What is an MCI?

MCI = Mid-Circuit Interrupter

The Tesla Mid-Circuit Interrupter (MCI) is used within PV strings and arrays to meet the Rapid Shutdown requirements of NEC Article 690.12 (2017 and 2020 editions). When paired with Tesla inverter products, solar array shutdown is initiated by any loss of AC power or system shutdown switch activation.

What does an MCI do?  

The MCI and PV Inverter function together to provide efficient Rapid Shutdown functionality on PV installations.

When solar shutdown is initiated, a relay inside of each MCI is opened, which interrupts the circuit and limits the total voltage present both inside and outside of an array.

MCI Placement

Code Requirements for MCIs

The position of the MCIs within a string is determined by the National Electrical Code. Article 690.12 states that the voltage inside of an array must be less than 80 VDC within 30 seconds of initiating rapid shutdown.

PV Hazard Control Systems

The PV Hazard Control Systems (PVHCS) listing is an additional safety certification that, when met, allows for greater voltage inside-the-array, which will require fewer MCIs.

When Tesla Solar Inverter and MCIs are installed together with listed modules and racking as a PV Hazard Control System (PVHCS), our inside-the-array boundary voltage limit is 165 VDC.

                   Requirements:

• PV Module listed to UL1703 and/or UL 61730-1 and UL 61730

• PV modules that use Staubli MC4 connectors

• Racking listing UL 2703, and rated for use with modules above

In the 2020 National Electric Code, the term "PV Hazard Control System" replaces the previously used terminology of "PV Rapid Shutdown Array" in the 2017 NEC.

How many MCIs are needed for a PVHCS array? 

An MCI must be installed at one end of each string and sub-array. It does not matter if the MCI is installed on the positive or negative end.

For a PV Hazard Control System, we are required to install 1 MCI after no more than every third module. 

• Each MCI is installed “in-line” with up to 3 PV modules, although less modules can be between MCIs depending on the array layout and needs. 

• Ensure no more than 5 MCIs are connected per PV string.

NOTE: In extremely cold climates where the VOC for 3 modules is greater than 165 VDC, MCIs must be installed after every 2 modules.

Rules and Best Practices

• One MCI must be after the first or the last module of each string.

• PVHCS systems require one MCI after no more than every 3 modules.

• No more than 5 MCIs total per PV string.

• Each array and subarray require one MCI on the first or last module of that string. 

• Always keep a string diagram with you during layout and installation.

• Mark the location of all MCIs on your string diagram for future service.

There is no minimum number of MCIs that are required for Tesla inverter products to function. MCIs are used solely to meet code compliance requirements of the NEC.

Installing MCIs 

The MCI features three mounting holes. These mounting holes allow the MCI to be mounted with screws or bolts. 

MCIs may be mounted:

• Directly to the rail

• Bolted to module frame holes

• Using an appropriate module hardware bracket

Key points to remember: 

• 1. Systems connected to Tesla inverter products, with Tesla MCIs, using Tesla modules, mounted with Zep hardware are considered a listed Photovoltaic Hazard Control System (PVHCS).

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• 2. For a UL-3741 listed rapid shutdown array, the maximum inside-the-array voltage is 165 VDC after initiating rapid shutdown.

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• 3. The maximum number of MCIs per string is 5.   

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• 4. There could be up to 3 modules between each MCI. 

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• 5. An MCI must be placed at either the beginning or end of each string and array.

You are now ready to install Tesla MCIs!