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Siemens QO breakers DC rating?

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  • #16
    Originally posted by Sunking View Post
    Dead serious, I am a professional electrical engineer with 40 years of experience. A solar panel is a Current Source, not a voltage source. With one or two strings of panels, makes it impossible for a solar panel system to operate a fuse. A 250 watt panel with a dead bolt fault will only deliver roughly 8 to 10-amps of current. 8 to 10 amps of current on a 15 or 20 amps breaker or fuse is a normal load current and will never operate a fuse.That is why electrical codes do not require a fuse on solar panels unless you cross the line with 3 or more strings.

    Look at any panel specification and look for the spec Isc and learn what it means. Isc = Current Short Circuit. A 200 watt panel Isc = roughly 8 to 10 amps. That is the absolute maximum current that panel can deliver with its output shorted out (two output wires twisted together tightly.) Please explain how 8 to 10 amps is going to operate a 15 or 20 amp fuse or breaker.

    You can certainly use OCPD on the panels if you wish, but they are not required and an unnecessary expense.
    In my own situation I have 4 strings that run about 8.5A max so I used 10A breakers.

    Last edited by PNW_Steve; 11-12-2020, 11:58 PM.


    • #17
      Breakers make a handy switch for when you need to reboot the charge controller, but in real world, your 8A panels can't trip a 10A breaker, except for a Edge of Cloud lensing event.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A