Panel Limit Feature

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Note
The Panel Limit feature is not automatically enabled for Powerwall systems without a Powerwall+.

Site Controller Software

UL 1741 Power Control System requirements and the “Panel Limits” feature are implemented in the Tesla Site Controller software. Tesla’s Site Controller software is used across all Tesla Powerwall sites. Site Controller software controls the entire energy storage site and communicates over CAN and Wi-Fi. The Backup Switch and Neurio (W1/W2) meter (if present) are part of a Powerwall system and contain two current sensors, one per phase, to act as a site and/or solar meter, which measure the grid and uncontrolled power production sources.

Feature Introduction

As per requirement from NFPA 70: 2020, 705.13, Tesla Site Controller software has implemented a Panel Limit feature that monitors the amperage flowing into an electric 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, then limits the output of solar. 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.

Notes

  • The “Virtual Panel” may include several panels and conductors, and is considered as one single panel with respect to the Panel Limit feature.
  • Grid is measured by one or more physical site meters to measure the full current flowing into the virtual panel from the grid.
  • Uncontrolled Sources include external Solar Inverters or similar sources to the virtual panel
  • Controlled Sources include Tesla ACPW and Tesla Powerwall+ in any quantity and combination (e.g. two ACPW and one Powerwall+)

Example Single Line Diagrams for Common Installation Configurations

  • Powerwall+ and Backup Switch for Whole Home Backup

Monitoring (2020 NEC, 705.13 (A))

  • Tesla Site Controller receives meter data from the Backup Switch or Backup Gateway 2 (meter X and/or meter Y), and/or Neurio (W1 / W2)
  • Tesla Site Controller receives controlled current data of the AC output of the Powerwall+ and the AC output from the AC Powerwall.
  • Current flowing into the Virtual Panel (positive current in all three meter types) is added together for comparison with the Panel Limit.
  • Implementing panel limits does not require dedicated meters.
  • Where communications are lost with any meter, all controlled power production sources are curtailed.

Settings (2020 NEC, 705.13 (B))

  • The panel site limit setting is restricted to Tesla personnel only and can be set in the config file with “panel_max_current” option.
  • As there may be more than one physical panel and conductors included, the panel limit is set to the rating of the smallest busbar within the “Virtual Panel”.
  • By default, all Backup Switch installs are automatically configured with a 100A Panel Limit. As 100A is the minimum allowable size for a single family dwelling per NEC 230.79 (C), this protects any main panel busbar for a single family dwelling with any quantity of Tesla Powerwall.
  • The maximum allowed Panel Limit setting (in Amps) can be verified by viewing the Summary page in the device setup interface in Tesla One.

Overcurrent Protection (2020 NEC, 705.13 (C))

  • The Tesla Site Controller software is certified under UL 1741 PCS as secondary overcurrent protection.
  • Overcurrent devices are still required for each circuit as required by the National Electrical Code.
  • Software curtails the output of the battery and solar as load approaches the Panel Limit. Once battery and solar have been curtailed, loads would be fed exclusively from the grid. Thus the busbar is protected by the main breaker, or the breaker in an upstream panel, as intended prior to the addition of secondary power sources.

Single Power Source Rating (2020 NEC, 705.13 (D))

  • Each Powerwall connection to the electrical panel requires an independent 30 A circuit breaker.
  • Each Powerwall+ connection to the electrical panel requires an independent 50 A circuit breaker.
  • This breaker serves as the disconnect for the unit, and must be wired in accordance with local wiring codes and regulations.
  • This breaker is by design smaller than the smallest panel used within the system, and smaller than the panel limit.
  • The service disconnect overcurrent rating is unchanged based on the Panel Limit.

Access to Settings (2020 NEC, 705.13 (E))

  • The Panel Limit feature is only adjustable by Tesla personnel.
  • The PCS controlled current setting will be auto-enabled and the only method to change it will be to call Tesla. A Tier 2 support technician will adjust the setting via Tesla software programs that are only accessible with a Tesla username and password.