luppensg avatar image
luppensg asked

Two strings parallel on one MPPT input ==> Isc too high. Use of stringfuse allowed?

Hi all,

I will use the MPPT RS 450/200 solar charger with 4 inputs, but due to the open circuit voltage of the solar modules (Voc), the limit is 8 panels per string.
If i'm correct, this is due to the short circuit current (Isc) which surpasses the maximum of 20A that is noted on the datasheet of the charger.

Mostly, this limit is due to the internal cable section current limit. But the problem only occurs in case of a short circuited PV-string.

Would it be possible to put two strings in parallel (Isc > 20A ! ) if the mppt input was protected with a 20A stringfuse?



mppt rs
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1 Answer
Alexandra avatar image
Alexandra answered ·


The limit is 18A tracking, short circuit is 20A : the ultimate limit is 30A.

But if you reverse polarity it.... There is no protection warranty is void.

See the manual here.

The TR versions can take up to 16mm² cable in them so cable size is not the actual issue.

Use separate trackers. That is the best course of action.

Connect each string to its own tracker. Each tracker has a limit as well of 4000w (tech specs max DC output charging). So while you can connect more (unless you have need for it such as very low light levels), you won't get more power.

Don't forget it is 8x battery voltage for VOC.

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luppensg avatar image luppensg commented ·
Thanks a lot @Alexandra . I missed that part of the manual and just looked at the datasheet.

I have indeed a setup where we expect low irradiation so we want to oversize the PV-array and connect more then 4 strings of 8 solarpanels.
With the 30A max limit and our max Isc of 21,2A we don't risk damaging the inverter and we even do not have to use a stringfuse if I understand you correctly.

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Alexandra avatar image Alexandra ♦ luppensg commented ·
I would still fuse individual strings. And ways to disconnect individual strings.

The combination of you fuse in one place, you could get annoyance blowing.

Just be over paranoid on PV polarity before going live you should be ok there.

Confirm it's the mppt rs you are using not the inverter?

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