Wiring size LiFePo4 confirmation

[lifepo4_powerwall]

I am installinga Lifepo4 24v system to run my 6000w/18,000W surgeAIMs Power inverter to run my house when I lose power after the next hurricane in Florida.
I have mounted battery banks on the wall and calculated the wiring sizes but needed someone to check it over before I order the wire. I checked out the FAQ on wire sizing.
If you look at the picture each bankof cells is 12v, so I am putting 2 banks in series to get 24v. Each bank will provide 228ah or 452ah per pair (based on 3ah per cell x 76cells per bank).
The pair of banks on the left will be connected to each bank pair until the wiring gets to the inverter so the amperage will increase.
So for leg 1 I calculated 452ah / 24v = 19amps, so a 12AWG wire will work, the distance from one leg to anther is 1ft.
leg 2 = 19 +19 = 38amps = 10AWG wire, 1ft red and black
leg 3 = 38 +19 = 57amps = 10AWGwire, 1ft, red and black
leg 4= 57+19 = 76amps = 8AWGwire, 1ft, red and black

leg 5:I also have a 40 cell bank of headway 38120 cells under the inverter which provides 10A each or 400ah with 24v which generates 16A and needs a 10g wire, 3ft red and black


Now I need to join the leg 4 wire to the leg 5 for a total of 76+16 = 92A which needs8AWG.
The AIMS inverter docs states itneeds 4/0 wire for 6000w at 24v which is a big jump up from 8AWG!

Are my calculations correct? What happens when I get a surge demand of 18,000W through the inverter? Will the calculated wiring gauge handle the load for 20 seconds?

 

[Redpacket]

Picture didn't make it - you have to click on the pic you want to make it upload now....

Without a pic I'm guessing a bit but it sounds like you're wiring daisy chaining the battery bus?
If not done right, this can lead to pack imbalance due to the currents you're talking about.
6kW for 24V system does make for huge operating currents!

I think your current calcs are out a bit:
If you have 4 "legs" = 4x banks of 2x 12V packs each, then the current at 6kW will be (ignoring losses)
6,000W/24V = 250A total
4 banks makes it 250A/4 - 62.5A per bank

Surging to 18kW, the current is a huge 750A.... + losses get worse due battery voltage sag, resistance, etc so this might hit 800A or 850A as the inverter regulates the output.

If you surge to 18kW for 20 secs, yes correctly rated wiring (ie for 250A above) will cope, ratings are about long term heating so a 29 sec surge won't heat enough to be an issue.

Are the 38120 cells Li-ion or LiFePo4 too?

 

[lifepo4_powerwall]

Sorry, here is the picture.





Correct, trying to keep cabling linear. Should I instead run all cabling to a busbar with studs that I can attach the cabling lugs to and from there to the inverter?

All cells are LiFePo4.

 

[Redpacket]

Sorry, here is the picture.





Correct, trying to keep cabling linear. Should I instead run all cabling to a busbar with studs that I can attach the cabling lugs to and from there to the inverter?

All cells are LiFePo4.

Especially at the very high currents you're heading for, cable runs need to be equal or arranged so that cable resistances are equally spread out & all banks see equal current & equal voltage.
A good way to do this is with the terminal strips you've suggested - you'd run equal lengths of cable from each bank to the terminal strip, eg say 1m from each banks +ve to the +ve strip & 1m from each banks -ve to the -ve strip.

Another good way is to locate the terminal strips at the diagonally opposite corners. So say the +ve strip is at the top right of the bank & you run varying cable lengths as needed to the banks +ve ends. The -ve strip then goes at the bottom left end & similar cabling to the banks. Result is same total cable length (add the +ve part & -ve part) for each bank.
You'd have one shorter cable to the inverter & one longer one (both large sized, see below)

The busbars need to be pretty solid & sized for the current.

Connect the other pack to the busbars too.

Don't forget fusing &/or breakers for safety & isolation when servicing, etc - one fuse per bank here might be good.

re the currents (again ignoring losses) the pic shows 10 packs, you'd said 2x 12V pack per bank in series, so that's 5 banks right?
6,000W/24 = 250A (same as above)
250A/5 banks = 50A per bank (lower)
76 cells per pack at 12V means 19 cells in parallel.
So 50A per bank / 19 cells = ~2.63A/cell, quite high.
At 18kW surge that'd be a massive 7.9A cell, very high! With various losses this could easily exceed 10A/cell.

