ECN Forum Forums General Topics of the Trade General Discussion Area PV Disconnect and 6 Disconnect Rule

Harold:
I would say about 75-80% of the resi solar installs are grid tied via a backfed CB, as opposed to line side taps on the service feeders. As Tesla said, the inverters shut down output when there is a POCO disruption, and the required utility exterior AC disconnect is in place.

As to the disco being a possible '#7'....SABrown has that covered.

John and Tesla,

I understand that when there is "No light" the inverters will shut down. However IF you tap the main service entrance conductors. There will always be current/voltage on the feeder between the last connection of the PV system and the service entrance. Right?

Even if the feeder that runs from service panel to PV system, there will still power on the feeder but this will be protected by the CB in the service.

Harold:

The 'tap' being ahead of the main OCP will have power unless there is a POCO interuption. That goes for the panel, and the utility side of the inverter.

Most, if not all of the inverters will shut OFF when the circuitry senses 'no power' from the POCO grid tie, like a power failure, or pulling the meter IF the line tap is ahead of the main OCP.

You can achieve the inverter shut down by opening the POCO required disconnect (AC).

The NEC, like all codes, can only react. Technology will ALWAYS be ahead of our rulebooks. Why do you think I keep advocating for good design, rather than 'code complaince?'

It seems obvious: everything should have an 'off' switch. With solar panels, IMO this switch needs to be as close to the source of the power as possible. If your only disconnect is at the panel, a fault in the supply line will never be shut off - at least, as long as the sun shines.

Getting back to our 'six throws' rule: I don't want to have to flip more than six switches to kill power to every branch circuit in the building. Whether that means I flip six branch circuit breakers, six supply disconnects, or some combination of the two doesn't matter: I want no more than six switches.

Can we count on the inverter as a disconnect, simply because it disconnects the solar feed in the absence of utility power? IMO, no. Make that "Hell No." Microchips and relays fail, or 'leak.' I want a real switch.

Since I want my six disconnects grouped together, I guess that means I want a disconnect on both ends of the solar feeder. Redundant? I don't see it as such. Rather, I see them as serving different purposes. One is there to contain a fault, while the other is there to remove the supply.

John and Reno,

Beside the six switch rule. My first concern is, if you tap the service entrance conductors and run to PV equipment. Do you need a disconnect or over current protection once it leaves the service panel?

What happens if someone cuts that wire or pipe? If it is dark and the POCO has a black out the answer would be nothing.

If the POCO blacks out during the day, the inverter will turn off. All is safe.

BUT if the PV goes down and the Poco is on, then that wire/pipe is energized.

I have successfully installed PV disconnects in addition to the six existing 'utility' service disconnects twice. In one case our PV disconnect was the seventh disconnect and in another we had two 800A PV disconnects in addition to the six 'utility' disconnects. Each service 'source' is allowed to have six disconnects according to the code, utility, wind, solar, cogen, etc. I would recommend confirming that your AHJ is up to date on this allowance AND will allow it before proceeding though as many have their own requirements and/or interpretation of 230.2(A), 230.71(A) and 230.82. 230.71(A) being the most critical because it allows up to six disconnects PER service and 230.2 defines the utility and PV as two separate services which would therefore allow up to 12 disconnects.

You can not rely on the inverters required anti-islanding features to shut down the PV service supply when it is tapped on the line side of the utility disconnects as most all commercial systems should be.

Most of the PV disconnects we install are in the 600A to 2000A range and are subject to the 10-25 foot tap rules, BUT some AHJ's and inspectors have asked why we're installing the disconnects so close to the utility service since we had inverter disconnects 200-800 feet away, they are actually considering the feeder a utility lateral and as such do not see a need for the additional fusible disconnect since the utility is not required to do it. We install them regardless of that though as the fire department is usually more interested in having the disconnects as close together as possible.

Even AHJ's that want the PV disconnects within 10-25 feet have allowed us some leeway when there was NO way to get a 1600A-2000A visible blade fusible bolt switch that close, as close as possible was still acceptable even if 50-75 feet was the best we could do. We have not done this, but they have been free with expressing their allowance of this.

I cannot agree that the code would allow you to, say, have twelve branch circuit disconnects as the onlt means of disconnecting the building simply because there are two "services."

As far as I'm concerned, there's only one 'service' when there's only one distribution system within the building - np matter how many sources of supply there might be to that system. I will concede that the code language is conflicting on this point.

The conflict is seen in the wording of the section that discussed the number of means of disconnection. It is clear that the authors imagined separate systems - for example, where a 480/277 service served 3-phase loads and a 120/240 service served single-phase loads. I cannot accept that the authors ever imagined both services feeding the same panel.

A similar situation might be where a generator is used- except that the traditional transfer switch was designed to ensure that only one of the sources was ever used at one time.

Harold, I cannot speak for NEC requirements. I don't think the code panel itself has come to grips with the various design issues- and the solution to that gets into the basic arrangement of the NEC. Personally, I think ALL sources of power need to be grouped into their own section, and treated as a whole.

As for overcurrent protection, I see that as a design issue. That is, there ought to be overcurrent protection at the point of generation.

Where the two systems come together, there needs to be some way to limit power from 'going the wrong way.' This might very well mean a breaker at that point as well. Otherwise, you have the potential of a fault tripping the feed from one direction - but still have the risk of a backfeed to the fault.

I note that the UL-listed arrays with 'micro-inverters' at the array and a simple circuit to the panel require that you backfeed the power through a breaker into the panel.

Vindeceptor ... you really got me thinking about this over lunch ....

Let's agree with your interpretation. That would mean that I could instal seven of the micro-inverter panels, backfeeding into six breakers in the panel- and not have any requirement to add a main disconnect, even if the 42-circuit panel was now full. (7 x 6 = 42).

After all, each of the micro-inverter units would be an independent service. Yet, none of the branch circuits would be powered only by a specific unit.

Add to that the main breaker, or the back-fed breaker on the Utility side, and you now need to flip EIGHT breakers to ensure that all of the branch circuits are dead.

I have a problem with this scenario. I can't speak for others - but I intend to have the next code cycle address this topic.

Reno:
I would have to take issue with the above scenario. I don't know what to base it on real quick, but that sounds like an issue to me also.

TBC