# Simple calculation for fuse/circuit breaker size?

Posts: 1,889Solar Expert ✭✭✭✭
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.
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

• Posts: 310Registered Users ✭✭✭✭
I'm the rookie here, but...  If it is safe to assume you've sized the battery bank appropriately for your inverter, you could just take the nominal rating of your inverter (or the 30 min surge rating of the inverter), divide by 0.85 (or whatever is appropriate for the efficiency of your inverter) and then by the voltage of your battery bank. Then multiply by the 1.25 for a good max current limit for the breaker.

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
Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
• Posts: 1,889Solar Expert ✭✭✭✭
edited November 2016 #3
Thanks....now I realize that I have possibly seen this formula a dozen times and failed to memorize it.

Lots of leeway considering that the inverter nominal rating all the way to surge rating can be plugged in.
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
• Posts: 310Registered Users ✭✭✭✭
Best wait for BB to chime in. I'm still speculating....
Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
You guys pretty much have it:
• 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)
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 310Registered Users ✭✭✭✭
Thanks Bill.  I guess I've probably seen it before, but why do you use 10.5V for the battery. That's 100% DoD for a nominal 12V battery, right?  Or when you say "battery cutoff voltage" are you referring to the inverter's low voltage cutoff?

Steve
Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
If you allow for 1.0 volt drop for wiring/breaker--That gives you 11.5 volts "at the battery" and 10.5 volts at the inverter DC input.

The 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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 751Solar Expert ✭✭✭✭
edited November 2016 #8
You also have 2 types of fuses and breakers,
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.

• Posts: 1,889Solar Expert ✭✭✭✭
Pretty sure that the better inverters have built in circuit breakers. I'd check but tapped out at the moment....from looking for lost drones. i am not the worlds greatest drone operator as it turns out.
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
• Posts: 2,203Solar Expert ✭✭✭✭✭
edited December 2016 #10
The purpose of a fuse is not to protect the load, it is sized to protect the cable feeding the load, so in the case of an overload or short the cables do not become the fuse.
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 )
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank

• Posts: 1,889Solar Expert ✭✭✭✭
Even a massive fuse....say 400/450 amps....would *likely* pop before 4/0 cables caught on fire. I'm not advocating for oversize fuses. Just thinking that an over sized fuse would be far preferable to no fuse at all.
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
• Posts: 3Registered Users
Horsefly said:
I'm the rookie here, but...  If it is safe to assume you've sized the battery bank appropriately for your inverter, you could just take the nominal rating of your inverter (or the 30 min surge rating of the inverter), divide by 0.85 (or whatever is appropriate for the efficiency of your inverter) and then by the voltage of your battery bank. Then multiply by the 1.25 for a good max current limit for the breaker.

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
Sorry im a bit lost, does this formula is to size the breaker between the charge controller and the battery ?
For the fusing/breaker between the solar charge controller and the battery bank--The sizing is a bit more fuzzy.

What 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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 3Registered Users
edited October 3 #14
BB. said:
For the fusing/breaker between the solar charge controller and the battery bank--The sizing is a bit more fuzzy.

What 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
I'm using three  SMA SUNNY ISLAND 6M ,https://www.sma.de/en/products/battery-inverters/sunny-island-44m-60h-80h.html
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.
• Posts: 7,901Solar Expert ✭✭✭✭
You do not use a "formula" to calculate the breaker size. Breaker size is determined by the wire used. NEC publishes tables to use to look up wire size and what breaker to use with that wire. Here is a link to one. https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/ You use the "Maximum amps for power transmission column 8 ga wire needs a breaker / fuse 40A or less You select a wire size that can carry your charge controller output, a 40A controller would want at least 8 ga wire
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 ,

edited October 4 #16
I think the OP is in Indonesia... So our US/North American AWG sizing will confuse stuff a bit--But the following is at least a "known" set of safety standards (at least to me).

There is the National Electric Code (USA) which is fairly conservative (and the actual tables are more complex)--But to give you an idea:

And there is a US Marine set of wiring codes--Note that SAE "gauge" is very slightly smaller than American Wire "gauge" (AWG) (SAE is much less conservative):

http://www.boatus.com/boattech/articles/electrical-wiring-on-boats.asp (1/2 way down web page--Engine room vs "inside" wiring)

Metric vs US wire sizes (I guess there are two standards for metric wire--One is simply diameter the other is the square mm of the wire)

http://wiringfordcc.com/wire_sizes.pdf

In the US, you need to understand the maximum current rating of the charge controller (MPPT controllers can limit their maximum output current, so the 40 amp limit, or whatever, is a "hard" limit. If you use a PWM controller or the size of your solar array, the maximum current should probably be "upped" by 1.25 times to allow for variations in sun, refection, etc.).

As always refer to the manual for your system:

Section 7.4.1 has some cable sizing... 14 mm minimum diameter (50 mm^2) which is around 1 to 1/0 AWG wire gauge... And per 10.3 there is a max of 110 Amps of charging current (I am just taking a quick look at the manual... for the 6H unit... You need to read the manual cover to cover).

In the US, roughly most standard fuses and breaker are rated at 100% of current--Fuse/Breaker will eventually trip; at ~80% of rated current, the breaker/fuse should not trip.