At 250A busbar to inverter cables would need to be AWG "2/0" or 70mm each.
At 50A/bank, bank to busbar cables would need to be just larger than 4mm, I'd go 6mm if metric, AWG 10 (= 5mm).
See this table, 4th column from right next value > 250A = 289A = 70mm on left.
http://electrialstandards.blogspot.com/2015/11/cable-size-and-current-carrying-capacity.html
Beware of "auto industry" sizes which are often under undersized.

And I'm running 48V so currents are half yours - my 5kW inverter manufacturer recommended 2x 70mm cables in parallel each for the +ve & the -ve !

 

[lifepo4_powerwall]

Thanks for the information, it puts it all in perspective. I do not plan to use anything like the max rating of the inverter to avoid over discharging the cells, but wanted the cabling to be adequate.
I'll probably upsize the cabling a little from 10awg to 8awg to minimize losses and use a bus bar like the one attached to connect the battery banks to the inverter.




I hadn't given much thought to the fuse rating on each of the fivebattery bank.What rating do you recommend?
I have purchased a disconnect for the positivecable, do I also need one for the negative ?

 

[daromer]

Didnt read it all but same length of wires from each pack to a busbar and then a thicker one to the inverter is recommended. Then you dont have to calculate anything more than max current

Dont forget fuses

 

[Redpacket]

Thanks for the information, it puts it all in perspective. I do not plan to use anything like the max rating of the inverter to avoid over discharging the cells, but wanted the cabling to be adequate.
I'll probably upsize the cabling a little from 10awg to 8awg to minimize losses and use a bus bar like the one attached to connect the battery banks to the inverter.




I hadn't given much thought to the fuse rating on each of the fivebattery bank.What rating do you recommend?
I have purchased a disconnect for the positivecable, do I also need one for the negative ?

Not sure that busbar is rated for enough current? Did you have a link?

For the each bank fuse, standard approach is 20% higher than expected steady max current, so for 6kW = 50A x 1.2, so size = 60A or next biggest size.

One disconnect is enough unless local regulations say you need both switched.

 

[lifepo4_powerwall]

Thanks for the information, it puts it all in perspective. I do not plan to use anything like the max rating of the inverter to avoid over discharging the cells, but wanted the cabling to be adequate.
I'll probably upsize the cabling a little from 10awg to 8awg to minimize losses and use a bus bar like the one attached to connect the battery banks to the inverter.




I hadn't given much thought to the fuse rating on each of the fivebattery bank.What rating do you recommend?
I have purchased a disconnect for the positivecable, do I also need one for the negative ?

Not sure that busbar is rated for enough current? Did you have a link?

For the each bank fuse, standard approach is 20% higher than expected steady max current, so for 6kW = 50A x 1.2, so size = 60A or next biggest size.

One disconnect is enough unless local regulations say you need both switched.

Here is the link for the busbar:
https://smile.amazon.com/gp/product/B00821YF1E/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
Its rated for 250A

 

[Redpacket]

That busbar will probably be OK, but putting 50A through those screws feels uncomfortable, I'd prefer bolts!
This one looks better but the $... https://www.amazon.com/Blue-Sea-Systems-600A-1000A-PowerBars/dp/B01FA3NSG4


Or there's heaps of options from electrical wholesalers by googling "brass neutral link" or similar
This style with the larger bolts for the inverter current are what I used:
https://insulect.com/products/brass-earth-bars
or
http://netec.com.au/plus/index.php?route=product/category&path=173_211 < find similar on alibaba for USA
or
https://www.civicsolar.com/product/outback-tbb-ground-ground-busbar

 

[brwainer]

That busbar will probably be OK, but putting 50A through those screws feels uncomfortable, I'd prefer bolts!

The current shouldn't be going through the screw - the screws are just holding the lug or whatever to the actual busbar, most of the current should be passed from one surface to the other directly.

 

[Raj55]

The rating of the cable and the bus bar must be of slightly higher rating to avoid overheating. This saves electricity and provides reliable operation. The jointing point on the terminal block must be bolted to avoid loose connections.