Fuse size Controller to batteries?

SwampYankee · October 2013

I have the following setup in my off grid camp and wanted some advice on fuse size from my charge controller to my battery bank.

12v system
1 / 250W 8.75A panel wired (10 gauge) to Midnite solar prewired combiner box with 15A breaker, 8 gauge running from the combiner box to the charge controller (should I fuse this??) (Sunforce 30A controller), 8 gauge running from the controller to the batteries (no fuse??), battery bank is 500AH. Inverter is 2000W with the recommended 4 gauge wire and 100A fuse.

Can someone help me out with the correct fuse size and or formula for sizing... and if you see anything else I am doing wrong with this setup advice is welcome.

Thanks

BB. · October 2013

Re: Fuse size Controller to batteries?

At the battery bus, there should be a fuse/breaker on every positive wire/cable that leaves the battery bus.

You can use the NEC to pick the size of fuse/breaker for the wire used. 8 AWG, depending on the insulation type, has a maximum current rating of 40 to 55 amps

In conduit, outside of conduit the wire will have better cooling and can carry more current. If you look at marine wiring charts, you will see they allow higher current ratings. NEC is conservative and usually the best way to go.

But, you have to look at the loads and sources to figure out the size of the wire. Wiring should always be larger than the rated load by a 1.25 NEC factor. So, a 30 amp controller * 1.25 = 37.5 minimum fuse/wiring size.

But back to the hardware you have chosen... The solar panel you have picked appears not to be a match for a typical Sun Force controller.

250 Watt panel / 8.75 amps Imp = 28.6 volts Vmp

I believe the Sun Force is a PWM (pulse width modulation) type charge controller (not MPPT-maximum power point tracking). The Vmp voltage for a 12 volt battery bank on a PWM controller should be ~17.5 volts or so.

You will have two problems here... Because you are using a PWM controller with your 28 volt Vmp panel will have an effective rating of:

8.75 amps Imp * 17.5 volts Vmp = 153 Watts

And the charge controller will see Voc of ~36 volts (standard test conditions). That is way more than the ~21-22 volts that the Sunforce controller is designed for (possibly--Some controllers can manage higher Voc voltages).

You can get 12/24 volt dual voltage PWM charge controllers and they will manage the higher Voc voltage--But they will still lose the same amount of power (due to PWM type charge controller with panel that has Vmp>>17.5 volts).

You can get a MPPT controller that will work well with this panel:

Morningstar SunSaver 15 Amp MPPT Solar Charge Controller

And if you want to go with a larger charge controller (two 250 watt panels)

Rogue 12/24/48 volt 30 amp MPPT charge controller

As you go larger, you will have more options (and spend more $$$).

Questions?

-Bill

zoneblue · October 2013

Re: Fuse size Controller to batteries?

Also be aware that you have quite a bit of inverter and not much pv. You dont say battery specs, but either way, it will be chronically easy to use and abuse that battery bank. Also if as bill mentioned it is a 12 volt battery, then 2KW/11V drop out/0.85 inverter efficency is 213 amps at full load, Twice that at surge. 100A fuse/4AWG dosent sound right.

If OTOH your battery is 24volts, the panel wont put out enough volts enough of the time to charge the bank.

Maybe theres things we dont know yet, but ought we better go back to the begiining on this one? ... How many kWh per day do you need to power?

SwampYankee · October 2013

Re: Fuse size Controller to batteries?

My mistake on the inverter its fused with 300A and using 2 gauge wire.
This setup is nothing high speed and is only used twice a year. I started out with 60 watts just to keep the batteries up while I was gone. I plan on 1000 watts and to double the battery bank. As for power consumption I am in the process of figuring that now but I'm currently out of the country. It is not much I can tell you that. I also have a generator backup to run the larger camper 13amp AC if needed. My plan is to run my small 5000 BTU AC during the day. I have tested this out in the middle of the day and ran it for 4 hrs without issue until the clouds roll in.

I have three additional 250W panels I haven't hooked up yet due to the charge controller.

SwampYankee · October 2013

Re: Fuse size Controller to batteries?

Bill thank you for the detailed response.

