Need help with fuse sizing

[WilliamC]

I have some rather basic questions about system wire sizing and fuse sizing.

Batteries

BigBattery———BigBattery———liFePOWER4———IiFePower4
24v 170Ah———24v 170Ah———24v 200Ah————24v 200Ah
Built-in—————Built-in———— Built-in——————Built-in
300a Fuse ———300a fuse ——— 100a breaker ———100a breaker

Cables to———Cables to ———— Cables to ———— Cables to
BussBar ——— BussBar ———— BussBar ————— BussBar
6 Awg ———— 6 Awg ————— 6 Awg —————— 6 Awg

300 Amp 4 terminal BussBars

Main Cables from BussBar to Inverter 1/0 Gauge

Inverter;
Aims Power 4000w continuous 12000w surge
24vdc in
240 vac out

My question is what additional fusing should I add?
Between each battery and the BussBar?
Between the BussBar and the inverter?

 

[WilliamC]

Sorry about the dashes above. I had it all neat looking with columns using tabs but it got scrunched together when published.
I get confused looking at the fuse size charts.
They say for 6awg cable to use an 80amp fuse.
But the LiFePower4 battery came with 6awg cables and a 100 amp breaker.
And the Big battery is even more. It came with 6awg cables but uses a 300 amp fuse.
How is a 300 amp fuse going to protect 6awg cable?

 

[MisterSandals]

You need to add wire lengths.

This is pretty handy to get close. Shoot for less than 3% drop.

Final wire selection should be based on wire specifics and ampacity:

American wire gauge - Wikipedia

en.wikipedia.org

Give it a go... trying to teach a man to fish...

 

[WilliamC]

Oh sorry all 6awg cables are 36” and the 1/0 guage cables are 12”.

 

[WilliamC]

For the BigBatteries which are 24v 170Ah. They came internally fused with a 300amp type-t fuse. They also came with a 48” 6 awg cable. Charts say that the 6 awg cable should be fused at 80 amps. Would it make sense to swap out the internal 300 amp fuse with a smaller 100 amp or 80 amp fuse? Or just add an additional 80 amp fuse to the cable? The only load will be from a 4000 wat inverter so roughly 170 amps total. Shared between 4 batteries each battery with its own 6awg cable from BussBar to battery. Im thinking That the draw from each battery at full power should be less than 40 amps.

 

[WilliamC]

Similar question for the LiFePOWER4 batteries. 24v and 200 Ah. Internal 100amp breaker. Charts say use an 80 amp fuse for the 48” 6awg cable. But maybe a fuse more than 100 amp is more practical so the breaker will trip first? Or is the breaker alone enough protection?

 

[Sennen]

Should fuse from Bus Bar to Load. Base that on the inverter's max draw. (Example, Solark 15k will pull up to 275a; Wire and fuse would be 300a)

 

[WilliamC]

Thanks for the help guys. One last question for sizing the fuse going from buss bar to inverter. Inverter is a low freq 4000 wat with a surge capable of 12000 wats. My well pump will pull close to 12000 wats during startup which is very brief, less than a second. I am considering using an MBRF fuse off the BussBar. How quickly will this blow? What size fuse is needed? Do I need to size for the surge wattage? Or just the rated wattage?

 

[Sennen]

Fuses are rated for amps. How many volts at that 4kw?

 

[WilliamC]

24 volts. 4000 watts. But my question is not specific to a particular voltage or wattage. What I want to know is should MBRF fusing be sized at continuous amps or at surge amps.

 

[Sennen]

Your 4000W is 166 amps.

 

[MisterSandals]

Your 4000W is 166 amps.

Yes but the 4000W (output) inverter draws more with roughly 85% efficiency. And, the inverter can draw more amps to meet the output as the battery drops. The inverter cutoff voltage, probably around 20V (use an actual value if you have it in the specs).

4000W / .85 efficiency / 20V cutoff = 235A