same continuity to ground as it had in the previous example, if you use standard locknuts alone. But the other side no longer has zero continuity, instead it has insufficient continuity. So would you, or would you not need a bonding bushing on enclosure 2?
Box 2 would be covered by the exceptions to 250.97 so no bond bushing would be needed ninety-nine times out of a hundred.
 
250.4(B) has the rules for ungrounded systems. 250.4(B)(2) has the wording about bonding in a manner capable of max voltage to ground.

Thank you very much.

Bill Brooks made some mention at the NABCEP conference earlier this month to some changes coming to the ungrounded inverters in 2017 but I don't recall him mentioning any specifics. Hopefully this is addressed.

Thanks everyone.
 

don_resqcapt19

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Do you know where that is at?

I thought 690.35 or .41 would have addressed this but I don't see it there.

The highest line to line makes sense to me, I'd just like to know where it is at so I can share it with the rest of our team.
The following is from Article 100.
Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit
that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit
 
Exactly! Ungrounded inverters are called such because none of the PV circuit lines are bonded to ground. This has nothing to do with equipment grounding nor the AC side. The industry and manufacturers call them ungrounded inverters and that's what we need to stick with.

I disagree with you. There is a big difference between an isolated and ungrounded system and a non-isolated "ungrounded" (as we call it) system. Perhaps a good analogy is that the former is like a system with an islolating transformer, and the latter is a system from an autotransformer.
 

jaggedben

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Further, note that we call transformerless inverters "ungrounded" when in fact they are a grounded system (assuming the AC side is grounded which essentially always is). I think I recall some chatter about the 2017 introducing a new term for transformerless inverter systems: "reference grounded."

The 2nd draft changed the phrase to 'functionally grounded'. I've forgotten what the comments said about why.

Good lord! How many different terms can we have for the exact same PV system topology? Ungrounded/non-isolated/transformerless/type-TL/floating / it never ends....

Except that ungrounded and non-isolated are arguably different topologies. And the new NEC term 'functionally grounded' is a category of topologies including everything that is not solidly grounded or truly ungrounded.

It would be easier if reality were simpler, but it's not so simple. Fortunately the new code will make the requirements much simpler.

The following is from Article 100.

Right. Kind of sad it took so many posts to answer the actual original question. :lol:
 

pv_n00b

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Location
CA, USA
Based on what I have seen in the 2017 NEC there will be very few grounded PV inverters being installed in the US in the future. A grounded inverter would have to have a transformer for isolation between the AC and DC side and the grounded DC conductor would be solidly connected to ground with no GFDI or other ground fault circuit that disconnects it from ground during a fault. That's the old school inverters before GFDI circuits were required and EGCs were large to handle the fault current that would go on as long as the sun was out.

Everything will be pretty much treated the way we do ungrounded systems now.
 

jaggedben

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Northern California
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Based on what I have seen in the 2017 NEC there will be very few grounded PV inverters being installed in the US in the future.

True. The exceptions would probably be some off-grid capable inverters (e.g. Outback). Also PV systems with no inverter (i.e. DC utilization, not grid-tied at all) might be solidly grounded, but that again will be a very small percentage of installations.

A grounded inverter would have to have a transformer for isolation between the AC and DC side and the grounded DC conductor would be solidly connected to ground with no GFDI or other ground fault circuit that disconnects it from ground during a fault. That's the old school inverters before GFDI circuits were required and EGCs were large to handle the fault current that would go on as long as the sun was out.

Actually GFDI was always required. And the solution of grounding one DC conductor (but not solidly, only through a fuse) was a simple way to engineer that requirement. But it made putting in a transformer necessary, which lowered efficiency, and it wasn't really that reliable. Once manufacturers figured out how to make reliable electronic GFDI that would get listed by UL without using a grounded conductor, transformerless inverters took over quickly due to lower cost and higher efficiency. They are also safer for installation crews and others, if perhaps more prone to nuisance tripping and destructive failure in the case of faults.

Everything will be pretty much treated the way we do ungrounded systems now.

True.
 
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