JK14
New Member
Agree with ShaneC above for 50 Amps.
The thing that is similar in and out of the SCC is power (watts) and not current (amps). With 4 x 260 watt panels the peak power is 1040 watts at STC (and could even be a bit higher on a cold bright day). Vmp for the 2S arrangement is 61.4 volts so the max power would be 16.9 amps. The SCC adjusts the voltage to get max power (MPPT).
However, the voltage heading out of the SCC is fixed to the voltage of the battery which for a "24volt" LFP battery can range from 29.2 volts (3.65v per cell) to 20 volts (2.5v per cell) at the extremes. So if 1040 watts are coming out of the SCC (it will be less, but 100% efficient for simplicity) it will push a range of 36.6 amps at 29.2 volts or 52 amps! at 20 volts. (Note that this is the same "20 volts low cutoff" in the first line of John Frum's information-packed post #11 above)
An SCC with max 40 amps is fine because a) you are unlikely to hit conditions of 52 amps IRL and b) the SCC will just peak out at 40 amps and won't hurt anything, and c) a SCC with higher amps is more expensive. However, it is very possible that your system will occasionally hit the 40 amp output limit so a 50 Amp minimum fuse is reasonable.
When sizing fuses and breakers, we can make just 2 main mistakes: 1) If the fuse rating is too big, a too-small wire could be the weak point of the circuit when the amps are high and and burn (which is dangerous) or 2) If the fuse is too small, it can blow when it did not need to (which is a nuisance). From this perspective, there is often a rather large rage of "acceptable fuse/breaker values. The 20 amp breaker on the input side of the SCC is getting close to nuisance (as 17 amps is likely) so bumping up to 25 or 30 amps is all fine because the wires can handle it as ShaneC noted.
Using the same principles John Frum's posts for the inverter amperage are valuable because while 83.33 amps is the right math for 2000 watts by 24 volts, each of the adjustments he notes are trying to avoid possible nuisance situations. If you attach 2 heat guns to the inverter and draw 2000 watts, it will do what it has to provide 2000 AC watts, but because it is not efficient (0.85 std or 0.92 per ShaneC) and the battery may be low (20 volts in the extreme) and the inverter is actually capable of surging to 3000 watts, John's calculation will help avoid ever having a nuisance pop of the breaker.
