Strings straight to inverter

hhsting

Senior Member
I have three strings in parallel. String 1 has 20 modules, string 2 has 30 modules and string 3 has 40 modules. String 1 circuit goes straight to inverter. String 2 circuit goes straight to the inverter and so does string 3 i.e there is NO combiner. Each module voc is 9V.

Inverter input voltage rated for 600V dc. String 1 voc is 180V, string 2 voc 270V, steing 3 voc is 360V. Which string voltage would be used to determine the final voltage to compare against inverter rated input voltage based on code?
 

jaggedben

Senior Member
I hope those strings are connected to separate MPPT inputs.

I'm confused about your question vis-a-vis the code. I think it is merely an issue of following listing instructions. I'm not aware of what NEC requirement is at play here, other than the highest voltage would be used for max voltage
 

electrofelon

Senior Member
What kind of modules are these? If you havent done so, you just need to check that each string is within the inverters voltage window, VMP at hottest temp, and VOC at coldest temps. Use a string sizing tool.
 

electrofelon

Senior Member
How can mismatched voltages be paralleled?
That is why JB asked if they were to separate MPPT's. Inverters typically have several different inputs that are independent so they can take different voltages. If that is the case, using the term "parallel" isnt really the correct term.
 

hhsting

Senior Member
I hope those strings are connected to separate MPPT inputs.

I'm confused about your question vis-a-vis the code. I think it is merely an issue of following listing instructions. I'm not aware of what NEC requirement is at play here, other than the highest voltage would be used for max voltage
MPPT are they also called optimizers and installed at each module?

Sent from my SM-G935U using Tapatalk
 

jaggedben

Senior Member
MPPT are they also called optimizers and installed at each module?
Could be, or the inverter could have multiple inputs like electrofelon mentioned.

MPPT is 'maximum power point tracker', which is a component that finds the voltage at which the solar panels deliver the most power. If multiple voltages are attached to it then it won't work right.

You really have to follow the specs and instructions on the equipment you're using.
 

ggunn

PE (Electrical), NABCEP certified
I have three strings in parallel. String 1 has 20 modules, string 2 has 30 modules and string 3 has 40 modules. String 1 circuit goes straight to inverter. String 2 circuit goes straight to the inverter and so does string 3 i.e there is NO combiner. Each module voc is 9V.

Inverter input voltage rated for 600V dc. String 1 voc is 180V, string 2 voc 270V, steing 3 voc is 360V. Which string voltage would be used to determine the final voltage to compare against inverter rated input voltage based on code?
Assuming you are going into three separate MPPT inputs, the highest string voltage corrected for the lowest temperature for the area is the one you use.

If there is only one MPPT input and if you aren't using optimizers you will need to restring your system so that all strings are the same length and in the same orientation.

A module Voc of 9.0V seems strange to me, but I haven't seen it all.
 

jaggedben

Senior Member
One other thing:

If you're using optimizers then the optimizer output spec is what your string voltage is based on. The module Voc only relates to the optimizer input in that case.
 
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