Panel Limit Feature

Feature Overview

As per the requirement from in the NEC, Tesla Site Controller software has implemented a Panel Limit feature that monitors the amperage flowing into an electrical panel / busbar from all controlled and uncontrolled sources (Grid, Solar Inverter, and Powerwall). As the measured current approaches the configured Panel Limit, the Site Controller first reduces the current contribution of batteries (at 90% of the Limit), then limits the output of controlled solar (at 95% of the Limit). If the measured current of all sources combined exceeds the configured Panel Limit for the site, the output of all PCS-controlled power production sources will be curtailed to 0.

For the purposes of Panel Limits, the Site Controller treats negative Site and Battery values as 0, therefore ignoring Site/Grid Export and Battery Charging and only factoring those values into the observed Panel Limit calculation if those values are positive. The result is that the Panel Limit effectively behaves like a Site Export Limit, as it prevents the controlled sources from contributing to an export value that is greater than the Panel Limit. If this behavior presents an issue for the system, and a Panel Limit is not required for busbar protection, the Panel Limit can be disabled (see Configuring Panel Limits below).

As detailed in Site Limit Feature, Site Export Limits are useful when a utility enforces restrictions on the maximum power that can be sent to the grid, and requires proof that the restriction is being followed. Site Export Limits also provide the following “best-effort” behavior that is not present in Panel Limits:

The Site Export Limit will cause Powerwalls to charge to try to avoid exceeding the limit, if the Powerwalls are able to charge.

Note

All current flowing to the loads will be fed from the grid supply, along with any sources uncontrolled by the Tesla Site Controller (which in turn need to be compliant with NEC Article 705.12 or their own PCS controls).

Note

The default overcurrent protection rating for the PCS Panel Limit is 80 A. The Panel Limit can be set by Qualified Personnel to a maximum of 200 A.

Warning

The maximum PCS operating current setting shall not exceed the panel rating of any PCS controlled panel.

Example Single Line Diagrams for Common Installation Configurations

Figure 1. Powerwall+ and Backup Switch for Whole Home Backup

Figure 2. Powerwall 3 and Gateway 3 for Whole Home Backup

Figure 3. Powerwall 2 and Gateway 2 for Partial Home Backup

Application of the Panel Limits Feature

The Panel Limits feature is most functional when using Powerwall+ or Powerwall 3 since the solar and battery currents are “controllable.” If using Panel Limits with any standalone inverter (Tesla Solar Inverter or third-party), this solar current is not controllable. There is still benefit to using Panel Limits to limit the contribution of backfeed from the energy storage system (Powerwall 2, Powerwall+, or Powerwall 3)

Typical Setting: Amperage of main panel rating, or any panel on site that requires protection from backfeed.

Example:

A system has a 200A main panel rating with a 200A main breaker.
When following the 120% rule in 705.12 of the NEC, the 200A main panel can have 240A of combined supply source breakers, which includes the 200A main and a 40A breaker for new generation.
The system design calls for 2 Powerwall+ units, 1 Powerwall 2, and 1 40A standalone PV inverter.

Solution:

Backfeed breakers land in a backup load center. Panel Limit Setting: 160A.
The 40A from uncontrolled PV is acceptable by the 120% rule. Load centers will not see >160A from controllable sources.

Note

This feature impacts the maximum contribution of backfeed from controllable sources. It will not restrict the energy that can be drawn from the utility. The setting should not be used to undersize load centers or overcurrent protection; equipment should always be sized based on loads.

Note

No additional CTs are required for this, other than those already needed to capture the whole site and all solar.

Configuring Panel Limits

The Panel Limit can be configured by Qualified Personnel in the Tesla One app; to change the default limit of 80 A (between 10 and 200 A), select Settings > Advanced Settings > PCS Settings. This setting or changes to the setting should always be aligned with the value approved by the local jurisdictional authorities.

Note

Only Qualified Personnel may edit the Panel Limit field. The installer will need to enter their license number to verify appropriate access to the setting. The license number does not need to be re-entered until the installer signs out of Tesla One, then signs back in.

To disable the Panel Limit, delete the value in the Continuous Current field.