PV system into 1200 Main Distro Panel with No Service Disconnect (I think)

[Xamacho]

I'm asking the PV forum because of the nature of this project. This is in Texas, which is all NEC 2020 now. I will say, when we started this project, the particular city was in NEC 2014.

I have been engineering a bi-facial module "140" kW solar system for what seems like a year now, and am trying to get some clarification about an interconnect. I have designed it two ways: a supply side interconnect [705.11] into a new 1200 A Service Disconnect OR a load side interconnect into a feeder conductor [705.12(B)(1)(b)] from a Main Panel (listed as suitable for service disconnect) but without a service disconnect on the premises. Or at least, I and the Master Electrician have been saying there is no service disconnect means. Due to Covid, I have not inspected the sight in person only via facetime with the Master Elec. and the PV Install Company.

First, the details about the location. The service entrance conductors leave a 225 kVA, 208/120 Wye Secondary, Utility Transformer (with CT Meter) 10' from the building wall, go underground, and enter inside the building into the bottom lugs of an ITE 1200 ("suitable for service") switch distro board. There are 5 switches in this panel - two 600A Siemens Vacu Switches, one 300A switch, and two 200A switches. On the inside of the panel I can see "suitable for service" but none of the 5 switches have any such labeling. Let's say the original electrical install happened in the mid to late 70's as I see a stamp that says "inspected in 1974".

The big question - are these "service disconnects"??? In my opinion NO, this is main switch board whose bus bar and service entrance conductors has no OCPD, no fault protection, and no disconnecting means. Thoughts?

The City in question has a (and I'm paraphrasing) - if you supply side interconnect, you have to bring up all equipment up to NEC 2020. This is the route I am trying to make the clients (PV Install Company and Building Owner) take. Actually install a 1200 Amp service disconnecting means (with OCPD and rated for available AIC) and make the PV supply side interconnect into this new panel.

I have told the clients that I believe the install of a service disconnecting means is paramount to safety and my preferred design. BUT if those 5 switches are considered service disconnecting means, and the AHJ agrees that they are, then I will advise the clients to do a 705.12(B)(1)(b) Feeder Interconnect (which requires the install of a new 600 Amp fusible switch).

Last question: the PoCo told me the available fault current was 19,893 Amps at their transformer (and I calculated 19,721 Amps at the line side of the new service disconnecting means). Since I don't know the motor contribution, should I make worst case assumption and require the fuse or breaker rating to be 30,000 or 40,000 AIC without having done any calculations? Is there an accurate method for determining existing motor load as it pertains to calculating AIC? This building had a bunch of exhaust fans in a previous incarnation, but they have been taken off line.

PS the City has been a bit unresponsive to these "no service disconnect" inquiries (via email and phone), and so I look forward to your responses.

 

[don_resqcapt19]

You were permitted to have a panel like that without a main and up to six service disconnects in it. Those five switches are the service disconnects.

The 2020 code makes that type of panel illegal as each service disconnect must be in its own compartment or enclosure.

 

[jaggedben]

Two thoughts...

1) Can you add two disconnects on the outside wall? One would be 1200A and feed the existing switchboard. The other would be for your PV system (400A?). You'd have to dig up the conduit from the transformer to intercept it and tap it to both disconnects, plus re-feed the inside switchboard. But it would meet code.

2) Can you add a main breaker to the switchboard? It would then be subject to the 120% rule, which I gather is going to reduce the size of your PV system, or at least force you to accept a very high DC-to-AC ratio. But still probably worth looking at.

Everything Don said is correct. Those 5 switches are definitely your service discos, and that was fine before. Kindof lame for the AHJ to not grandfather that panel but that doesn't sound like a battle you can win.

 

[Carultch]

Kindof lame for the AHJ to not grandfather that panel but that doesn't sound like a battle you can win.

Also unfortunate 705.11 doesn't have a clause to clarify that you can add a line side interconnection at a 6th disconnect, to grandfathered MLO service equipment that was built prior to 2020. That would settle the issue so you aren't at the mercy of the AHJ's opinion. Adding a 6th disconnect presents no new safety hazard that wasn't already present when the service equipment was first built.

It would also be great if NEC 705.11 would clarify the issue of neutral-to-ground bonding in line side interconnections, so that enforcement isn't so inconsistent.

 

[don_resqcapt19]

250.25 in the 2020 code addresses the bonding for permitted line side connections.

250.25 ...
(A) Grounded System.
If the utility supply system is grounded, the grounding of systems permitted to be connected on the supply side of the service disconnect and are installed in one or more separate enclosures from the service equipment enclosure shall comply with the requirements of 250.24(A) through (D).

 

[jaggedben]

250.25 in the 2020 code addresses the bonding for permitted line side connections.

I hadn't noticed or recalled that. Thanks.

 

[don_resqcapt19]

... Kind of lame for the AHJ to not grandfather that panel but that doesn't sound like a battle you can win.

There was a PI for an exception to address this in the 2023 code. It would grandfather existing service equipment and permit adding disconnects up to a total of six. We will get an idea on how this PI fared when the First Revision is published on 7/2/21.

 

[Xamacho]

Thank you for your responses. I was concerned because I never saw any of those switches labeled as service discos.

To answer some of your questions/points in random order:

It is a 5 switch panel, and there is no room for another. The inverter output current at 125% = 347 Amps and so we cannot use any of the busbar interconnection rules (1200 amp bus bar). Because of this, a supply side interconnect was what I was considering our best option. The PoCo granted us permission to supply side interconnect into the service conductors inside their transformer, only if we paid for the upgrade of the transformer from 225 to 250 kVA.

At this point, I designed a system based on 705.12(B)(1)(b) into one of the 600 Amp Feeders. I was under the impression that if we used this rule, we weren't subject to the busbar requirements. I have never designed a system using this rule, and assumed, using Kirckhoff's CL as my logic, that the conductors were sized accordingly and the busbar ampacity didn't need to be taken into consideration. I just read this mike holt forum from last year that two persons in this thread responded to and I still must ask the same question for clarity sake - is the 1200 amp busbar subject to the rules of 705.12(B)(3) (busbar ratings) when we're attempting a 705.12(B)(1)(b) interconnection?

I see in the 705.12 preamble that the equipment shall comply with 705.12(A) though (E). But I still ask this forum for their advice.

 

[jaggedben]

... I still must ask the same question for clarity sake - is the 1200 amp busbar subject to the rules of 705.12(B)(3) (busbar ratings) when we're attempting a 705.12(B)(1)(b) interconnection?

...

No, that busbar is supply side and is therefore not subject to any of the load side rules. This is clear. And the design you are describing is absolutely fine, in the parts you've described.

It is somewhat semantic, but the way I interpret things is that you are describing a supply-side connection in series with a load side (705.12(B)(1)) feeder connection. One can have any number of connections in series so long as each one follows an appropriate rule in its own context and location.

That leaves open the question of whether you are doing a supply-side connection in such a manner that you're triggering your AHJ's rules that the service panel has be brought up to the 2020 NEC. Maybe if you put it to them that you aren't touching the service panel, and you are only doing the connection at a feeder, then they will grandfather the existing service panel. After all, there's a difference between proposing to add a new service disconnect (that would not be compliant with the current code) and simply backfeeding an existing one you are not modifying in anyway. But the NEC is silent on what triggers this sort of stuff, so you'll have to abide by what the AHJ says. Hopefully you haven't prejudiced your conversation with them too much already.

(BTW, you are using 2017 NEC references. Maybe switch to 2020 references if that's what the AHJ is now going by.)

 

[Xamacho]

Thank you jaggedben, I always appreciate your insight.