# Choosing The Right Size DC Cables for Battery to Inverter?

Posts: 57Registered Users ✭✭
edited September 18 #1
Hello,

I am in the process of hooking up my Battery Bank to my Inverter and just want to ensure I have adequate cable size.

My Schneider XW+ comes with a 250 Amp breaker. I also have a Midnite Solar DC Disconnect on my battery box also with a 250 Amp Breaker.

I will be using some equip from Blue Sea to connect and fuse (200 Amp per string) my three strings of batteries (not the best I know- but its what I got - in the future I will be doing ONE STRING ).

I have not verified exactly but will ASAP. For now I think my required DC cable length from Inverter to Battery Bank Bus Bars is around 20' or 6 Meters one way. A little long I know.......

[ update - approx. 17 feet max cable length - a little better than I thought ]

All Battery Inter-connect Cables and Cables running to positive and negative Bus Bars are 4/0 as well.

Questions:

1. Do I need to calculate one way or round trip for my cable length?

2. Does my Length measurement start from the actuall battery terminal or the 3 string Bus Bars?

3. Do my Battery Cables need to accommodate 250 amp?

(I wouldn't think so since I believe my whole battery bank will only accept around 55 Amps from the Inverter/Charger and this is when it is equalizing and I don't anticipate putting heavy loads in the battery bank either so I thnk the Amps drawn from the Battery Bank will be even less)

Inverter does 6800 watts continuous
8500 watts for 30 min
11000 watts for 5 min
12000 watts for 1 min

P=VA

A =P/V

=12000 Amps / 48 Volts

= 250 Amp Breaker

Some calculators say I need cable 3 sizes up from 4/0 and some say my voltage drop is under 1% with 4/0 cable?

I'm scratching my head a bit? As usual I know it is simple but not so simple until you fully understand it

Looking forward to hearing back from all who can offer some clarity on this!
John

• Posts: 2,205Solar Expert ✭✭✭✭✭
Based on full load, 4/0 conductorswould be 0.72%, at maximum surge slightly over 1%, so 4/0 is well suited for the purpose.

1   One way see linked calculator
2   Since there is a healthy margin in voltage drop, really insignificant
3  If batteries are a single string the interconnects will carry full current, if series parallel they could be smaller, depending on the number of      parrallel strings but larger is better.

Rarely would the maximum rated output be used, especially off grid, even grid connected 6.8Kw is pretty sizeable a load capacity, as not all loads are used simultaneously, actual draw may be significantly less, having less voltage drop however is always a good thing.

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.1608&voltage=48&phase=dc&noofconductor=1&distance=6&distanceunit=meters&amperes=180&x=20&y=20
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank

• Posts: 1,892Solar Expert ✭✭✭✭
Been here awhile, have yet to see an application where 4/0 was too small for connecting a 48 volt inverter to batteries. I'm sure it exists but would be on the "rare" side.
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: 57Registered Users ✭✭
Hi guys, and thank you so much for your responses!

So if I plug 180 amps into this calculator it says I need a conductor that is 329.4 mm2 ???

And according to this link, 4/0 is approx. 107.16mm2.

4. Why are above links/calculator  so grossly over-killing the recommended cable sizes?

5. Also, how critical is it that the positive and negative cables be the EXACT same length?

Reason I ask, is that my positive and negative bus bars in the battery box may be up to 2 or 3 feet apart due to keeping all the paralleling cables the same length and as short as possible and still look professional while giving clear access to maintain the batteries. (I can make them the same length and have the longer cable just do a "U" behind the batteries - just wanted to know if I could not have the "U" behind the batteries and use less cable as well)

6. Has anyone ever bothered installing Ferrite Core Noise Suppressors on longer runs of cables?
I have not been able to find them for 4/0 but I am sure they may be out there. Might be neat to experiment with ring wave, ripple, noise etc on the cables before and after installing the Suppressors? (not exactly sure what I am talking about - I am sure you guys do though

Thanks again for you time,
Always appreciated !!!
John

• Posts: 7,902Solar Expert ✭✭✭✭
You need to  insure that the voltage drop in the cable, does not rob so much voltage the inverter shuts down on surge loads, like starting a well pump, or making the lights flicker when the fridge starts.
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 ,

• Posts: 2,839Registered Users ✭✭✭✭✭
IMHO, keeping the length of the +/- wires the same isn't necessary. You want to keep the total length of the circuit as short as possible, and in the case of parallel battery connections, you want to have the total length of +/- wire the same (eg. all -ive wires 2', all +ive wires 4', so all parallel connections are 6' total).

