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PV fuse and cable


Jacques1s
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Hi – I have a Kodak 5kw ( Axpert ) inverter. I have 2 strings of 5 JA Solar 390w Panels.

Each sting has a 6mm cable to 32amp fuse. All is working just great. I now would like to add a second

Inverter in parallel and add another identical PV. The PV is on a very high roof and getting the 2 cable stings up in

Runking is a huge mission. I am now thinking of combining the existing 2 strings and parallel them on the roof so I can

use the other cable for the new PV. I think the cable 6mm is good at 24M. The question I have is, do I need to put inline fuses

on the roof or can I just combine 2 stings ( Parallel ) and will the 32A fuse be adequate?

Panel is IMPP 9,7A  VOC 49,35.

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Hi Jaques

A single 6mm sq cable (pair) is sufficient for the 2 strings in parallel as the source is 2 x 9.7A, so less than 20Amp. Even a 4mm is enough but yes the losses are less in the larger diameter and the given distance. In my installation I've connected the parallel strings on the roof without fuses using a connector box (self made, but I see you do get the MC4 type PV parallel connectors). It then passes through a single fuse and 30Amp Isolator switch before it goes into the Inverter. The 32amp fuse is sufficient, as stated the current will be below 20amp and less. (I do run 2 Inverters in parallel, with 2 different PV sets)

I had both fuses and isolators, so I installed them but in my mind, the fuse will never blow : if you spec the fuse correctly for the load (the current draw into the inverter) in this case 20amp, then the current in the PV cable will/can not exceed that, as the source (PV) cannot give more. The 9.7A in the spec sheet is the short circuit (fault) current and the two strings in parallel gives 19.4amp. So effectively you cannot short-circuit the cable at any point before or after the fuse to blow it, as the max current is lower than the fuse rating.
The only possibility for the fuse to blow is if the fuse is a poor design or rated lower than source (PV) capability and perhaps lightning - but if a lightning surge blow a fuse, chances are there is damage all over. You may consider Surge Protection on the 2 PV inputs on the Inverters - this is more sensible to me. (surge arresters of higher than 246.75V in your case)

But what you have in mind will work, you have enough cable capacity for 2 PV sets already 😃 

Kind Regards
Jaco P

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