FlexBoss21 - Inline Fuse for Panels in Parallel

[AlyGreen]

I am learning about inline fuse requirements so a beginner question.

26.00A - FlexBoss21 MPPT 1/2 (max current)
8.91A - Short Circuit Current Isc (panel spec sheet)
25.00A - Max Series Fuse Rating (panel spec sheet)
13.90A - Fuse Size Calculation (NEC Rule 1.56 x Isc)
17.82A - Dead Short Current Calculation (8.91 x 2)

Two strings of panels in series, in the same roof array (identical type and number of panels per string) but these two strings are then connected in parallel at the FlexBoss21 inverter on MPPT1 or MPPT2 (allowing two strings in parallel), here is my question about inline fuses...

Question 1: As the two strings are paralleled at the MPPT charge controller, rather than via a branch circuit on the roof, do you still need to put in an OCPD (Overcurrent Protection Device) inline fuse?

Question: 2: Does the act of paralleling at the MPPT charge controller change how a dead short can potentially damage the second string of panels in series?

Questions 3: Does the 26A MTTP max current capacity mitigate that need for an inline fuse?

Thanks for the help.

Aly

 

[timselectric]

No OCP needed for two identical strings in parallel. Doesn't matter where the parallel connection is.

 

[chrisski]

Can you describe the string set up?

Sounds to me as if you have two identical strings, each going to a different MPPT, so these would not be parallel.

 

[AlyGreen]

@timselectric Thanks for the guidance.
Why would having only two identical strings prevent the need for protection against a dead short current of 17.82A when the the panel short circuit current capacity of 8.91A on the second string? This is what I am not understanding, why the exception?

@chrisski Yes these are two identical strings. On the FlexBoss21, MTTP1 & MPPT2 allow for two strings in series to be paralleled at the inverter, as long as you don't go over the maximum 26A capacity of the MPPT charge controller. To give more details here are the inverter & panel array specs here is a PDF document.

 

[timselectric]

@timselectric Thanks for the guidance.
Why would having only two identical strings prevent the need for protection against a dead short current of 17.82A when the the panel short circuit current capacity of 8.91A on the second string? This is what I am not understanding, why the exception?

When 1 string develops a short, the other string can send its current to that short. If both strings are identical, the shorted string is rated to carry the current of the other string, so there's no concern with a fire.
If you had 3 strings, then a shorted string wouldn't be capable of carrying the current from the other 2 strings. And there would be a possibility of fire. And would require OCP for each string to protect the shorted string from the current of the others.

 

[AlyGreen]

@timselectric Got it! The lightbulb just went on in my brain. Thanks so much for helping me understand the principle.

That makes it easy with the wiring to the inverter, there being no rooftop fuses needed, fantastic.