I knew I was close with the Sunforce controller and have plans to purchase a Midnite solar controller this week from a friend you bought the wrong item so I got a good deal.
I have had the system in place for six months now and so far I haven't seen any issues. I am not in the US now so I cant get out there to take a look and see how its doing but I did leave a small LED burning that I can see from my cell game camera and its still on so its appears luck is with me but the numbers don't lie and it will burn up soon.

The controller I got my hands on is the MS classic 150 which can handle 96A 150V and my planned array of 1000W. My 8 gauge wire from the bank to controller is rated at 55A. With this new info what is a safe fuse size for the Midnite controller 50?
Also, do I need another fuse from the combiner box to the controller? Combiner box is actually the prewired MS unit that has 15A breakers. Batteries are 4 deep cycle rated at 115AH (20hr rate)
My running watts will be around 650 with the small AC and a few other things running.

Im just starting out so this is mostly greek to me.

Vic · October 2013

Re: Fuse size Controller to batteries?

Hi SY,

Looking at the 300A Inverter fuse, the #2 AWG cable is too small.

As you probably know, Fuses/Breakers are there to protect the cable/wiring. #2 in Free Air is rated at 190 A without the NEC safety factor.
Looks to me that the 300A fuse would require 4/0 90 degree C cable with some safety factor.

Appears that the #2 cable would support about a 150 A Fuse/Breaker. Am not a code guru. YMMV, FWIW, Vic

SwampYankee · October 2013

Re: Fuse size Controller to batteries?

Vic wrote: »

Hi SY,

Looking at the 300A Inverter fuse, the #2 AWG cable is too small.

As you probably know, Fuses/Breakers are there to protect the cable/wiring. #2 in Free Air is rated at 190 A without the NEC safety factor.
Looks to me that the 300A fuse would require 4/0 90 degree C cable with some safety factor.

Appears that the #2 cable would support about a 150 A Fuse/Breaker. Am not a code guru. YMMV, FWIW, Vic

That makes sense. I really hate to buy another set of cables maybe I will put a 200A fuse on it and call it a day. I was going off the manufacturers recommendation which was for a 4-6 ft run using 2 gauge wire and a 300A fuse with the inverter and battery bank being no further than 4 ft. Just checked my wire to be sure and its rated at 200 amps.

I didn't think about questioning what was in the manual until you mentioned it thank you.

Vic · October 2013

Re: Fuse size Controller to batteries?

Hi SY,

Will from a practical view, the inverter manual is probably correct. #2 90 C cable is rated at 190A in free air. A direct short at the inverter would probably blow a 300A fuse quickly, before the cable would heat significantly.

Perhaps I was being a bit too nit-picky. There have been times when I have crowded the NEC specs a bit. Just all about comfort level with large current sources, like batteries. Vic

SwampYankee · October 2013

Re: Fuse size Controller to batteries?

Vic wrote: »

Hi SY,

Will from a practical view, the inverter manual is probably correct. #2 90 C cable is rated at 190A in free air. A direct short at the inverter would probably blow a 300A fuse quickly, before the cable would heat significantly.

Perhaps I was being a bit too nit-picky. There have been times when I have crowded the NEC specs a bit. Just all about comfort level with large current sources, like batteries. Vic

I should have read up on it more before buying the wire. I actually just came across some 0Gauge on Amazon and I will probably grab it and use that with the 300A in a shorter run than what I have. I just looked at the photos I have of my setup and realized my total run is probably too long. Thanks for the advice its welcomed!

zoneblue · October 2013

Re: Fuse size Controller to batteries?

Additional to cable ratings, the voltage drop is important as well. With a 12v 3kW inverter your most precious resource is voltage drop, or, greatly desired lack of it. A 4ft 2AWG at 300A is gonna sag something wicked. Most folk around here would say that a 3kW 12 inverter was better avoided for these and related reasons.

The trick to working out cable sizing is to work out the voltage drop on the cable, and keep it real small. Use an on line wire size calcluator to figure it out. If your drop is small (under 1%) you stay so far away from the cable rating, that its a non issue.

Fuse size Controller to batteries?

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