Fuse sizing

landyacht.318 Solar Expert Posts: 82 ✭✭✭✭
My charge controller(sb2512i) says to use up to a 30 amp fuse maximum.

As I only have one 130 watt panel, I'm only using a 10 amp fuse.

Will having 2 10 amp inline fuses, one before the controller and one after cause any significant voltage drop?

Is there any benefit to using Fuses underrated for the controller?



  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Fuse sizing

    what is the make of panel and what are its specs?

    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Fuse sizing

    The KC-130’s short circuit current (Isc) spec is 8.02 A. The in-line fuse between the module and the controller should be rated at 8.02 A x 156% = 12.51 A, or rounded up to the next standard size (13 A? 15 A?).

    The controller output’s maximum fuse size spec of 30 A is probably derived from ~125% of its rated output (25 A), which is turn probably has a 125% surge factor applied. In fact, your controller’s Isc input limit is 20 A (manual page 9).

    The primary purpose of a fuse or circuit breaker is to protect the conductor between the energy source and the load. Personally, I think it’s OK to use a fuse size matched to expected operating conditions. For example, your 2512i doesn’t have load connections, and if a 15 A fuse between your controller and battery were to blow, that would suggest to me that there was a bit of a problem somewhere.

    Jim / crewzer
  • BB.
    BB. Super Moderators, Administrators Posts: 33,493 admin
    Re: Fuse sizing

    Fuses are, basically, resistive heating devices... And the lower the amp rating, the higher the resistance.

    You can just use a volt meter and measure the voltage drop across the fuse(s) in your circuit to see if they are costing you much power.

    For PWM controllers, the fuses will not cost you any power (as long as the PV array voltage is high enough to charge the batteries plus all of the system voltage drops).

    Your Blue Sky controller is a MPPT type--and the fuses will reduce the overal system power by their voltage*current drop...

    Fuses, of course, should never be larger than the rating of the wiring and the devices they connect will allow.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset