Max Backfeed on a Line-Side Tap

[tallgirl]

That doesn't mean those installations are truly compliant. It just means those AHJ are either unaware of the violation or simply choose to ignore it... take your pick... but I certainly would not take any action to make them aware either.

I'm still not convinced the install isn't compliant. And just because the code seems to be fairly silent on a point doesn't mean something is forbidden.

Power conversion equipment is inherently current-limited, both by the amount of available input power (PV or whatever) and by the design of the equipment. Presumably the lab doing the testing has verified that the device won't exceed its rated output -- I have clients who do bad designs (... because they don't listen to me ...) and even they can't exceed (by much) the rated output of the equipment they install. So, that rules out the situation with some inverter going nutso and violating several laws of thermodynamics on the supply side.

Then you've got the issue, as was raised, that a UL1741 certified inverter CANNOT feed a short. Heck, they can barely feed overloads if they cause significant voltage drop at the inverter terminals. This is definitely tested for UL1741 certification and the response times are faster than many trip curves -- which is its own set of problems (inverters that trip out too fast when presented with a surge).

The only scenario that can lead to any problem is one in which an inverter suddenly presents a load that is "large", but not a bolted short. The flip side -- an inverter suddenly producing more power out than it receives as power in is ruled out by the First Law of Thermodynamics.

 

[Smart $]

I'm still not convinced the install isn't compliant. And just because the code seems to be fairly silent on a point doesn't mean something is forbidden.

Not entirely silent is as much the point as the Code is not explicit. Not only do we have criteria which leaves no question as to compliance, we have enough to say compliance is questionable under certain conditions. To restate the focus here, we are discussing an aggregation panel (PVAP) between multiple PV inverters installed to comply with a supply side interconnection per 705.12(A).

I see no issue with an MLO panel used as a PVAP, where the busbars are connected by service conductor "tap", and it meets the six-disconnect rule. This installation, IMO, leaves no question as to compliance, regardless of whether there are unused spaces for more breakers. The only question here in my mind is whether this PVAP has to be service rated or not... and I would simply use one to remove any doubt.

I also see no issue if in the preceding scenario there is an interposing disconnect without OCP.

Now say we have a PVAP with an interposing disconnecting means with OCP. This (arguably) establishes a service disconnecting means. Per 705.12(A), each "electric power production source" is not "connected to the supply side of the service disconnecting means as permitted in 230.82(6)". However, the "loophole", IMO, is the highlighted part of the following sentence from 705.12(D):

Where distribution equipment including switchboards and panelboards is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters, and where this distribution equipment is capable of supplying multiple branch circuits or feeders or both, the interconnecting provisions for the utility-interactive inverter(s) shall comply with (D)(1) through (D)(7).

One method of rendering the distribution equipment incapable is to use all spaces. Any question regarding compliance is effectively mitigated. Yet if all spaces are not utilized, how do we effectively render the PVAP incapable without violating its listing???

Power conversion equipment is inherently current-limited, both by the amount of available input power (PV or whatever) and by the design of the equipment. Presumably the lab doing the testing has verified that the device won't exceed its rated output -- I have clients who do bad designs (... because they don't listen to me ...) and even they can't exceed (by much) the rated output of the equipment they install. So, that rules out the situation with some inverter going nutso and violating several laws of thermodynamics on the supply side.

Then you've got the issue, as was raised, that a UL1741 certified inverter CANNOT feed a short. Heck, they can barely feed overloads if they cause significant voltage drop at the inverter terminals. This is definitely tested for UL1741 certification and the response times are faster than many trip curves -- which is its own set of problems (inverters that trip out too fast when presented with a surge).

The only scenario that can lead to any problem is one in which an inverter suddenly presents a load that is "large", but not a bolted short. The flip side -- an inverter suddenly producing more power out than it receives as power in is ruled out by the First Law of Thermodynamics.

Noteworthy... but has little if not nothing to do with Code compliance.

 

[tallgirl]

I do understand that some people seem fixated on "capable" in the way that it is being interpreted.

Short of welding the cover to the panel shut and outlawing metal cutting tools, a panel with all spaces utilized is still "capable" of being modified at some point in the future. This tells me that the interpretation of "capable" that's being advanced is absurd.

 

[Smart $]

I do understand that some people seem fixated on "capable" in the way that it is being interpreted.

Short of welding the cover to the panel shut and outlawing metal cutting tools, a panel with all spaces utilized is still "capable" of being modified at some point in the future. This tells me that the interpretation of "capable" that's being advanced is absurd.

Granted, taking "capable" to the extreme is absurd... but are there less-than-absurd interpretations? If yes, how do we qualify these?

 

[ggunn]

I also see no issue if in the preceding scenario there is an interposing disconnect without OCP.

Let me get this straight; quite possibly I am misunderstanding you. Are you saying that something (anything) installed without OCP can be safer than with OCP?

The point of the code is safety, and in my estimation a safe installation has OCP as close as possible to the point of interconnect. If it's through a backfed breaker, the breaker is the OCPD, and if it's a tap there needs to be a fused disconnect right next to it irrespective of whether there's a single inverter or a combiner panel feeding the tap. If there is a way to interpret the code which allows for or dictates an unsafe installation, then that interpretation is incorrect.

 

[tallgirl]

Let me get this straight; quite possibly I am misunderstanding you. Are you saying that something (anything) installed without OCP can be safer than with OCP?

