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I need to connect 3 similar 180W solar panels to my MPPT. Each one outputs max 9A. I would like to send a + wire and a - wire for each panel back to either a bus bar or a fuse box... and then use a 4AWG jumper from the bus/box to the MPPT controller. if the fuse box is even necessary.

So to clarify, I want to 'gang' up all of the + and all of the - at one point, either the bus bar or fuse box.

The MPPT can handle input of 50A max.

Is this possible, and do I need fusing at each hot line carrying 9A? It should be well within my MPPT to handle all 3 panels unfused. I'm just worried if I gang up all of the hot lines can a 27A short somehow appear at one panel and ruin it. Please advise.

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You could directly connect all three panels in parallel to the MPPT, but I would probably fuse the input unless it has significant input protection of its own. I would isolate each hot line with its own fuse using a 15A fuse. Typically the rule is to use input protection 20% higher than your max expected current because heat at the protection can make it trip sooner (though this applies much more to breakers than fuses). If you get a short anywhere from positive to negative all 3 panels will be shorted at the output. I don't know if the panels have any general output protection or short circuit protection but I would rather be safe than sorry. It's much cheaper to add some fuses than to replace damaged panels.

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