Transformer less inverter to GEC?

[NYSolar]

In the 2014 code it's my understanding that with transformer less inverters we can just use anEGC for grounding the AC equipment. From what I've read that there is no connection to the pre-existing GEC with a utility connected inverter. So does this mean when I do a supply side tap there does not need to be a GEC from the disconnect? That there is no connection to a grounding electrode?

 

[Carultch]

In the 2014 code it's my understanding that with transformer less inverters we can just use anEGC for grounding the AC equipment. From what I've read that there is no connection to the pre-existing GEC with a utility connected inverter. So does this mean when I do a supply side tap there does not need to be a GEC from the disconnect? That there is no connection to a grounding electrode?

I'd like to follow this discussion. It is my understanding that standard EGC practices are all that is required with any and all transformerless inverters. And that the GEC is only required where you derive a grounded conductor, which we do not do for transformerless inverters.

Do realize that when you make a supply-side tap, your EGC between that disconnect and the tap interconnection point must be sized based on the feeder/service to which it connects. So tapping 200A of PV onto a 400A service or feeder, will require a #3 Cu ground wire. Not a #6 Cu, as you are using for your PV system feeder up to that disconnect.

 

[electrofelon]

In the 2014 code it's my understanding that with transformer less inverters we can just use anEGC for grounding the AC equipment. From what I've read that there is no connection to the pre-existing GEC with a utility connected inverter. So does this mean when I do a supply side tap there does not need to be a GEC from the disconnect? That there is no connection to a grounding electrode?

you still need to connect the non current carrying metal parts of a PV system to the GES (or establish one), but the AC EGC is allowed to make this connection. So in practical terms, you just think of the inverter as a normal piece of electrical equipment as far as grounding. Note that the PV service disconnect will require a connection (usually a tap from the GEC) to the GES just like a "normal" service with multiple service enclosures.

 

[NYSolar]

See the reason I ask this is I recently received an email from NYSERDA (who is the governing body for all PV systems in NY. They state that for transformer less utility interactive inverters that both AC and DC sides are isolated from GEC.

 

[Carultch]

See the reason I ask this is I recently received an email from NYSERDA (who is the governing body for all PV systems in NY. They state that for transformer less utility interactive inverters that both AC and DC sides are isolated from GEC.

Do they give a schematic that shows how this can be so? Or what they expect you to do, when connecting the EGC of a transformerless inverter to the premises equipment ground system?

Transformerless utility interactive inverters are also known as non-isolated inverters.

 

[electrofelon]

See the reason I ask this is I recently received an email from NYSERDA (who is the governing body for all PV systems in NY. They state that for transformer less utility interactive inverters that both AC and DC sides are isolated from GEC.

Well sort of......but not really. I am guessing that is not exactly what they said. A transformer in a transformer based inverter isolates the DC side from the AC side. Although the NEC allows grounded or ungrounded PV systems, most transformer inverters operate the DC side as grounded. If/when this is the case, we connect one of the PV output circuit conductors and all the metal stuff to the grounding electrode system (actually the inverter usually does it through a fuse as part of its GFP scheme). So, we have a grounding electrode conductor running to the inverter per one of the methods described in 690.47(C) (2014, as you know, NY is on 2008 still, I dont recall what 2008 says). So note in this case, we are SYSTEM grounding and EQUIPMENT grounding. In a transformerless inverter, there is no isolation between the AC and DC sides so we CANT ground the DC system because its already grounded and doing so would cause a direct short every half cycle. So we call this "ungrounded" which is silly and wrong IMO. The NEC is obsessed with the idea that the system grounding connection is of paramount importance, so my theory is that is why the GEC requirements for grounded PV systems are more stringent (no splices) than ungrounded systems - with a transformerless inverter you just use the AC egc and it can have splices and all.

 

[electrofelon]

Also I would just provide a nit picky correction that NYSERDA operates the state subsidy program. They would have nothing to do with off grid systems or grid tie systems where one is not going after the state subsidy.

