Newbie small system design advice.

micahvmicahv Registered Users Posts: 17
I am trying to design a small system using some already acquired hardware. 3 12V panels with Voc at 21.5 and Isc at 7.67. Then the SunBoost 2000E CC. The CC controller limits me to all modules in parallel because it have a max Voc of 30v.

So my max current will be around 7.67*3*1.25 = 28.8A
and then min conductor ampacity is 28.8*1.25 =36A

So with this I look at record high temp for the area, 45Deg C which gives me a .87 derate factor for ampacity. and then if I run some cable in conduit on the roof 0.5 - 3.5 inches I add 22 to temp 45+22 so 67 this gives me a derate of .58! per NEC 310.16. 36/.58 = 62A for conductor on the roof or 6# cable.

If I'm dealing with free air conductor 36*.87=42A which puts me at 10#.

We are trying to run the cable around the eve instead of poking a hole in the roof. Do I need to worry about roof conduit derate if just running 1-2 ft of conduit on the roof? I was hoping to but a junction/combiner box under the eve, but using conduit on the roof for cable protect seems to exceed the amapcity of the conductor then. Is my only option to have a junction box on the roof?

Total run from the array to Charge controller is like 15'*2 or 30' 10# will have about 7% Vdrop and 6# is about 1/3 that. So probably better to convert to 6# for sake of efficiency. At one point I thought of just running 10# all the way from the array down to circuit breaker before the charge controller.

So as far as my junction/combiner box, can this just be a splice? 10# to 6#? My plan is to have it run into a 35A breaker. Do I need fuses for each module?

I'm also planning on using the Midnight Solar Mini DC 125A disconnect which will contain the 35A breaker/disconnect for the array. It will also contain the 30A breaker(instead of fuse) for the CC to battery. The 125A breaker for batteries to Inverter (200/400W surge), and two 15A breakers for fridge and DC outlets.

Daily Load is about 1kw, with plan for about 350AH 12 Volt battery system. 125A limited by main breaker would seem to imply I could use fairly small battery interconnect and inverter cables? Was just going to go with 2/0 which seems overkill but good if we change out the inverter. Safe to say I could safely go with 1# in conduit.

Why use fuses? I heard somewhere cost, but if I'm going with this midnight Mini DC which comes with 125A breaker, and then purchase additional 35A and 15A breakers for around $10 why do I want to mess with fuses? link to some of my ramblings trying to explain to someone not familiar with this stuff my design rational

Thanks a lot!


  • BB.BB. Super Moderators, Administrators Posts: 30,219 admin
    Re: Newbie small system design advice.

    By the way--the tiny URL is to a text google doc posting about the system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.