PV String OCPD requirement

[ohmti787]

If I have a 2+ string system and each string lands on a separate MPPT in the inverter, is that still consider a parallel connection point? If so, then each string would require a OCPD, correct? Just a little confused on what exactly is considered a parallel connection. Exception 1 on Article 690.9(A) states that OCPD's are not required if "there are no external sources such as parallel-connected source circuits, batteries, or backfeed from inverters". So if I'm reading this correctly, even if separate MPPT's are not considered parallel connections, that parallel connected source will always be present since all these MPPT's are integrated to the inverter, correct? Please advise. Thanks!

 

[Carultch]

If you have multiple MPPT zones on the same inverter, it is not your concern what the inverter does to parallel the MPPT zones inside it. That scope of the circuit paralleling and buck/boost voltage conversion of each zone is all part of the UL listing of the inverter, and the inverter manufacturer will do what they need to do to make it work.

Treat each MPPT zone as if it were independent of all the other MPPT zones on the inverter. Make a code-compliant connection of your DC strings to the MPPT zone as if it were its own inverter. Once you do that, you are all set.

Given 2 circuit inputs per MPPT zone, this is a special case where you aren't required to have OCPD's to parallel them, and it is very common that multi-MPPT inverters are built for 2 strings on each zone, precisely with this application in mind. It is when you "force fit" 3 strings on one zone, that you need fusing on each string, to parallel each pair of them, and to parallel the pair with the sole string. You might do this if you have particularly low-current modules compared to the current of modules that the inverter manufacturer anticipated.

 

[pv_n00b]

As an example, the CPS 275kVA inverter has two input options, 24 and 36 strings. It has 12 independent MPPTs. With the 24-input option, each MPPT gets two strings, and no fusing is required. With the 36 input option, each MPPT gets 3 strings and fusing is required.

 

[ohmti787]

That makes sense. Thank you guys

 

[ggunn]

As an example, the CPS 275kVA inverter has two input options, 24 and 36 strings. It has 12 independent MPPTs. With the 24-input option, each MPPT gets two strings, and no fusing is required. With the 36 input option, each MPPT gets 3 strings and fusing is required.

I just wanted to point out the obvious.

It doesn't have to be either 24 or 36 strings; each MPPT input operates independently of the others; you can have one, two, or three strings (or none) on each MPPT with as many or as few modules in a string that fits between the inverter's maximum and minimum. The only restriction is that all the strings connected to a single input need to be the same length.

 

[pv_n00b]

I just wanted to point out the obvious.

It doesn't have to be either 24 or 36 strings; each MPPT input operates independently of the others; you can have one, two, or three strings (or none) on each MPPT with as many or as few modules in a string that fits between the inverter's maximum and minimum. The only restriction is that all the strings connected to a single input need to be the same length.

Well, CPS only has two options in the manual and two part numbers for the 275KTL inverter. They make an input section compatible with each option. It's true that in general, you can make up any number of strings and string lengths as long as it's compatible with the inverter. For this particular inverter only two are compatible, 24 and 36 strings.

 

[ggunn]

Well, CPS only has two options in the manual and two part numbers for the 275KTL inverter. They make an input section compatible with each option. It's true that in general, you can make up any number of strings and string lengths as long as it's compatible with the inverter. For this particular inverter only two are compatible, 24 and 36 strings.

Of course, but someone might incorrectly infer that they could only have have 24 or 36 strings. In the 36 string configuration any of the 12 MPPT inputs that can accept 3 strings can also accept two strings, one string, or no strings. Also, only the strings that are connected to the same MPPT channel have to be the same length.