NEC PCS rule for uncontrolled/unmonitored sources

[hitehm]

All - Please see the attached SLD of the backfed PW3 system we're installing with an existing 20A backfed unmonitored system and refer to 705.12 and 702.13 for this discussion. Our issue is as follows: The engineering team is setting the PW3 Panel Limit feeding the 100A main panel bus to 60A. They limited the max monitored and controlled pcs sources (the pw3 and solar) to 80% of the bus rating. This is based on Tesla's own doc that states (in so many words) you must consider the bus to be 80% of its rating because of the way manufacturers test their equipment. That's fine, so that would make the max PCS output 80A. However, they are also subtracting the 20A unmonitored existing system from the PCS making the max PCS panel limit 60A. Although this seems logical in several ways, we can find absolutely no code or AHJ rule that says to do this. In all of 705.13 the only language that deals with unmonitored sources on the busbar is 705.13A that states:

705.13A Any busbar or conductor on the load side of the service disconnecting means that is not monitored by the PCS shall comply with 705.12


There is no mention of subtracting this value from the monitored controlled source. The engineering team highlight 705.13B as their reasoning but it vaguely at best only states the PCS can be no more than the OCP it's connected to if connected to an unmonitored bus. Tesla considers this bus monitored if it has CTs on the utility consumption input to the bus and of course its own output feeding the bus.

705.13B - Where the PCS is connected to an overcurrent device protecting any busbar or conductor not monitored by the PCS, the setting of the PCS controller shall be set within the ratings of that overcurrent device.

Therefore, we believe by the CODE, the PCS Panel Limit setting should be 80A not 60A and let the unmonitored existing system follow the 120% rule of 705.12. But this brings up the obvious problem that the existing system can still apply 20A in addition to the 80A pcs from the PW3 which combined is more than 80% of the bus rating. Also, without monitoring the existing system, even though you are monitoring the main bus source inputs from both the utility and the PW3, is the bus FULLY monitored if you don't monitor ALL the other source on that bus?

So the question for all is, how is this interpreted correctly and should the unmonitored source be subtracted from the PW3 panel limit?

 

[Elect117]

The PCS + Existing Solar would need to met 705.12(B). I don't know if I am understanding the situation correctly, but you can not treat each separately. That is why the code uses the word "source(s)". Implying if there are more than one source you need to consider that when doing the calculation on the service.

P.S. Is there no production meter on the existing? The utility isn't going to mind having 3 meters?

 

[hitehm]

The PCS + Existing Solar would need to met 705.12(B). I don't know if I am understanding the situation correctly, but you can not treat each separately. That is why the code uses the word "source(s)". Implying if there are more than one source you need to consider that when doing the calculation on the service.

P.S. Is there no production meter on the existing? The utility isn't going to mind having 3 meters?

So by what you're saying, having any unmonitored system on the main bus would negate the ability to use PCS on any system since it would force you to use 705.12 rules for all the systems connected to the common bus. Respectfully, I can't see that being correct although I might be misunderstanding you. 705.13 allows you use PCS in place of 705.12 for PCS controlled sources as long as you follow its rules. If not, you must refer back to 705.12 for bus protection. In fact, this in itself is a grey area they should've been more clear on but here is an article where the author states that 705.13 can effectively bypass 705.12

Tech Guide: Designing with panel-level PCS in Powerwall Systems

Adrian Hood, May 2024 Part 1 - What is panel-level PCS? POWER CONTROL SYSTEM (PCS) technology represents one of the most significant breakthroughs for installers and homeowners in the residential solar and battery industry in recent years, overcoming one of the biggest restraints that kept solar fro

www.linkedin.com

I assume the existing system has production metering, not sure why the utility would mind.

 

[Elect117]

So by what you're saying, having any unmonitored system on the main bus would negate the ability to use PCS on any system since it would force you to use 705.12 rules for all the systems connected to the common bus. Respectfully, I can't see that being correct although I might be misunderstanding you. 705.13 allows you use PCS in place of 705.12 for PCS controlled sources as long as you follow its rules. If not, you must refer back to 705.12 for bus protection. In fact, this in itself is a grey area they should've been more clear on but here is an article where the author states that 705.13 can effectively bypass 705.12

Tech Guide: Designing with panel-level PCS in Powerwall Systems

Adrian Hood, May 2024 Part 1 - What is panel-level PCS? POWER CONTROL SYSTEM (PCS) technology represents one of the most significant breakthroughs for installers and homeowners in the residential solar and battery industry in recent years, overcoming one of the biggest restraints that kept solar fro

www.linkedin.com

I assume the existing system has production metering, not sure why the utility would mind.

I got busy yesterday so I couldn't respond timely, but let me try to stab at it.

PCS monitors the amps seen at the service or feeder to ensure that that panel never exceeds it's rating.

If you have more than one source your concern is now the direction of current flow and if any direction could exceed the panel's rating and the PCS would not catch it because it only checks the "Main breaker" side.

I wonder, then, can the 20A of uncontrolled solar + the PCS controlled solar ever exceed the panels rating with 100A of current going to the loads. I believe the answer is yes.

The PCS will not see the contribution of the uncontrolled solar. It will say, "ahhh there is only 40A of load on this board from the utility. So the PCS will say we can supply 40A of continuous or 60A noncontinuous". But it doesn't know whether or not the uncontrolled solar is offsetting an exisitng 20A already. So your current seen near the uncontrolled and PCS controlled could accidentally exceed the panel boards continuous limit.

That is just how I see it.

 

[Elect117]

I should add. If the PCS is smart enough to monitor both existing sources to ensure it never exceeds their combination than you should be fine.

You would essentially have 3 interconnected sources at different locations in one panelboard.