Battery to Inverter Wire Size

Joe J
Joe J Solar Expert Posts: 49 ✭✭
in Off Grid Solar & Battery Systems #1
So I'm running my 4- 6 volt batteries in series than parallel to give me 12 volts at 440 amp hours. I connected them using 4/0 wire. Going out from the batteries to the inverter I was going to use 2/0 wire with a 250 amp fuse. It's a 6 foot run. My xantrex 12 volt inverter manual said they recommend to use "0" wire for low line loss (I went bigger). It said to use a 250 amp fuse. So here is my question. Is even a 2/0 wire big enough for a 250 amp fuse? I like the idea of 2/0 wire (easy to handle Vs the 4/0) Does this sound right?? What size wire is neccesary? Can a 2/0 wire handle the 440 amp battery bank? Confused:confused:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Battery to Inverter Wire Size

    What matters is the current, not the battery bank size. 2/0 can handle up to 275 Amps (you will see different ratings depending on application).
    How big is the inverter? Roughly 2kW judging by the recommended fuse size. That will draw about 167 Amps under most circumstances, up to 200 if the Voltage is allowed to go down to 10, and momentary surge above 200. Why so many different current numbers? Because of the variable input Voltage, adjustments in Ampacity rating for application, NEC fuse size calculation, et cetera.

    So rather than try to learn all this stuff in one go (like trying to swallow the whole bottle of aspirin at once) accept that the people who made the inverter already know it. If they recommend a 250 Amp fuse, use one. If they recommend a minimum 0 AWG wire, go with that or larger. Don't go with a larger fuse size and don't expect to see 200 Amps coursing through the wires either. :D
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #3
    Re: Battery to Inverter Wire Size

    The Xantrex Pro watt 2000 manual is not very clear. It tells you can use 4 AWG battery cables as the smallest size. They recommend "0". They were talking more for voltage drop. My concern was that the wire would melt before the the 250 amp breaker would trip if a dead short should happen. That's why I went with a 2/0 gauge wire .But later looking at some information on a/c house panel wire size it said 2/0 wire was good to 200 amps only. This concerned me. So if I got this right....Your saying that the draw will never be 275 amps. Correct? And that for a 6 foot run the 2/0 wire should handle up to 275 amps with this application. Correct? Let me also say the inverter can surge to 3000 watts. That's why the 250 amp breaker.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Battery to Inverter Wire Size

    It's a difference of continuous current vs. surge current. A wire that's good for 200 Amps continuous may take 300 Amps momentarily. The fuse that's good for 250 Amps continuous should blow before the wire does. 275 Amps @ 10 Volts is 2750 Watts, so it is highly unlikely this condition will ever come up. If you look at two different wire charts you could see two different Amp ratings for the same size wire. What's worse, the type of insulation and how it is installed has an affect on its current carrying ability.

    Somewhere on the forum this conversation has taken place before, as it is a common issue. But the mention of the Prowatt's manual being a bit vague rings a bell. I think you will be fine with the 2/0 wire and a 250 Amp fuse.
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #5
    Re: Battery to Inverter Wire Size

    Thanks a lot Cariboocoot. I understand now. You really are a SUPER MODERATOR! Below is a fast drawing I made up of my system. See anything out of the way? Does not show grounding at this point. Just a rough drawing.

    Attachment not found.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Battery to Inverter Wire Size

    joej,
    your concerns for the wiring is founded. i use #2/0 on my inverter and it's a 1kw inverter. at 1000w for 12v this is 1000w/12v=83.3a continuous and i have a 125a cb. this allows surges up to that point of 125a.
    by the chart here,
    http://www.armstrongssupply.com/wire_chart.htm
    at 125a i should use a minimum of #1/0. anything better than that is good for voltage drops.
    now you are at 2kw and double the currents. this is 166.7a continuous and as 250a cb. what that says is to you would need #350 sized wire or parallel 2 #1 wires minimally. you are using #2/0 so you should match that. many would suggest to put a cb on each wire for 125a in case one wire should break or have a bad connection.

    the alternative to all of this is to limit yourself to say 150a and use a 150a cb. technically it should be #3/0 as #2/0 is good to 145a. at 150a this is 1800w.

