Need help with fuse sizing for camper solar system

gphoto120

I’m relatively new to solar systems and would like some advice from those of you who have a much better understanding of them…..I’m setting up a simple, small solar system for our camper….System includes: Victron 75/15 SCC with a 10awg wire with 20amp fuse inline leading to battery. A LiTime 100ah BMS/Bluetooth 12v battery and 2 Renogy suitcase 100w panels that I want to use either just the one, or if needed add the second which both would be wired in parallel. I have a 45amp converter with 8awg wire and a 40 amp breaker between battery and converter….No inverter.
 I’m confused as to what size mrbf fuse on battery terminal to install. I have minimal dc usage…led lights, propane furnace, fridge, maxxffan, propane water heater etc. total amperage use at any one time is not more than 15amps. What amperage mrbf fuse should I use…..I have 100amp fuse but when using an online fuse calculator it said recommended fuse size should be 55amps. Any advice is greatly appreciated!

Thanks…

GP

BB.

First question is what is the current you wish to use... A 100 AH can output somewhere between 50-100 Amps continuous loading, and probably 50+ amps charging. Typically, a 100 AH Li battery with BMS will shutdown with >100 Amps of current.

Guessing that your 15 amp max charging current and with 10 AWG cable, rated at upwards of 30 Amps... For battery systems, I suggest setting your fusing/loads at a maximum of 80% of "rated current". For 30 Amp rated cable:

30 * 0.80 derating = 24 amp maximum fuse suggest

20 amp fuse would be fine too. For a 15 amp max continuous current:

15 amps * 1/0.80 = 18.75 ~ 20 amp fuse/breaker

The 80% derating is for North America rated fuses and wiring. Typically the 80% derating is used for "continuous current" operations. The 100% rating is for "normal" on/off and "home loading" (lights, washer, etc.). Batteries can take full charging current for hours and overheat wiring/trip over current protective devices after hour+ continuous current operations.

There is also the choice of using a circuit breaker vs fusing... Breakers are physically larger (make sure you get "DC" rated breaker as "AC" breakers don't work safely with DC current) but give you the manual on/off switching when needed (working on power, turning off battery for winter storage, etc.).

As a side note, storage batteries can frequently surge a lot of current (>>100 Amps)... Make sure the fuse/breaker you get is rated for DC power, and maximum surge current rating.

Another thing with Lithium batteries, They really like to charge at >41F and not discharge below 32F (freezing)--If you will be winter camping, you will need to keep the battery warm for best operation (BMS, depending on model, can shut down the battery when over current/over voltage/out of temperature range).

Regarding your solar panels... Ideally your Victron 75/15 controller will work best with >~30 volts from your solar array (max when panels are cold is 75 VDC for Controller). Your panels are rated for 21.6 volts, and for an MPPT controller, they are "less efficient" (lest harvest power) with lower solar array voltage.

Placing your two solar panels in series for ~44 VDC nominal (~Vmp?), would be optimum for your setup.

Remember that fusing/breakers protect down stream wiring... While 20-30 Amp protects your 10 AWG cable, if you use smaller AWG cabling to lights, radio, USB chargers, etc., you need to fuse those "branch ciruits" with appropriate sized fuse/breakers. For example, using 14 AWG cable, 15 amp fuse/breaker is typical rating.

https://www.helukabel.us/HELUKABEL/Publications/Technical-Documents/Allowable-Ampacity-Tables.pdf

-Bill

mike760

Great information above by BB.

But, I'm not sure what your power distribution looks like to the "led lights, propane furnace, fridge, maxxffan, propane water heater" so it is hard to say.  It doesn't necessarily matter how much or little power a device consumes.  The circuit protection is generally determined by the ampacity of the conductor supplying the device.  The conductor size is determined by the draw of the device(s) supplied by that conductor.  If the conductors are individually and directly connected to the battery an inline fuse on each wire can be used.  If you are distributing the power from some type of bus, the bus will have to be fused in addition to the individual "branch" circuits connecting the devices.  So in this scenario you may have a #8 with a 50A fuse supplying the bus from the battery and then a #12 connected to the bus with a 20A fuse supplying the water heater and then maybe a #14 with a 15A fuse supplying the lights.

Basic guideline is to make sure the fuse is rated at the same ampacity or less than that of the conductor that it is protecting.  If you are making use of that 100A fuse that you have, you need to run no less than a #3 wire.  Fuses are cheaper than wire so if you do not need anywhere near that amperage, buy some smaller wire and an appropriate fuse.

Keep in mind there are other factors which can change some of the amperage references we make on some of these posts but that gets a little more detailed and most likely beyond the scope here.