So, if you have a system rated at 110 Amps maximum charging current, then you should up size the wiring/fuses/breakers by 1/0.80 or 1.25 times or 110A*1.25=138 amps minimum. In the US, 1 AWG cable is rated for ~110 to 150 Amps (depending on temperature and insulation type).

Remember that fuses/breaker protect the system wiring, not the end devices. You want the cables and fuses/breakers to be large enough that they do not 'false trip' during normal operation (in the US, that is the 1.25x operating current = "branch circuit/fuse/breaker" rated current minimum.

Remember there is another sizing for battery wiring... More or less for a 48 volt battery bank, you want no more than (suggested) ~0.2 to 0.4 volts drop between the charge controller and battery bank... Too much wiring drop will slow down the battery charging (i.e., charger "sees" 58.0 volts charging, but battery sees less than ~57.6 volts and accepts less current).

You can use a voltage drop calculator to estimate the voltage drop... For example, say you use ~50 mm^2 wire (between 1 and 1/0 AWG wiring), how far can you run the wire before you have "too much" voltage drop. Using a simple calculator:

Enter 110 AH and 1 AWG, play around with distance to get a maximum 0.4 volt drop:

Voltage drop: 0.36
Voltage drop percentage: 0.62%
Voltage at the end: 57.64

4 meters gives us ~0.36 volt drop at 110 Amps and 1 AWG cable.

If you have more questions, I suggest that you start a new "thread" about your systems and needs. We can then focus on your questions and you can control where you wish the discussions to proceed.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 3Registered Users
BB. said:
I think the OP is in Indonesia... So our US/North American AWG sizing will confuse stuff a bit--But the following is at least a "known" safety set of safety standards (at least to me).

There is the National Electric Code (USA) which is fairly conservative (and the actual tables are more complex)--But to give you an idea:

And there is a US Marine set of wiring codes--Note that SAE "gauge" is very slightly smaller than American Wire "gauge" (AWG) (SAE is much less conservative):

http://www.boatus.com/boattech/articles/electrical-wiring-on-boats.asp (1/2 way down web page--Engine room vs "inside" wiring)

Metric vs US wire sizes (I guess there are two standards for metric wire--One is simply diameter the other is the square mm of the wire)

http://wiringfordcc.com/wire_sizes.pdf

In the US, you need to understand the maximum current rating of the charge controller (MPPT controllers can limit their maximum output current, so the 40 amp limit, or whatever, is a "hard" limit. If you use a PWM controller or the size of your solar array, the maximum current should probably be "upped" by 1.25 times to allow for variations in sun, refection, etc.).

As always refer to the manual for your system:

Section 7.4.1 has some cable sizing... 14 mm minimum diameter (50 mm^2) which is around 1 to 1/0 AWG wire gauge... And per 10.3 there is a max of 110 Amps of charging current (I am just taking a quick look at the manual... for the 6H unit... You need to read the manual cover to cover).

In the US, roughly most standard fuses and breaker are rated at 100% of current--Fuse/Breaker will eventually trip; at ~80% of rated current, the breaker/fuse should not trip.

So, if you have a system rated at 110 Amps maximum charging current, then you should up size the wiring/fuses/breakers by 1/0.80 or 1.25 times or 110A*1.25=138 amps minimum. In the US, 1 AWG cable is rated for ~110 to 150 Amps (depending on temperature and insulation type).

Remember that fuses/breaker protect the system wiring, not the end devices. You want the cables and fuses/breakers to be large enough that they do not 'false trip' during normal operation (in the US, that is the 1.25x operating current = "branch circuit/fuse/breaker" rated current minimum.

Remember there is another sizing for battery wiring... More or less for a 48 volt battery bank, you want no more than (suggested) ~0.2 to 0.4 volts drop between the charge controller and battery bank... Too much wiring drop will slow down the battery charging (i.e., charger "sees" 58.0 volts charging, but battery sees less than ~57.6 volts and accepts less current).

You can use a voltage drop calculator to estimate the voltage drop... For example, say you use ~50 mm^2 wire (between 1 and 1/0 AWG wiring), how far can you run the wire before you have "too much" voltage drop. Using a simple calculator:

Enter 110 AH and 1 AWG, play around with distance to get a maximum 0.4 volt drop:

Voltage drop: 0.36
Voltage drop percentage: 0.62%
Voltage at the end: 57.64

4 meters gives us ~0.36 volt drop at 110 Amps and 1 AWG cable.

If you have more questions, I suggest that you start a new "thread" about your systems and needs. We can then focus on your questions and you can control where you wish the discussions to proceed.

-Bill
Thanks a lot Bill, ur informations was so helpful for me. i got it. and it all make sense.

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.pdf

thanks a gain
You are correct... We are talking about the DC input side to the DC to AC inverter. Several notes... We use the Inverter Watt rating (ike 1,200 Watts) usually as the "design" wattage. However, there is nothing really that stops you from saying "I only want to use a maximum of 600 Watt loads"... And you use the smaller number (the fuse/breaker will simply trip if you try to pull 1,200 Watts for any length of time).

Also, 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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset