How to size the main battery bank fuse?

[rmaddy]

I have a basic understanding of how to size fuses used for individual components within the system. But I want to verify the proper way to size the main fuse that is between the positive cable off of the battery bank and the main positive busbar. Attached to my busbar will be two cutoff switches - one leads to the inverter and the other leads to the DC fuse boxes (for my 24V system I will have a 24V fuse box and a 12V fuse box). The MPPT charge controller is also connected to the busbar. There are of course separate fuses for each of those components (which I better understand how to size).

What is the general formula and set of variables needed to calculate the main fuse? Is it the usual calculation but based on a total sum of all possible loads? Is it a sum of the other fuses (indirectly) attached to the busbar? Is it based on the batteries in some way? And of course the wire size has a role since that is what the fuse protects.

I'm not looking for a specific number, but the set of variables and general formula used to determine the fuse size.

 

[Supervstech]

I have a basic understanding of how to size fuses used for individual components within the system. But I want to verify the proper way to size the main fuse that is between the positive cable off of the battery bank and the main positive busbar. Attached to my busbar will be two cutoff switches - one leads to the inverter and the other leads to the DC fuse boxes (for my 24V system I will have a 24V fuse box and a 12V fuse box). The MPPT charge controller is also connected to the busbar. There are of course separate fuses for each of those components (which I better understand how to size).

What is the general formula and set of variables needed to calculate the main fuse? Is it the usual calculation but based on a total sum of all possible loads? Is it a sum of the other fuses (indirectly) attached to the busbar? Is it based on the batteries in some way? And of course the wire size has a role since that is what the fuse protects.

I'm not looking for a specific number, but the set of variables and general formula used to determine the fuse size.

If you have a massive buss bar connected to the bank, and loads off it are fused, I would think the best fusing method here would be see the combined max amps the batteries are rated for, and size the fuse just below that.

For instance, if you have 4 battleborn sets, and each one is rated to provide 100 amps max continuous, size the fuse, AND the main conductors feeding the bussbar, for 400 amps.

 

[Supervstech]

Or, if the bank is far in capability over your designs max load, size the conductors and the main fuse not to exceed that maximum.

Many lead acid batteries are capable of thousands of amps each, a huge bank of them could easily exceed 5000 amps! So, in that case, I would want to set a maximum draw and rate the fuse and conductor to my combined continuous draw plus 25%... so, a pair of 10,000 watt inverters at 48v, pulling max demand could pull
417 amps x 1.25 = 521 amps... I would size the feed conductors based on this demand, and fuse it at 520...

 

[rmaddy]

For instance, if you have 4 battleborn sets, and each one is rated to provide 100 amps max continuous, size the fuse, AND the main conductors feeding the bussbar, for 400 amps.

In my case I'm looking at Battle Born, probably four 12V in 2S2P so that would be 200A, right?

I will just have one 2000W inverter which will have its own 200A fuse. So 2000W / 24V x 1.25 = 105A. 200A sounds better.

 

[Supervstech]

As long as you size the wire to handle 125% of loads, you are good.

 

[Supervstech]

As long as you size the wire to handle 125% of loads, you are good.

Apparently 156% is needed on loads.
My bad.

 

[rmaddy]

I believe the 1.56 (1.25 x 1.25) is only for calculating the amps from the solar panels to the charge controller. The typical 1.25 can be used for other calculations within the system. See (starting at 8:38)

for an explanation (though the whole video is worth a watch).