    it becomes too difficult to handle current ratings of inverters up around 2kw because of the wire sizes involved, but if you don't have to have 2kw or surges up around 3kw you can cut back on the cb size to accommodate wires up to #4/0 which are more common.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Battery to Inverter Wire Size

    Okay, let's try answering again only this time without the blue screen of death coming up and wiping everything out! :grr

    At the top it looks like you are expecting 60 Amps from the panels. The panel current in this configuration will be 2X Imp. This is not the same as the output current from the (MPPT) charge controller. As such you probably do not need 8 AWG wire to go 15 feet from array to controller, as few panels have an Imp of 25 (8 AWG can handle 50 Amps of current).

    870 Watts on a 12 Volt system will produce just under 60 Amps peak current (depending on efficiency). You've size the controller's fuse for full 60 Amp output according to NEC rules which is fine. The 2 AWG wire probably won't fit in the controller's terminals; 6 AWG is usually the max they will take and is all that is necessary.

    The battery wiring has one flaw: you have two parallel strings; you need to connect the positive of the inverter and controller to one string and the negative to the other string (diagonal wiring). This will help keep the current flow even between both strings.
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #8
    Re: Battery to Inverter Wire Size
    Cariboocoot wrote: »
    The battery wiring has one flaw: you have two parallel strings; you need to connect the positive of the inverter and controller to one string and the negative to the other string (diagonal wiring). This will help keep the current flow even between both strings.

    I'm not getting that. I'm not sure what you mean and how to do it. Can you fix it on my drawing???
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #9
    Re: Battery to Inverter Wire Size

    Hi Niel,

    I'm really a worry wort. But better safe than sorry........Thanks for chiming in.

    Joe
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Battery to Inverter Wire Size

    Try this link, Joe: http://www.smartgauge.co.uk/batt_con.html
    Look at method #2. Instead of four single batteries in parallel you have two (strings) but the principle is the same.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #11
    Re: Battery to Inverter Wire Size

    and here is another diagram: http://forum.solar-electric.com/showthread.php?17203

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #12
    Re: Battery to Inverter Wire Size
    Cariboocoot wrote: »
    The 2 AWG wire probably won't fit in the controller's terminals; 6 AWG is usually the max they will take and is all that is necessary.

    This seems like a design flaw. A #6 wire is not rated for 100 amps. More like 75 amps. Number 4 wire is rated 95 amps or so. Now I know in my situation I won't come close to this but someone else could. So in fact why even use a 100 amp fuse at all with a # 6 wire? Doesn't make sense. You have no protection. I see a lot of people in the solar world using fuses rated for way more than their wire can handle. In the ac world this should not pass code. I understand what you told me about current draw and for my situation I will be well below that but still don't make it correct, with all due respect. I base this on the potential of the wire and breaker not what may or may not be drawing at the time.

    Cariboocoot wrote: »
    The battery wiring has one flaw: you have two parallel strings; you need to connect the positive of the inverter and controller to one string and the negative to the other string (diagonal wiring). This will help keep the current flow even between both strings.

    Ok. I think I understand. How does this look? I'm also going to use 4/0 from the inverter to the battery bank.

    Attachment not found.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #13
    Re: Battery to Inverter Wire Size

    Put the fuses/breaker "near the battery terminal" for best protection against short circuits (don't want a short to your heavy wire before the fuse).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Battery to Inverter Wire Size

    That's the right change on the battery hook-up. :D

    The thing with the controller wiring is the difference between the continuous (operating) current (which the 6 AWG will handle because it is 60 Amps) and damaging amount of sustain over-current which should trip the breaker/blow the fuse. 6 AWG will actually handle 100 Amps long enough for the circuit protection to trip first. If you feel nervous about this you can put in a smaller fuse (80 Amp) than required but never a larger one.

    I should mention that not all charge controllers are limited to 6 AWG: the MidNite Classic for example has DC terminals capable of 4 AWG (and it has output up to 96 Amps depending on model/system Voltage).
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #15
    Re: Battery to Inverter Wire Size

    A couple things, First the fusing is a straight calculation;

    Peak wattage of the inverter / minimum voltage to run = Amps to protect

    So for you 3000 watts / 11.5 Min voltage = @260 Amps

    On the wiring, 4/0 to the invereter is Ok due to minimum time the inverter will allow max wattage, The wire between the batteries will never carry much more than 1/2 the load and over a short distance, so they could be a gauge smaller.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #16
    Re: Battery to Inverter Wire Size

    And in case anyone is casually reading this thread, anyone who runs a 2kW 12 Volt inverter near its limit continuously has picked the wrong size inverter and the wrong system Voltage.