Some ham radio types do noise suppression but not sure how many here do (likely some). You might try starting a new thread on that if interested.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Posts: 1,892Solar Expert ✭✭✭✭
Excel Energy runs #4AWG aluminum from the utility poles to the homes in my Denver area neighborhood. Ties into the 2/0 that the homeowners provide. #4 alum is roughly #6 copper. #6 copper at 120 volts is very roughly #2-4 copper at 48-60 volts. DC vs AC though? Big difference there - thankfully Tesla beat Edison for AC over DC.

What inverter does 6800 watts continuous? Thats a monster.

Subject Change: Now why in the world is Neil DeGrasse Tyson comparing Elon Musk to Edison? Edison hasn't fared so well in the age of the internet.
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: 57Registered Users ✭✭
edited September 17 #8
Schneider XW+ 6848
page 126 of 148

I reached out to energymatters.com to inquire as to why their calculator is recommending such huge cable??
I'll post what I get back.....

I'll test voltage drop, but hopefully I should not have any issues given that the drop should be less than 1%.... We'll see

• Posts: 7,902Solar Expert ✭✭✭✭
softdown said:
......
What inverter does 6800 watts continuous? Thats a monster.
The old XW6048 does honest 6Kw, and it's new brother 6848 should do 6.8Kw, no question
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 ,

• Posts: 2,906Solar Expert ✭✭✭✭

It is customary to look to the Inverter manufacturer's recommendations for breaker sizing,   and connecting cable size.

Would bet that Schneider specs a 250 A breaker and 4/0 for connections to the battery bank.

The Ampacity tables in use here show that 4/0 Cu cable  has an Ampacity of 230A for 75 degree C (for three or fewer current-carrying conductors in a raceway ..).   Most of the breakers in common use in our systems spec 75 C maximum for breaker terminals.

There are Ferrites that will accommodate fine strand 4/0 cable with MTW ratings.   Outback uses such ferrites in some Radian inverters that ship to the USA,   and probably to Canada,   as well.

John,   why do you wish to add Ferrites to your system?   Schneider probably already uses some internally on the battery connections.   Am sure that the XW+ 6848 meets FCC Part 15, Class (b),   already.

Are you doing HF Communications?

Would be surprised if the Schneider Charge Controllers comply with any Line Conducted Emissions reduction (Emissions testing up to 30.0 MHz) is not required by the FCC for electronic devices that do not connect to the AC Mains.   Perhaps,  by now,   there might be an EU standard that requires Emission reduction in the HF spectrum.

FWIW,    Vic

Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Posts: 57Registered Users ✭✭
Installation guide says battery cable length Should not exceed 10 feet. (but its gonna have to be just a little longer-I'll measure it out exactly in the morning and call the Engineers at Schneider to see if they say yah it'll work or absolutely Not) I'm sure it will be fine as I will not be pushing the inverter anywhere near what it can do and the battery bank can only accept so many amps at 5 %to 13% of C20 AH rating. Hoping for cold cables and under 1% Voltage drop which the calcs do confirm.

pg 44 of 174

No I am not doing HF Communications, just trying to do this install (the first one I have ever done on my own) to the very best of my ability and as such am leaving no stone un-turned.

When I ran the LED lights w/ dimmer in the sea can that the system is built in I got a bit of noise coming from the inverter (might be normal- might be from the lights/dimmer and was wondering if Ferrites might quite that down. Mind you I did have 20 foot leeds about 1 foot apart from each other as I temporarily charged up and tested the system. I'll get it all set up with as short as possible cables (equal length where they need to be) and tape them together before I install in the conduit and test it then and worry about Ferrites down the road maybe. (Might be already built into the system like you say but I will find out and set things up the best I can given my current situation.

Still curious as to why Energy Matters.com calculator is recommending such HUGE cable. Like I said, I'll post their response if any.

Thanks again Guys for all your feedback. Absolutely Invaluable.
John