Sizing Fuses

Jan 10, 2011 02:16 pm
Sizing Fuses

I'm wondering if I could get some guidance in regards to sizing fuses for a few solar systems, as I've never done it before and I'd like to avoid fires. I figure there needs to be one between the battery & inverter, the battery & charge controller, and the PV panel & charge controller. I'm not sure if they're all necessary, or if there will be any others required. This is an off-grid system.

The first system is for a 30W application:
1 x 135W Panel
1 x 200 Ah lead acid battery
1 x Morningstar Sunsaver charge controller
1 x Morningstar SureSine Inverter 300W

The second system is a 100W application:
2 x 210W Panel
4 x 200 Ah lead acid battery
1 x Morningstar TriStar charge controller
1 x Morningstar SureSine Inverter 300W

Any help would be greatly appreciated.
220 Posts
Jan 16, 2011 04:15 am
Re: Sizing Fuses

I'd like to avoid fires.

we are not getting much input here Bradley, could be folks are afraid of making some kind of code (NEC) mistake in their response.

you say "off grid system" with a goal of avoiding fires? we should be able to help with some of the steps without getting too mired down with the big blue book (still good as a guide though).

let's look at system one first.

you say:

The first system is for a 30W application:
1 x 135W Panel
1 x 200 Ah lead acid battery
1 x Morningstar Sunsaver charge controller
1 x Morningstar SureSine Inverter 300W

i'm the curious sort and wonder about that module model#, also curious as to how long each day you might run that 30w application? i'm in hartford county on the east coast of the u.s.a in the state of connecticut. how about you? happy to try to help and want to be sure we are going to have a shot at a working system before we get too carried away.

let's look at a typical 135w module. the kyocera kd135 that has an isc (current short circuit) of isc 8.73A. a good guide for controller sizing would be to take the isc X 1.56 to give us 8.73 X 1.56 = 13.6amps, that would indicate that we should be using the SS-20. is that the controller you are using? or were you wanting to use the SS-10..does it get cold where you are?. WOW we have just started and it's time for an executive decision. Morningstar says we don't need that second derating, that brings us to 8.73 X 1.25% =10.9amps..getting closer, how far will the wire run from the pv module to the controller be?..that module has an imp (current max power) of about 7.5amps. if our wire run were 25feet i would reccomend a wire size of awg#8 and a breaker of 15 amps (the fuse rating of that module)..i would choose the breaker because of it's disconnect abilities not so much for the overcurrent abilities..nice to have both.

yikes! getting long winded already with all these questions and have to take a break for now... and we have only just started with step one of system one!

cheers for now, dave
22 Posts
Jan 18, 2011 04:33 pm
Re: Sizing Fuses

Dave, thanks for getting us started. I'll continue on from where you were, using the same assumptions, Kyocera 135 and Morningstar SS-20.  I'm going to give broad figures, there are always variables that I can't get into here.

You'll want a breaker or fuse between the module and the charge controller.  Either using the math or the spec sheet. isc X 1.56 to give us 8.73 X 1.56 = 13.6amps, round up to the nearest available of 15A.  Or use the Max Series Fuse listed in the specs of, drumroll, 15A.

Coming out of the charge controller into the battery, you want another one.  If it is the 20A charge controller, 20A x 1.25 = 25A.  Not sure if they make one at 25A, may need to round up to 30A.

Now the second system, that's tricky without knowing what the model module is.  Let's keep it simple with Evergreen 210W 12V modules.  Since you are using the 12V inverter, I am assuming the modules are in parallel to keep at 12V, so you would have a combiner box with a 20A breaker on each output of the module (11.23Isc x 1.56).  The output of the breakers would be combined.

Assuming Morningstar TriStar 45A PWM charge controller, 45A x 1.25 = 56.25A, round up to 60A breaker.

Between the inverter and the battery, 300W can work as low as 10V, 300W / 10V = 30A potential x 1.25 = 37.5A, round up to 40A.

Now I was making a lot of assumptions, why don't you give us a call here and we can go through it with facts.

AltE Store
Solar Thermal Queen
« Last Edit: Jan 18, 2011 05:49 pm by Sascha Deri »

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