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

Registered Users Posts: 172 ✭✭
edited September 2018 #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!
Thank you in advance
John

• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
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 with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Solar Expert Posts: 3,590 ✭✭✭✭
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
• Registered Users Posts: 172 ✭✭
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

• Solar Expert Posts: 9,460 ✭✭✭✭✭
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 ,

• Registered Users Posts: 4,495 ✭✭✭✭✭
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
• Solar Expert Posts: 3,590 ✭✭✭✭
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
• Registered Users Posts: 172 ✭✭
edited September 2018 #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

• Solar Expert Posts: 9,460 ✭✭✭✭✭
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 ,

• Solar Expert Posts: 3,124 ✭✭✭✭

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.
• Registered Users Posts: 172 ✭✭
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

• Registered Users Posts: 172 ✭✭
edited October 2018 #12
"Estragon said:
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."

Hello all,
Re: lengths of negative and positive battery cables, I am soon having my final battery cables made up and as such want to make certain I am doing the right thing. I have measured to ensure all positive parallel cables are equal and all negative parallel cables are equal.

1. How critical is it that they are the exact same length (can they be out by 1 inch?)
2. Just confirming: Can the positive and negative cables going the buss bars be different lengths? Due to my battery negative terminal posts being at the front edge of my battery box and the positive terminal posts at the back, the positive posts can get by with approx. 1 foot less cable on each parallel connection.
3. Kind of the same question as #2: Can the positive and negative cable going from the buss bars to the disconnect be about 3 feet different in length?

Does DC electron flow just behave in accordance with total circuit resistance? ie. a 1 foot negative cable and a 20 foot positive cable is fine in that in will only affect total resistance and not cause any damage or imbalance on the batteries?

I will continue to research on my own but just wanted to repost this question for further discussion.

I think this post answers my question but fell free to re-clarify for us slower learners
https://forum.solar-electric.com/discussion/13704/battery-cable-length

• Super Moderators, Administrators Posts: 32,205 admin
For balancing "equal resistance", the equal length overall is the correct answer.

For reduction in radio frequency radiation, there are several ways of doing this.

One, equal lengths for each wire, bundled next to each other, or better twisted pair (twists per foot depends on the frequency of interest, low frequencies a twist per foot is probably OK (AM radio). For FM radio and TV, several twists per foot to much more (like 6-12 twists per foot for 1 GHz frequencies--Probably not practical for heavy power cables).

Do not Loop cables (if two cables take different paths, they act like a loop antenna). If you have extra cable (variable length needed like extension cord), "loop" the cable in figure "8" pattern (the two counter loops cancel eachother out).

Or install the cables/wiring in metal conduit... Pretty much 10 feet of conduit from source of noise (DC input to inverter, AC output of inverter, etc.) -- The metal conduit acts like a capacitor and bleeds off the excess energy... Most of the time, the rest of the cable run can be run outside of conduit (or shielded cable) to elsewhere without problems.

Regarding ferrites... There are special ones for EMI reduction. For AC lines, you can run a single wire through a ferrite core (or run several turns around the core for more coupling).

There are Two types of radiation... Common mode (the same noise signal is on both wires). And Differential mode signals (+ phase on one cable, - phase or no signal on the other cable). Common mode chokes you run the parallel wires together through the core. Differential noise, you run each wire through its own core/filter.

For DC cables (power cables), most of the time, you need to run them in pairs (+/- cable sets) through the ferrite core. If you run one cable through the core, the core will saturate (reach maximum magnetic field strength) and not function as a filter any more... Just a few amps can be enough to saturate a core. Sending +/- both through the core, the common magnetic fields cancel each other out--and the cores do not saturate).

https://www.mouser.com/new/fairrite/fair-rite-ferrite-kits/

More or less, under 30 MHz signals are carried on the cables between sources (conducted noise). And 30-180 MHz is were most cables "radiate" Radio Frequencies.

Over >180 MHz, slots in electronic enclosures tend to radiate energy (slot antenna).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 172 ✭✭
edited October 2018 #14
Whoa.... Thanks Bill. I just read that twice slowly

The first line of your response is what I was looking for but I absolutely do appreciate the in depth (from my perspective) response addressing RF radiation as that is an area of interest for me and should be for anyone. I am running most of my PV wire and 4/0 battery cables in EMT as that is code in Canada. I own a meter that tests non-ionizing RF and magnetic field strength and do what I can to keep distance from anything over 1 mW per square meter.

So just to confirm, "For balancing "equal resistance", the equal length overall is the correct answer."

Does that mean all cables equal length is preferable or does that mean with cables of paralleled battery strings, all positives the same lengths and all negatives the same lengths, but positives and negatives can be different lengths?
Sorry to beat it to death here, just want to be clear.
• Super Moderators, Administrators Posts: 32,205 admin
For DC issues, you don't care if one string is 4'+2'+2' and another string is 2'+2'+4' and a third string is 3'+3'+2'... Just also long as they all add up to the same overall (i.e., 8' in this "made up" example).

Although, it is usually easier to have equal cable lengths so that everything is built the same (and a some extras for spare parts).

Some folks believe that cross connects between parallel strings ("laddering") is useful... However, I hate it. If the battery bank is running correctly, there will be no current in the cross connects. If there is a failed cell/battery for the cross connect to bypass current through the ladder connection(s), you still have a failed cell/battery and reduced capacity. And the failed cell/battery will typically be "hidden" from easy diagnosing by the cross connects.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 172 ✭✭
Got it! Thanks Bill. Will finalize cable lengths in the morning and put in the order.
John
• Solar Expert Posts: 1,386 ✭✭✭✭
> If the battery bank is running correctly,

I'd say "if all the batteries are perfectly matched".   And they never are.

> cross connects between parallel strings ("laddering") is useful

It's hard to argue with the math, which shows that on average, it provides better balancing of series voltages.

> And the failed cell/battery will typically be "hidden" from easy diagnosing by the cross connects.

You could simply clamp an ammeter on the cross connects.

I am available for custom hardware/firmware development

• Solar Expert Posts: 3,124 ✭✭✭✭

ligwyd,

One approach which will help keep total length of battery interconnects and inverter cable lengths at a minimum,   is to layout the battery bank as a U  --  half of the batteries going away from the inverter/s  and the other half returning to a point near the inverter/s.   Of course,   ideally,   this will be a single string of appropriately-sized batteries.

In your Cargo Containers  this might be a bit difficult,   due to the relatively narrow nature of these containers.   Our power rooms  are all Cargo containers,  with U-shaped battery banks.

We DO do Hammie Radio HF Comms.   Have not needed to take additional steps to attenuate HF interference from the Inverters.   Most MPPT CCs ARE HF-noisy,   and do respond well to conventional (Common Mode)  emission attenuation approaches.  Stand-alone MPPT CCs do are not customarily tested for Line Conducted emission compliance,   and are usually quite noisy in the lower HF frequency ranges.

Just good practices for all AC wiring,   run AM mains cables in the ground,   about two feet deep (plastic conduit seems fine),   and  you should have nothing to worry about.

Even without ground-bonding cargo container doors to the rest of the shell,   they are great shields for what goes on,   inside of them.

YMMV,   have fun,   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.