Recommended number of MCI units

[offandonagain]

I'm dealing with a Tesla solar panel issue: production drops to 0 with a PVS_a044_FaultStatePvStringSafety alert. Tesla replaced my Powerwall+ inverter which didn't solve the issue, so they're now scheduled to replace MCI units.

One of the Tesla technicians that was here previously told me to talk to the MCI crew and ask them to only keep a single MCI unit per string of panels. He said that's how they do it now, it's simpler to service, and up to code. That made sense to me initially, although after looking at Tesla's documentation I'm not sure anymore.

The docs suggest 1 MCI per 3 modules for my panels (T400H). They also mention:

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.

I currently have 3 strings of ~10 panels each, and each string has 4 MCI units according to plans.

Has anyone here dealt with MCI replacement and has thoughts on the best number of modules? It seems reasonable that fewer units would be easier to maintain and cause fewer issues, but I'd like to understand the trade-off. (Also, I'd normally just let the service crew figure this out, but given how long it's taking to resolve my issue, I need to be more involved.)

 

[yblaser]

Not an expert but my understanding is that the number of MCIs required is primarily a result of a safety requirement to keep the max voltage on the roof below a certain value when the system is shutdown. Decreasing the number of MCIs would increase the voltage on the roof when shutdown and wouldn't meet Rapid Shutdown requirements.

 

[offandonagain]

Not an expert but my understanding is that the number of MCIs required is primarily a result of a safety requirement to keep the max voltage on the roof below a certain value when the system is shutdown. Decreasing the number of MCIs would increase the voltage on the roof when shutdown and wouldn't meet Rapid Shutdown requirements.

Good point, all of this is assuming a single MCI unit per string is up to code. The Tesla technician seemed convinced that's the case, but I'll double check with the MCI crew.

 

[offandonagain]

Update here: Tesla crew came and replaced all the 12 MCI units with the newer versions (they kept the same number of units). One of the units was bad, it was clear a fuse blew just by looking at the unit.

This seems like a pretty bad single point of failure and a servicing nightmare. They had to rip out all the panels, replace the MCIs and put the panels back. It took two servicing visits (and multiple visits prior to that to even diagnose the problem) and months of little to no solar production.

The MCI unit failed after less than two years in operation. I really hope the new units are more reliable :/

 

[BGbreeder]

 

[Vines]

Update here: Tesla crew came and replaced all the 12 MCI units with the newer versions (they kept the same number of units). One of the units was bad, it was clear a fuse blew just by looking at the unit.

This seems like a pretty bad single point of failure and a servicing nightmare. They had to rip out all the panels, replace the MCIs and put the panels back. It took two servicing visits (and multiple visits prior to that to even diagnose the problem) and months of little to no solar production.

The MCI unit failed after less than two years in operation. I really hope the new units are more reliable :/

Glad you got the correct parts replaced without sacrificing safety.

It could be that some jurisdictions have not adopted the portions of the code requiring module level rapid shutdown. Regardless of that I think its best that you have the recommended number of MCI on your roof.

MCI have a 25 year warranty and are pretty easy to replace. I hope the replacements are more robust.