what size fuse/ breaker
breealee
Registered Users Posts: 4 ✭
i'm putting together a small pv system on my 25' travel trailer.This will be a DC only system no inverter.
Specs:
Specs:
 2 BB 100ah li fe po ,
 1 sunpower 360 panel,
 100/30 victron mppt,
 victron batt. monitor.
 15a breaker switch on the panel side of mppt.
 40 amp on the battery side.
Comments

Welcome to the forum Breealee,
I edited your post into a list of what you haveEasier (at least for me) to follow.
The battery is usually the "heart" of your system. All wiring leaving the battery (the source of high surge current into loads and/or short circuits). Wiring and fuses/breaker are sized for your loads. And fuses/breakers are there to protect your wiring (from overheating/starting a fire). Fuses/Breakers are not there to "protect your equipment" from overcurrent (usually any electronics and other loads will fail before a fuse/breaker trips).
So, what are your DC loads... Maximum continuous current (like LED lighting, running a DC CPAP machine, HAM radios, or what?).
If I recall correctly, the Battle Born batteries will supply a maximum of 100 Amps before their internal Battery Management System (BMS) will shut them down.
Assuming that you would draw 100200 Amps from your battery bank for your loadsProbably not what you are planningIt would take your battery bank "dead" in 12 hoursNot very useful.
To take a stab at what "reasonable" loadsLet's look at your solar panel. Say 360 Watts, in Tucson AZ, flat on roof:
http://www.solarelectricityhandbook.com/solarirradiance.htmlTucson
Measured in kWh/m2/day onto a horizontal surface:
Average Solar Insolation figures
And guess that most of your dry camping will be in months with >5 hours of sun per day (MarchSeptember). The daily harvest would be:Jan Feb Mar Apr May Jun 3.30
4.20
5.64
6.89
7.59
7.52
Jul Aug Sep Oct Nov Dec 6.43
5.87
5.55
4.57
3.62
3.05
 360 Watts * 0.77 panel+controller deratings * 5.0 hours minimum average sun per day = 1,386 WH per day (of 12 VDC energy)
 1,386 Watt*Hours per day harvest / 5 hours of use per day = 277 Watt average load
 277 Watt load / 12 volt battery bank = 23 amps
 23 amps * 5 hours per day load = 115 AH @ 12 volts from battery bank
You can use the NEC table to size the cabling for your loads:
https://lugsdirect.com/WireCurrentAmpacitiesNECTable30116.htm
You don't have too, but I like to derate the NEC wiring table by 80% (or 1.25x). So, for a 23 amp DC loading (assuming max continuous current): 23 Amps * 1/0.80 derating = 28.75 amps ~ 30 Amp branch wiring and fuse/breaker ratings
 360 Watt * 1/0.80 NEC NEC derating * 1/10.5 minimum battery bus voltage (worst case with MPPT controller) = 43 Amps
 30 Amps * 1/0.80 NEC derating = 37.5 Amps = 40 amp rated wiring/breaker
I don't know which 360 Watt solar panel you have, but to estimate its output current using Vmp~38 VDC: 360 Watts * 1/0.8 NEC derating * 1/38 volts = 11.8 Amps typical worst case current
Please note that all switches/breaker/fuses need to be rated for DC usage. The solar panel switch need to be rated to >Voc (around 50 VDC), and the DC battery bus breakers need to be rated for 12 VDC or higher. AC current is "easier to break" vs DC current (DC "sustains arcs" much easier).
Each load/charging source should be "home run" back to the battery + and  bus (you have a Battery Monitor, so the + bus is the battery common connections. The  bus is not the battery but the "ground side" of the precision Battery Monitor power resistor.
There is one other major issue to take into account. And that is voltage drop for your wiring. For example, say you want to send 23 amps 20 feet (one way run for this Voltage Drop calculator), and you want no more than 0.5 VDC drop:
For 8 AWG cable the voltage drop would be:
https://www.calculator.net/voltagedropcalculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=20&distanceunit=feet&eres=23&x=0&y=0Result
Voltage drop: 0.58
And if you used 10 AWG (which is perfectly OK for a 30 amp circuit):
Voltage drop percentage: 4.82%
Voltage at the end: 11.42
https://www.calculator.net/voltagedropcalculator.html?material=copper&wiresize=3.277&voltage=12&phase=dc&noofconductor=1&distance=20&distanceunit=feet&eres=23&x=54&y=22Result
Voltage drop: 0.92
Which is a lot of voltage drop for a 12 volt circuit. Sending significant amounts of power any distance at 12 VDC generally needs lots of copper.
Voltage drop percentage: 7.66%
Voltage at the end: 11.08
Anyway... Lots of typing and assumptions. Questions/clarifications/corrections?
