When to fuse modules?

I am confused about when to fuse the modules. I know I should do it if the Isc x 1.25 x #modules - 1 is more than the ampacity of wire. I know I don't have to if its just one string. I always use 150 vdc breakers when I do a battery system with a regular charge controller and a combiner. Im stumped on this Xantrex XW 80/600 charge controller installation. Its has a large input voltage rating much like a string inverter. I have my 2 (2 x 9 Suniva 260's) pv source strings going into a 600v 30 a dc disconnect 3 pole square D, and then into the 2 pv + and - inputs of the charge controller just like I would wire up a string inverter. Do i need to be worried about battery backfeed current? I have the 48 450 aH battteries on a 250A breaker. What if I retrofitted a string inverter that didnt need fusing with a battery inverter (retro fit ac coupling)? Would it require fusing on the PV because of the batteries?

What the heck is battery backfeed current and how would that happen? Obviously I dont want the battery current to try and go up through the PV wire but I dont see how it could do that with the 250A breaker there. Please help me understand how to do these systems right. The system is working fine right now PS


  • BrentBrent Solar Expert Posts: 64 ✭✭✭✭
    Re: When to fuse modules?

    There is no requirement to fuse one or two strings of modules. More than two strings requires protection. The purpose of the fuses/breakers it to limit the backfeed current into a string from the combined current of all the other strings in case of a fault or short in a string.

    The charge controller will take the high voltage from the strings and regulate it down to the battery voltage and provide MPPT regulation and battery charging capability. If you had 300 volts at 10 amps [3000 watts] going into the charge controller you would have something close to 48V at 62A [still 3000 watts] available for the batteries and inverter.

    There is no chance that the battery voltage would ever backfeed to the panels through this model charge controller due to the nature of how the controller works so I would not be concerned with that.

Sign In or Register to comment.