705.13 PCS panel rating

[TallTimber]

I have been dealing with some residential installations utilizing PCS protection of main service panel. The issue I am having a hard time making a decision on is if the PCS supply limit can be 100% of the bus bar rating.

For instance one installation is a 200A MSP with a 200A 80% main breaker. Pretty cut and dry standard home install. Solar is landed on the lowest position in the panel and would not comply with any of the 705.12 sections. For this reason they have opted to implement PCS to curtail solar supply at the predefined limit. This limit can be set at 100% of the bus bar rating correct(200A)? The PCS is just taking the sum of supply currents and curtailing the PV when load reaches the PCS limit. I cannot see how allowing the solar to operate between 160A-200A would cause issues on the panel as in this range the solar would only be taking load off the main breaker. I feel like there are a lot of moving parts here but I cannot find a corner of this problem that leaves the panel in a worse position than not having any solar, or maxing out the 120% rule.

 

[pv_n00b]

Since you are calling it a PCS I'm going to assume you are using the 2020 NEC. The PCS has to do a few things things for the service panel to fall under 705.13.

• Monitor the PV system output and the current being drawn from or sent to the utility.
• When the loads on the service panel are drawing current from the main and the PV system the PCS has to modulate the PV system output to maintain the total current supplied to the bus to 200A or less.
• When the system is exporting to the utility it has to monitor the current through the main CB and modulate the PV output to keep the current through the CB under 80% of the 200A CB rating and to the bus under the 200A bus rating.

If the PCS is only monitoring the PV output and not monitoring the current through the main CB to the service panel then the service panel falls under 705.12. If the PCS just limits the PV to a set maximum output then the service panel falls under 705.12 using that set output and not the full inverter output in the calculations.
For a 200A MSP with a 200A main and a PCS that only monitors and limits the PV output the limit for the PCS would be set to 32A. You could have an inverter with a higher output as long as the PCS was set to 32A. This limitation is why the PCS monitoring the current coming from the utility is important.

 

[TallTimber]

Since you are calling it a PCS I'm going to assume you are using the 2020 NEC. The PCS has to do a few things things for the service panel to fall under 705.13.

• Monitor the PV system output and the current being drawn from or sent to the utility.
• When the loads on the service panel are drawing current from the main and the PV system the PCS has to modulate the PV system output to maintain the total current supplied to the bus to 200A or less.
• When the system is exporting to the utility it has to monitor the current through the main CB and modulate the PV output to keep the current through the CB under 80% of the 200A CB rating and to the bus under the 200A bus rating.

If the PCS is only monitoring the PV output and not monitoring the current through the main CB to the service panel then the service panel falls under 705.12. If the PCS just limits the PV to a set maximum output then the service panel falls under 705.12 using that set output and not the full inverter output in the calculations.
For a 200A MSP with a 200A main and a PCS that only monitors and limits the PV output the limit for the PCS would be set to 32A. You could have an inverter with a higher output as long as the PCS was set to 32A. This limitation is why the PCS monitoring the current coming from the utility is important.

I maybe should have elaborated, tried to avoid a wall of text.

The systems I am dealing with have CTs on the PV and Utility side of the 200A MSP. This allows the PCS to take the sum of the PV supply and the Utility supply and keep that less than or equal to a set amount as required by 2020 NEC 705.13(B). My line in the sand here is if the bus bar rating that the PCS follows should be the full rating(200A) or 80%(160A). I am leaning towards the 200A rating as the only reason we have an 80% rating is because of the main breaker. A PCS set at 200A would comply with 705.13(B) as we would not be exceeding the rating of the bus bar. These systems are 20kW so back feed over 80% of the MSB could never be an issue.

As for the PV only monitoring, we typically refer to that as a "limited export" system. Which I would agree still falls under the requirements of 705.12.

 

[jaggedben]

For a listed PCS the instructions should be clear about what to enter and I think the PCS should account for any 80% factor needed on the solar. So if the instructions say to enter the busbar rating then enter the rating on the panelboard label (200A).

I believe the PCS will apply an 80% factor to the PV per 705.12, for cases where the actual draw was between 160 and 200A. For example at 180A draw, it could limit the PV to 80A so that 100A comes from the utility, and 100A+(80A×1.25)=200A. But this is accountable to the PCS listing and I don't think you should have to worry about it.

 

[PWDickerson]

We have started to install a bunch of residential PSC system just like this made by Tesla, SolarEdge, FranklinWH and others. With all of the systems we have done so far, there was an installer training and certification piece, and the PCS settings are entered during commissioning using the manufacturers app on a smart phone. Some are more straightforward than others, but by and large, not too complicated.

OP: "The issue I am having a hard time making a decision on is if the PCS supply limit can be 100% of the bus bar rating."

You shouldn't really need to concern yourself with making that decision. The App should do it for you. You should just enter in the values for the busbar rating, the main breaker rating, and the max output rating of the PV inverter. The PCS system takes care of the rest.

BTW, there is strictly no longer a need to place the solar breaker opposite the main when using PCS busbar control, but I still like to do it because some of the systems will default to limiting the inverter output to the 120% rule value if there is a PCS component failure and the install is configured with the PV breaker opposite the main.