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Thanks Bill for your quick response. I'm new to all this DC electric. Not sure if this will change anything with your response. My Sunpower 360 X22 has an efficiency of 22%. Volts 59, amps 6.0, open circuit 69.5. As I stated in my post, I'm not clear if I need to fuse/ breaker the main positive # 6 wire from pos. term. to the dc distribution center in my trailer. Distance is approx. 15 feet. I'v read/ heard that a 80 amp. bussman breaker will do the job. Thoughts? Also, my dc usage estimate is 85 amps. on a very busy day. As far the copper goes, from the panel down to the mppt iis a #10. From the mppt to the battery bank is a #8.

@breealee
You will need a breaker on the load side of the battery. It can be located at the distribution panel if so desired. The #6 AWG wire will carry 75 amps max if it is THHN. If you need 85 amp capability for loads, then recommend #4 AWG THHN minimum. Need to determine the max constant amperage use for wire size. Don't know enough about Bussman breakers to comment. The wiring to and from the charge controller sounds adequate. When it comes to DC wiring, over sizing a bit saves some power lost to resistance. Especially on the load side for runs longer than 10 feet.4480W PV, MNE175DRTR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset. 
Generally, you connect your battery bank to the "battery bus" using short/heavy cable. Sized to carry your maximum charging/discharging current. Technically, you should consider a fuse/breaker from each battery to the bus (2x if 2x 12 volt batteries to a 12 volt bus, 2x fuse/breaker to a 24 volt bus and the two batteries in series).
If I recall correctly, the BB batteries do shut down if >100 amp loadSo, wiring that can handle 100 amps into a dead/heavy loads for a short period of time could negate the need for a "battery fuse/breaker".
And every positive wire that leaves the battery bus (wire sized to branch circuit loads) should have its own appropriately sized fuse/breaker (breakers are nice, you can use them as on/off switches too).
As an example, say you have 5 amps worth of lighting. One 10 Amp "cigarette 12 volt socket", and 15 amps to a CPAP or HAM device.
You have to look at the load, length of wiring, and protective device. For example: 5 amp lighting * 1.25 NEC derating = 6.25 Amps. Use a 7 amp minimum breaker/fuse. And 18 AWG wiring (good for 14 amps).
 15 amp rated (14 AWG) * 1/1.25 NEC derating = 12 amp branch circuit rating (suggested maximum fuse/breaker)
And that 14 AWG would also work for the lighter socket (10 amps typical max rating).
Say your 15 Amp CPAP or HAM radio is mounted at the other end of the RV... 20 feet, and you want no more than 0.5 volt drop.
First derating: 15 Amps * 1.25 NEC derate = 18.75 ~ 20 amp branch circuit with 12 AWG minimum cable
https://www.calculator.net/voltagedropcalculator.html?material=copper&wiresize=5.211&voltage=12&phase=dc&noofconductor=1&distance=20&distanceunit=feet&eres=15&x=64&y=18
12 AWG:Result
Voltage drop: 0.95
8 AWG:
Voltage drop percentage: 7.94%
Voltage at the end: 11.05Result
Voltage drop: 0.38
For the solar panel specs... Need specifically Vmp and Imp... It sounds like those are Voc and Isc (open circuit and short circuit ratings).
Voltage drop percentage: 3.14%
Voltage at the end: 11.62
And you did not clarify if 12 or 24 volt Battery Bus.
85 Amp draw from 2x 100 AH batteries in parallel is a pretty hefty load. At that rate, it will drain the two batteries (12 volt bus) in something like a bit more than 2 hours.
If that supports your loads and your needs, that is fine... But that is out of the ordinary for off grid systems (which tend to be highly conservation minded, and spread out for 510 hours at a time between charges).
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
@Raj174
The load side of the battery is the wire that comes from the charger converter to the pos. terminal correct? When I have the trailer plugged in to shore power my charger will charge at 55 amps, 14.6 volt. 
The battery bank is generally the source of very high current (even smallish lead acid battery bank can output 100's of amps into a dead short).
So, every postive wire that leaves the battery bus should have a fuse/breaker on it.
For example, a 55 Amp charger: 55 amps * 1.25 NEC derating (my personal suggestion) = 68.75 amps ~ 70 amp rated branch circuit suggested
https://lugsdirect.com/WireCurrentAmpacitiesNECTable30116.htm
Somewhere between 6 and 4 AWG (depending on the insulation type, for copper wire).
I would guess that you have one or more wires leaving the battery + bus... Perhaps a heavy cable to a DC distribution box. Or possibly individual wires leaving to Lights/Fan/DC outlets for computer/cell phone charging... Each wire sized for it load. Each wire protected with the appropriate breaker/fuse.
In theory, the fuse/breaker/protection should be within ~1218 inches of the battery bankBut that is not always possible. You do want to make sure that all wiring is protected from damage (conduit, mounting to reduce bending/vibration, any sharp holes/edges protected with grommets, etc.).
Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
breealee said:@Raj174
The load side of the battery is the wire that comes from the charger converter to the pos. terminal correct? When I have the trailer plugged in to shore power my charger will charge at 55 amps, 14.6 volt.4480W PV, MNE175DRTR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset. 
thanks!
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