# Simple calculation for fuse/circuit breaker size?

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Solar Expert Posts:

**1,928**✭✭✭✭
BBs mental gymnastics to calculate batteries and panels and controllers? Awesome....but I want something fast and simple. If the calculation is off by 2.73 amps....so be it.

Referring to the fuses or circuit breakers between the batteries and controller/inverter.

Then what about in between the panels and controller?

Maybe I used to know. Have forgotten so much.

Referring to the fuses or circuit breakers between the batteries and controller/inverter.

Then what about in between the panels and controller?

Maybe I used to know. Have forgotten so much.

First Bank:16 180 watt Grape Solar with FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries

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## Comments

318✭✭✭✭As an example (based on my to-be-assemble set): 4000 watt peak 30 minute inverter and my 24V battery bank:

4000w / 0.85 (efficiency) / 24V * 1.25 = 245A, so I would probably round that up to a 250 A breaker (which is what I bought!).

Steve

1,928✭✭✭✭Lots of leeway considering that the inverter nominal rating all the way to surge rating can be plugged in.

318✭✭✭✭28,184admin- Rated Watts * 1/0.85 typical inverter eff * 1/battery cutoff voltage * 1.25 NEC derating for wiring/fuses/breakers = branch circuit rated current
- 1,200 Watts * 1/0.85 inverter eff * 1/10.5 battery cutoff * 1.25 NEC derating = 168 Amp circuit (1,200 Watt, 10.5 volt cutoff, wire+breaker rating)

-Bill318✭✭✭✭Steve

28,184adminThe 10.5 volts is a typical Inverter shutdown set point (for 12 volt battery bank). And is used to protect the inverter from excessive input current (Power = Voltage * Current --- Assuming the AC output power is "fixed", as the DC input voltage falls, the DC input current must rise).

At some point, the I^2*R heating exceeds the Inverter's component and heat sink ratings--And the inverter' ability to supply rated AC voltage (avoid AC brown out, which can damage attached AC loads such as refrigerator motor, etc.). Note that internal heating rises with the square of the current. A (for example) 2x increase in current means ~4x increase in waste heat.

-Bill

759✭✭✭✭Ones that fault at the rated current.

And ones that will run at the rated current.

For example my warn winch is rated to draw 300 amps max. 300 amps is over load but it can take that much over load for up to 30 seconds, but then requires a 10 minute rest. I put a 250 amp ANL type fuse on it.

It will run fine at 250 to 260 amps. But will only run for about 5 seconds at 300 amps and 1 or 2 seconds at 350 amps.

What ever fuse or breaker you are looking to use check out the fuse's trip curve or you may smoke the inverter before the fuse pops.

So for your 4000w inverter you may actually want to run as low as a 200 amp fuse, depending on the fuse or breaker if you want to actually protect the inverter from over load.

A 250 amp fuse or breaker may only protect the batteries and wiring from a short circuit.

Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

1,928✭✭✭✭2,423✭✭✭✭✭Edit: so in the case of an OVERCURRENT or short the conductors do not become the fuse. (overload is not the same as overcurrent, an overload device is to protect the load, a fuse is an overcurrent device )

900W 3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system.

5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.

1,928✭✭✭✭3✭28,184adminWhat brand/model of charge controller (MPPT, PWM? Voltage of battery bank, size of solar array, size and length of cable from charge controller to battery bank)?

-Bill

3✭Batteries are 48V. battery capacity is 552Ah.The solar array size is 52 Panels of 100w. using JA solar. Length is 3m and size im not sure about it yet.

Isn't there a formula that i can use to calculate the breaker size between the battery and the controller ?

In addition, can you please specify for which breaker is the formula u wrote above ?

sorry for the much question because im still a beginner.

8,030✭✭✭✭|| 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 ,

28,184adminhttp://wiringfordcc.com/wire_sizes.pdf

0.36Voltage drop percentage:

0.62%Voltage at the end:

57.643✭My last question is Regarding the formula listed in the conversation above by u which is,

- Rated Watts * 1/0.85 typical inverter eff * 1/battery cutoff voltage * 1.25 NEC derating for wiring/fuses/breakers = branch circuit rated current
- 1,200 Watts * 1/0.85 inverter eff * 1/10.5 battery cutoff * 1.25 NEC derating = 168 Amp circuit (1,200 Watt, 10.5 volt cutoff, wire+breaker rating)

is this formula to size the breaker between the inverter and the batteries? in this case the the inverter is the load of the batteries. So do u mean by the rated watts is the rated watts of the inverter? And obviously is should be at the DC side right? can you help me to find this number from my inverter ? http://files.sma.de/dl/31716/STP30-60-DEN1834-V13web.pdfthanks a gain

28,184adminAlso, notice that the Branch Circuit Rating (wiring and fusing) is ~168 amps minimum... Not a small amount of current. Generally, I/we suggest a maximum inverter for various battery bank voltages of:

- 12 volt battery bank > ~1,200 Watt typical maximum AC inverter rating (you can push to ~1,800 Watts if you are careful)
- 24 volt battery bank > ~2,400 to 3,600 Watts maximum
- 48 volt battery bank > Suggested for any system where you are over ~2,400 Watt to 2,600 Watt AC load

There are other practical limitations for AC inverters... The battery bank can only output so much current (and absorb a practical amount of current from the solar array)... For a flooded cell lead acid deep cycle battery bank, the maximum power per 100 AH of battery bank capacity at each bank voltage is (roughly):- 48 volt battery bus > ~1,000 Watts of AC inverter (solar array) maximum per 100 AH battery bank capacity
- 24 volt battery bus > ~1,000 Watts of AC inverter (solar array) maximum per 200 AH of battery bank capacity
- 12 volt battery bus > ~1,000 Watts of AC inverter (solar array) maximum per 400 AH of battery bank capacity

And, you have thrown another wrench into the mix:http://files.sma.de/dl/31716/STP30-60-DEN1834-V13web.pdf

This appears to be a 3 phase Grid Tied inverter--There is no DC battery bus connection. This inverter family simply takes Solar Power from your array and converts it to 180 to 280 VAC @ 50 Hz (230 VAC nominal).

The maximum current per 3 phase leg appears to be 3 x 9.1 Amps @ 230 VAC nominal (3 phase = 3 wire connection) for the 6,000 Watt model. So, you would design your AC branch circuit to be 9.1 amps maximum current, and in the US, we would multiply by 1.25x or ~11.4 amps minimum (round up to your country's next standard breaker rating). This is for a 6,000 Watt (VA) output rating and a maximum of 9,000 Watt solar array rating.

I do not know much about SMA products. They do have a lot of solar power (grid tied, off grid, hybrid AC inverters, etc.). Some very nice equipment and (in the USA) kind of on the expensive side (usually).

There are a lot of details to discuss about your needs and overall system design. I highly suggest you start your own thread/discussion and get going there.

Normally, we start with your loads (watts, Watt*Hours per day, voltage, any DC loads, where will the system be installed, seasonal power variations, etc.). Then design a system to support those needs.

Jumping into the "middle" of a system design can cause issues (we are guessing you know what you want/need--You may not). And there are many different ways to come up with an "optimal" solution. Starting from the beginning is usually more helpful and less frustrating.

-Bill