Install Mid-Circuit Interrupters in PV Array
The Tesla Mid-Circuit Interrupter (MCI) is used within PV strings and arrays to meet the Rapid Shutdown requirements of NEC Article 690.12. For information on how the Tesla MCI functions with Powerwall 3 as a Photovoltaic Rapid Shutdown System (PVRSS), see Appendix D: Solar Rapid Shutdown.
Note
The Tesla MCI is also referred to as the Tesla Solar Shutdown Device.
Note
There are two models of Tesla MCI,
MCI-1 and MCI-2. Both perform the same function, but they have unique specifications
(see Specifications) and
installation methods (outlined in the following installation instructions).
MCI Locations
The quantity and placement of MCIs necessary to meet Rapid Shutdown requirements in
NEC Article 690.12 (B) and Canadian Electrical Code (CE Code), C22.1, is governed by
the UL 3741 (PV Hazard Control System or PVHCS) listing applicable to various array
types. Refer to the UL 3741 Application Addendum for a complete
list of systems in the US and Canada that have been certified to UL 3741 when
installed together with Powerwall 3 and Tesla MCI. Always refer to
the applicable PVHCS datasheets and references included in the addendum for detailed
instructions to ensure adequate compliance and safe operation.
Warning
It is critical
that the minimum number of required MCIs is installed; without the appropriate
number of MCIs, the MCIs may not function as intended which could lead to
equipment damage.
Warning
Where it is allowable to install
only (1) MCI per string, MCI-1
must be used.
Installation Best Practices
- Ensure that Tesla MCIs are not stored prior to installation in environments with moisture or dirt
- Install the Tesla MCI in line with the PV modules using standard Staubli Type MC4 certified compatible connectors
- Ensure the MCI is minimum 12.7 mm (1/2 in) from glass or backside of a PV module
- For flat plate solar panel installations, ensure the MCIs do not directly contact the roof. For Solar Roof installations, see the Solar Roof Installation Manual for proper mounting guidance
- For easier servicing of flat plate solar panel installations, install the MCIs on the perimeter of the array if possible
- Always secure the MCI with appropriate wire management
- Ensure no more than 5 MCIs are connected per PV string
- Ensure that total DC circuit
length, from + MPPT terminal to - MPPT terminal (including module wire leads,
jumper wires and all wiring within the array boundary for any individual
string), does not exceed 160 m for single strings or
paralleled strings
- Total DC circuit length refers to the entire round trip wire length, from inverter to the roof, then back to the inverter
- Ensure paralleled strings are the same length, or as close in length as possible (if the paralleled strings are different lengths, it is likely the MCIs will not function properly; this likelihood increases the greater the difference in paralleled string lengths)
- Keep a completed string diagram indicating the layout of the array and the placement of each MCI (either a paper copy, electronic copy available in the field, or both)
Installing MCIs
- Connect the positive and negative leads of the MCI to the corresponding positive / negative MC4 terminals of the PV modules or solar roof modules.
- For flat plate solar panel
installations, attach the MCI under the module frame.
- To attach the MCI using ZEP Home Run wire clips, install the (2X) clips to the module frame and clip both ends of the MCI leads, then slot the MCI underneath the metal frame.
- To bolt the MCI to an existing hole in the frame, align the MCI through hole with the frame and use the appropriate mounting hardware as defined in MCI Mounting Hardware Sizing .
- To install MCI-1 on Solar Roof
installations, mount the MCIs directly to the roof, directly above the Tile
Footlaps and positioned at a slight angle as pictured below. Use the appropriate
mounting hardware as defined in the table below.
Table 1. MCI Mounting Hardware Sizing Through Hole Hardware A Wood Screw (No. 8) Screw (up to M4 (5/32")) B Bolt (up to M8 (5/16")) C Nail (8D) Screw (up to M4 (5/32")) - To install MCI-2 on Solar Roof installations, attach each lead of the MCI to the corresponding Staubli MC4 connector, and place an MC4 cap on each of the remaining MC4 connectors. Secure the cables in the wire management clips.