    Most of the time this inverter will be operating at less than half its capacity and so the constant-current will be far below the potential maximum making the Amperage ratings of wiring and fuse much less of an issue.

    With this set-up (as we have seen in the past with the exact inverter) the bigger wiring issue is to be sure it's sized sufficiently to keep the Voltage drop minimal in the event that the power demands are high (momentary surge demands as for starting a motor).

    Again, a wire or fuse/breaker rated for 'X' Amps will take higher current than that for a short period of time. This is how it is supposed to work.
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #17
    Re: Battery to Inverter Wire Size
    Cariboocoot wrote: »
    That's the right change on the battery hook-up.
    I should mention that not all charge controllers are limited to 6 AWG: the MidNite Classic for example has DC terminals capable of 4 AWG (and it has output up to 96 Amps depending on model/system Voltage).

    I do have the Midnite Classic ordered from this site:-) So I guess that 4 wire is close enough to work with that 100 amp fuse.

    I'm glad my battery bank is wired correct now. Much thanks! I'll post some pictures in the "Show Me Yours" sticky when finished.

    Joe
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #18
    Re: Battery to Inverter Wire Size
    Cariboocoot wrote: »
    the MidNite Classic for example has DC terminals capable of 4 AWG (and it has output up to 96 Amps depending on model/system Voltage)

    While the Classic will accept 4 AWG THHN, it takes painstaking care to get it in there. The Classic also only has holes for 1" conduit. The maximum number of 4 AWG THHN wires that will fit in 1" conduit is four. So if you want to wire your Classics with a "raceway" in the bottom for the wiring like this you would not believe how much beer it takes to get 4 AWG THHN to go around corners and into those little terminal blocks. :blush:
    --
    Chris
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #19
    Re: Battery to Inverter Wire Size

    And that, boys and girls, is the downside of a neat installation. :p
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #20
    Re: Battery to Inverter Wire Size

    Chris, is that a vote in favour of a 'not-a-gutter'?
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #21
    Re: Battery to Inverter Wire Size
    westbranch wrote: »
    Chris, is that a vote in favour of a 'not-a-gutter'?

    Well, kind of. But my way was ~$600 cheaper (not counting the beer expense) ;)
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #22
    Re: Battery to Inverter Wire Size
    ChrisOlson wrote: »
    While the Classic will accept 4 AWG THHN, it takes painstaking care to get it in there. The Classic also only has holes for 1" conduit. The maximum number of 4 AWG THHN wires that will fit in 1" conduit is four. So if you want to wire your Classics with a "raceway" in the bottom for the wiring like this you would not believe how much beer it takes to get 4 AWG THHN to go around corners and into those little terminal blocks. :blush:
    --
    Chris

    LOL - Yeah, I recently needed a few beers to get through my system addition. Added the Classic on the right for my second Array. A fair amount of blood, sweat and beer later and yes, 4 AWG x2, 3 AWG x1, 10 AWG x1, a network cable and a temp probe cable will all fit through 1 inch flex conduit.:roll:

    And I must say- Midnite's engineers deserve credit for building the Classics terminal block mounting to withstand the abuse of an unskilled amateur electricians hands.

    Attachment not found.
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #23
    Re: Battery to Inverter Wire Size
    mtdoc wrote: »
    4 AWG x2, 3 AWG x1, 10 AWG x1, a network cable and a temp probe cable will all fit through 1 inch flex conduit.:roll:

    LOL! I think I'll have a beer to admire that accomplishment! :cool:

    After fighting with four 4 AWG conductors in 1" rigid conduit I said to heck with it and ran the USB cables, network cables and batt temp sensor wires out the wire clamps on the bottom.
    --
    Chris
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #24
    Re: Battery to Inverter Wire Size
    ChrisOlson wrote: »
    LOL! I think I'll have a beer to admire that accomplishment! :cool:

    I'll join you. Here's to bellies full of beer and conduit full of copper! Cheers.;)
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #25
    Re: Battery to Inverter Wire Size

    This was my best conduit accomplishment. When I built my bus I ran all the incoming power in the left side and the 4/0's to the inverters went out the right - all in 2 1/2" rigid conduit. Well, turns out I could've used 3" on the incoming side. I put the Classic breakers inside the bus panel (especially for wind turbines so they can't be inadvertently turned off when the turbines are running in 30 mph winds).