The point of the code is safety, and in my estimation a safe installation has OCP as close as possible to the point of interconnect. If it's through a backfed breaker, the breaker is the OCPD, and if it's a tap there needs to be a fused disconnect right next to it irrespective of whether there's a single inverter or a combiner panel feeding the tap. If there is a way to interpret the code which allows for or dictates an unsafe installation, then that interpretation is incorrect.

If you read the referenced John Wiles article, one concern is the thermal load associate with having both a main OCP and the additional per-inverter OCPs. I'd like to see FLIR photos or something else before I'd be convinced, especially since a panel rated for some total ampacity, including the main OCP that Smart$ wants to remove, should already have had the thermal loads calculated for the worst case.

Where I =can= see an issue is if someone were to make a PVAC using a main lug panel. For example, SquareD makes a 6 space, 100A main lug panel. If someone were to put a 100A breaker on one end, and 5 20A breakers in the 5 remaining slots, the total thermal load would be from 200A worth of OCP devices, which is greater than the UL testing would have covered. And that is what I believe Wiles is discussing.

 

[tallgirl]

Granted, taking "capable" to the extreme is absurd... but are there less-than-absurd interpretations? If yes, how do we qualify these?

Ah, the joys of subjectivity.

Once you let the camel's nose under the tent flap with "someone might do something dumb in the future ...", you either whack the camel and make it go away, or you welcome it with open arms. I know people who've said camel is good eats, and I'm inclined to think slitting the critter's throat and being done with said camel is the right approach. Which is to say, the installation should be inspected as-built, rather than as-might-be-screwed-with-in-the-future.

 

[jaggedben]

I do understand that some people seem fixated on "capable" in the way that it is being interpreted.

Short of welding the cover to the panel shut and outlawing metal cutting tools, a panel with all spaces utilized is still "capable" of being modified at some point in the future. This tells me that the interpretation of "capable" that's being advanced is absurd.

Granted, taking "capable" to the extreme is absurd... but are there less-than-absurd interpretations? If yes, how do we qualify these?

Actually the interpretation of 'capable' isn't absurd at all whatsoever. As the John Wiles article (that I linked to above) makes clear, it's very much the interpretation that the CMP intends you to make. If they didn't want you to make that interpretation, they would have taken out 'is capable of supplying' and simply put 'supplies'. They've been asked multiple times to take it out or to make exceptions to it and have rejected all such proposals. So they really do intend exactly what the wording says.

Now whether it's absurd that the code is so conservative on this issue is a different question. I would agree that it is. I think the difficulty has been in coming up with concise enough language that allows more flexibility without allowing too much flexibility. There are proposals for the 2014 code that will completely change things around if they were adopted, and will hopefully make things a bit better overall.

 

[Smart $]

I also see no issue if in the preceding scenario there is an interposing disconnect without OCP.

Let me get this straight; quite possibly I am misunderstanding you. Are you saying that something (anything) installed without OCP can be safer than with OCP?

The point of the code is safety, and in my estimation a safe installation has OCP as close as possible to the point of interconnect. If it's through a backfed breaker, the breaker is the OCPD, and if it's a tap there needs to be a fused disconnect right next to it irrespective of whether there's a single inverter or a combiner panel feeding the tap. If there is a way to interpret the code which allows for or dictates an unsafe installation, then that interpretation is incorrect.

I agree with you regarding the safest installtion has OCP as close as possible to the service point, but there is no such stipulation in the Code. With the Code permitting supply side connection, it opens the door to installing a disconnect without OCP in that path. Understand that I'm not saying no OCPD at all between inverter and service point.... my comment was regarding a non-OCPD'd disconnect between the service point and the PVAP. Code does permit the OCPD's to be located remote to the service point... even in not-readily-accessible locations.

 

[Smart $]

Ah, the joys of subjectivity.

Once you let the camel's nose under the tent flap with "someone might do something dumb in the future ...", you either whack the camel and make it go away, or you welcome it with open arms. I know people who've said camel is good eats, and I'm inclined to think slitting the critter's throat and being done with said camel is the right approach. Which is to say, the installation should be inspected as-built, rather than as-might-be-screwed-with-in-the-future.

What one does to the camel will likely depend a bit on how hungry one is...

Analogies are not very good means for making a determination. I think we need something a bit more tangible. :happyyes:

 

[tallgirl]

Actually the interpretation of 'capable' isn't absurd at all whatsoever. As the John Wiles article (that I linked to above) makes clear, it's very much the interpretation that the CMP intends you to make. If they didn't want you to make that interpretation, they would have taken out 'is capable of supplying' and simply put 'supplies'. They've been asked multiple times to take it out or to make exceptions to it and have rejected all such proposals. So they really do intend exactly what the wording says.

Now whether it's absurd that the code is so conservative on this issue is a different question. I would agree that it is. I think the difficulty has been in coming up with concise enough language that allows more flexibility without allowing too much flexibility. There are proposals for the 2014 code that will completely change things around if they were adopted, and will hopefully make things a bit better overall.

Oh, I totally understand that the "capable" interpretation that is being given =is= what is intended. I just happen to fall into the "this is entirely too stupid" camp.

The assumption is someone will make one non-compliant modification, but not another equally non-compliant modification. Trust the installers, or don't trust the installers, but don't half-trust them. PV systems do get retrofitted in various ways that can suddenly make what was once a compliant installation into a non-compliant installation.