 

[MasterTheNEC]

I will assume (which might be a mistake) that you are talking about MicroInverters. Attached is a fairly good website which contains some whitepapers that will explain it in great detail rather than me regurgitating it here.

http://enphase.com/grounding/

 

[SolarPro]

In a transformerless inverter, there is no isolation between the AC and DC sides so we CANT ground the DC system because its already grounded and doing so would cause a direct short every half cycle. So we call this "ungrounded" which is silly and wrong IMO.

Grounding terminology may get fixed in the 2017 edition of the NEC. Even the systems we call "grounded" are not really solidly grounded, because they are grounded through a ground-fault fuse. This is what the new proposal looks like:

690.41(A) PV System Grounding Configurations.
One or more of the following employed:
(1) Reference grounded 2-wire PV systems with one conductor
referenced to ground.
(2) Reference grounded bipolar PV systems with the reference
(center tap) conductor referenced to ground
(3) PV systems not isolated from the inverter output circuit.
(4) Ungrounded PV systems
(5) Solidly grounded PV systems as permitted in 690.41(B)
Exception.
(6) PV systems that use other methods that accomplish
equivalent system protection in accordance with 250.4(A) with
equipment listed and identified for the use.

 

[jaggedben]

We had an 87 post thread about whether or not a supply side tap is a service disconnect a few threads back, which I'm not going to link to. :roll: You can find it, and a few others on the subject, if you search past threads.

The point is...

If the disconnecting means for the supply side tap is considered a service disconnect, then article 250 requires a GEC to be run from the electrode(s) to the disconnect.

If not, then there is no such requirement. You could run an EGC back to the location of the normal service disconnecting means.

My advice is to do it whichever way you prefer, as long as everything is safely bonded, but make provisions for doing it the other way in case your AHJ disagrees with you.

None of what I've just said should be construed as requiring a GEC from the inverter for DC system grounding. On a transformerless inverter, I agree that under the 2014 code only an EGC is required. The GEC I'm talking about would only go from the supply side disconnect to the electrode.

 

[NYSolar]

First I would just like to say that NYSERDA may not be the "governing body" but, if you want any incentive money on your PV installation you have to do exactly what their third party inspectors(CADMUS) want. 90% of the time it's more than what your AHJ wants. Like I said I got an email from them on transformer less inverters. Two things it states word for word is:

-the GEC is established at the MDP and not at the inverter. In this case those inverter is not considered a separately derived power supply.
-both AC and DC sides are isolated from the GEC.

What I don't understand is how can you not have or be connected to a GEC when you do a supply side tap? How can they say it's not a new service when you tap before OCP?

 

[jaggedben]

The AC hot conductors are isolated from the GEC but of course the AC neutral is not. However since the inverter output does not utilize the neutral and is not supposed to put any current on it (it only uses it for measuring voltage and phase) there's a sense in which what they say is correct with respect to the PV system.

Practically speaking, I would simply take it as meaning no solar GEC being required from the inverter to the service (unlike with grounded DC systems). And of course there is no field installed neutral to ground bond, but there isn't any for any type of listed grid-tie-only inverter if you are living in the twenty-first century. They are sort of just talking to themselves about how it works internally, and you as an installer just need to make the connections as specified in the manual and not worry about it.

Now again, since you are doing a supply side tap, you'll have to figure out if your various powers-that-be want you to do it as a new service disconnecting means or not, and thus whether a GEC is required for the disconnect. Which I think is a totally different subject from what they are trying to talk about in the language you quote. They are really talking DC system grounding.

 

[jaggedben]

How can they say it's not a new service when you tap before OCP?

Look up those threads I talked about. The code actually has a lot of vagueness on this point.

 

[NYSolar]

Thnx

 

[electrofelon]

First I would just like to say that NYSERDA may not be the "governing body" but, if you want any incentive money on your PV installation you have to do exactly what their third party inspectors(CADMUS) want. 90% of the time it's more than what your AHJ wants. Like I said I got an email from them on transformer less inverters. Two things it states word for word is:

-the GEC is established at the MDP and not at the inverter. In this case those inverter is not considered a separately derived power supply.
-both AC and DC sides are isolated from the GEC.

What I don't understand is how can you not have or be connected to a GEC when you do a supply side tap? How can they say it's not a new service when you tap before OCP?

Both of those statements are horribly worded. Who comes up with this stuff??