    But the time I got dual 2/0 feeds from the bank in there, plus four 4 AWG's for the Classics with the positives going to the breakers and the negatives going to a 500 amp shunt on the negative bus bar, it was about all that would fit thru that 2 1/2" hole

    Attachment not found.

    Attachment not found.

    And then I got it all in there and it suddenly hit me - our furnace controls, and all the relays for the wind turbine voltage clippers are 24 VDC and they have to go thru that input side too, and they have to be fused at the bus. So I managed to snaggle some 14 gauge extension cord wire thru there for that.

    And then I added two more Classic 150's later so I ended up hole sawing another hole in the top of the cover of the 2 1/2" 90 and got four more 4 AWG THHN wires thru there. In the end, the outlet coming out of the bottom of my generator panel got buried under a mass of conduit and is totally useless. It's there, but I can't plug a cord into it :blush:
    --
    Chris
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #26
    Re: Battery to Inverter Wire Size

    Gosh you guys are scaring me. I thought I was going to run 2 classics on the side of my E-Panel with through wiring the 2 with breakers in the E-Panel...

    Heck, I don't know what to do, I don't even drink beer...

    ... or is that a lubricant?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
    #27
    Re: Battery to Inverter Wire Size

    Beer gives you insight on why your original "vision" you had of how it was supposed to be is not working out ;)
    --
    Chris
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #28
    Re: Battery to Inverter Wire Size
    BB. wrote: »
    Put the fuses/breaker "near the battery terminal" for best protection against short circuits (don't want a short to your heavy wire before the fuse).

    Yes, I will do that. So lets take that a step further. The 100 amp fuse to the controller. Since the fuses are really protecting the wire is it better to put that fuse close to the battery bank as well to protect from the battery backfeeding to the controller?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #29
    Re: Battery to Inverter Wire Size

    For most wiring in an off grid power system--The battery is the source of "unlimited" current--So all fuses/breakers should be "close to" the battery/bus bar connections.

    The wiring+fuses are otherwise are rated 1.25x larger than the charging source or loads (standard breakers and fuses should trip if operated at 100% of rated load).

    So, if your Midnite controller is capable of 96 amp output, then the fuse+wiring should be capable of:

    96 amps * 1.25 NEC derating factor = 120 amp minimum

    Note--MPPT charge controllers are capable of regulating/limiting their output current... So using their rated output for wire sizing is appropriate. For smaller arrays/PWM controllers, NEC requires an extra 1.25x factor of derating of the array (always read the installation manuals--They do take some of the extra 1.25x deratings into account when they rate their controllers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Joe J
    Joe J Solar Expert Posts: 49 ✭✭
    #30
    Re: Battery to Inverter Wire Size
    BB. wrote: »
    So, if your Midnite controller is capable of 96 amp output, then the fuse+wiring should be capable of:

    96 amps * 1.25 NEC derating factor = 120 amp minimum

    By the letter of the law.....This is not possible with this controller. I have read that #4 wire is almost impossible to use with this controller most use #6. So let's say you get #4 to work. It's rated for 95 amps. 120 amps, It won't do. Forget about # 6 wire. Now I know #4 can take more than that on a surge for a short time but still in my opinion it's a flaw in the design using a 100 amp fuse with #4 wire.

    Thanks for answering my question. So I'm going to put my controller fuse close to the batery bank as well.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #31
    Re: Battery to Inverter Wire Size

    Yep--95 Amps is about the maximum current allowed by NEC for higher temp rated insulations and copper wire.

    So, 95 amps * 0.8 NEC derating = 76 amps maximum (per code). The wire gauge should be closer to #2 or larger.

    But if you use Boating wiring tables, 4 awg is good for 160 amps maximum...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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