20 amp fuse for 300watt Solar?

Registered Users Posts: 41 ✭✭
howdy!

I'm upgrading my wee little RV system to 300w of mono. (Mounted flat) I need a bigger fuse. According to this

equals (# of panels in parallel) x Panel ISC x 1.2

I came up with 21.6 amps so I guess a 20 amp fuse would be better than 25?

What is the AWG of the wiring? Fuses/breakers are there to protect the wiring. Too small of fuse can cause false trips. Using the NEC (relatively conservative), the maximum fuse size would be:
• 14 AWG = 15 amps max
• 12 AWG = 20 amps max
• 10 AWG = 30 amps max
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

In detail--Insulation rating, ambient temperatures, conduit fill, etc. can also adjust the maximum allowed current.

Fuses and breakers (in general in North America) will not blow at 80% of rated current and well eventually trip at 100%+ of ratted current.

Isc is short curicuit rated current for a solar panel... But I would suggest heavier wire and a bit larger fuse/breaker to limit false trips and keep voltage drop low.

Also, note that for ~3 or more panels (or series strings of panels) in parallel--Each needs a series protection fuse per panel/string to reduce the chances of a single shorted panel from being overheated by the other 2+ solar panel strings. In the panel specification, you should see a series protection fuse rating (usually somewhere around 10-15 amps depending on size/amp rating of panel)--Using a combiner box with multiple breakers.

There is usually no reason for a siingle breaker between the array and the charge controller--The combiner box protects the individual panels/strings and the wiring from the combiner box to the solar charge controller panel input should be large enough to handle the Isc-array current directly.

However, many people still like a to install a single breaker (or switch) near teh charge controller panel input for easy array on/off when servicing the solar charge controller (especially if the combiner box is some distance away at the array).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 41 ✭✭
Thanks Bill, here is my setup, thoughts?

• Registered Users Posts: 41 ✭✭
edited October 2020 #4
Hmmm... reading now... So you're saying I need a fuse for each panel now that I have 3 before or in a combiner box? Even if mine are in Parallel? I did find these cool inline MC4 fuses https://smile.amazon.com/Renogy-10-Waterproof-Line-Holder/dp/B00YG1DY20/ref=sr_1_3?crid=2PC9N8PLRQIQ3&amp;dchild=1&amp;keywords=mc4+inline+fuse+10+amp&amp;qid=1603398742&amp;sprefix=MC4+inline+fuse,aps,190&amp;sr=8-3

• Solar Expert Posts: 9,274 ✭✭✭✭✭
2 panels in parallel, no over current required.   3 or more in parallel, each panel needs a fuse or breaker as specified on the sticker on the back of the panel.  Breakers let you turn off a string at a time for comparison testing or troubleshooting. Fuses, you have to park in a dark place before you can install or remove one, as most fuse holders, while rated "touch safe" are not rated for "circuit interruption", like taking it open while there is light.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 41 ✭✭
mike95490 said:
"... Fuses, you have to park in a dark place before you can install or remove one..."
Thanks sir. I got it. Follow up. I put battery disconnect just before the CC. If I turn that OFF can I change the fuses without covering the panels/parking in a dark place? Since there would be no load? Or do I not get it. Thanks!
• Registered Users Posts: 41 ✭✭
edited October 2020 #7
I'm totally fusing each panel. Thanks

So, here is where I'm getting tossed around. This is what I got from "Tech Support" whom shall not be named after I sent in a pic of my setup. Maybe he doesn't think 300 watts is enough to worry about?

"Yes, you have too many things in your setup that will cause many issues.

First, remove all fuses from your setup.

Once the panels have been connected in a parallel connection connect them directly to the charge controller with the MC4 connectors you have installed.

If you install the system with the power converter and you find issues please remove the power converter from your system."

• Solar Expert Posts: 9,274 ✭✭✭✭✭
No.  Many charge controllers will fry, if the solar is pumping voltage into it, with no battery connected.  The power flow is stopped, but panel voltage still gets transferred thru, but no battery to sink the energy into.

Because so many fuses get installed poorly and cause problems, could be the reason the mfg says not to use them. More than 2 parallel strings or panels, requires protection on all the panel strings, per series fuse spec on the panel sticker;
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 1,599 ✭✭✭✭
edited October 2020 #9
In your case, the MC-4 inline fuse holder and appropriately sized fuses are pretty foolproof. you must have one per panel but on the negative side they aren't cheap.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 540 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

• Registered Users Posts: 41 ✭✭
edited October 2020 #10
Thanks again everyone!

Here is my modified setup. If any of you have a chance to peruse it. I think the fuse on my battery is probably too big. 80 amps?

FYI... whats missing from here is my 2000w pure sine wave inverter, and 600 watt subwoofer.

Cheers

Hmm... Paralleling charging sources is not the worst thing in the world... More or less, the charging source with the highest voltage set point "wins".

The chargers may not stay in sync (bulk vs absorb vs float), but that does not really matter.

Just set the voltages to your needs. AC charger lower set point and solar charger to the higher set point (to save utility costs)--Or whatever is best for you.

Some chargers have canned cycled

Too many fuses... Somebody that never had a short circuit to a battery bank. I have suggested, half joking, to have a fire ax and/or cable cutters handy in case the worst happens and you need to cut power to a glowing red hot cable.

Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 41 ✭✭
BB. said:
Paralleling charging sources is not the worst thing in the world...
When I park the trailer under it's carport I usually turn the switch between the panels and the CC to Off. Is that OK?
If you have AC power charging/floating the battery bank--Then it really does not matter if you switch the panels off or not. The AC charger should float the battery bank just fine.

Note: You should only connect the solar charge controller to the battery bank first, then connect the solar panels to the charge controller--Otherwise the charge controller can get confused, or possibly even damaged. When turning off--Turn panels off first, then turn off the charge controller second.

If you do not have AC charging for days/weeks/etc..., a PWM charge controller usually draws very little current when "sleeping" (no sun).
MPPT type charge controllers generally draw more current, and if left connected to a battery bank without solar power (typically snow covered panels or other issues), a MPPT controller can significantly discharge the battery bank if left connected for weeks or longer without solar power (running the solar charger's computer and electronics).

For "storing" Lead Acid batteries (no loads, no charging)... A fully charged FLA battery can go about 1 month between charging. An AGM type battery can go upwards of 6 months between chargers--Both at 25C/75F... Hot batteries have higher self discharge (the rule is for every 10C/18F increase, 2x faster self discharge... For every 10C/18F below room temperature, 1/2 the typical self discharge rate).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 41 ✭✭
Do you think that 80 amp fuse at the battery is too big?
• Solar Expert Posts: 5,576 ✭✭✭✭✭
Do you think that 80 amp fuse at the battery is too big?
Depends on your wire size and inverter rating.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 41 ✭✭
Thanks True. I've decided to make a detailed schematic with cable sizes and appliance ratings. I also got a clamp on DC amp Meter and am learning how to use it.