DC Cable Size For Solar Battery Bank
ALTERNATIVE ENERGY

Selecting The Right Cable Size For Your Battery Bank

Are you putting together a battery bank for an alternative energy system?

If you are, then you might be thinking about the size of interconnecting cables between the batteries themselves and the charger / inverter.

How big should the cables be? What size? More accurately, what gauge (AWG)?

Read on… or go straight to the chart (gauge versus amps) at bottom of page.

I highly recommend getting yourself a clamp meter that measures DC current! This will help verify what’s actually going through your cables! The most popular and reasonably priced clamp meter is this one (which I own)…

>> Meterk Digital Clamp Meter
(view on amzn)

The Battery Bank

A battery bank for an Off grid solar powered alternative energy system will consist of a number of batteries and their interconnecting terminal cables.

The batteries will be connected together in various series-parallel configurations to achieve a desired voltage and capacity to work best with the inverter (and charger).

The batteries will serve as a energy storage center to deliver power during periods when the solar panels themselves are either in the dark or under-delivering due to weather conditions.

I’m not going to get into recommendations for series-parallel battery wiring (another article). However I will cite my own current configuration as an example:

All 24 batteries are 12 volt 100 AH rated. They are configured in groups of 4 in series, making 48 volt strings. 6 strings are connected in parallel to comprise a battery bank of 48 volts @ 600 AH. That’s a total capacity of 28.8 KwH, although 14.4 KwH is safely usable (50% max). That said, my rule of thumb is to avoid draining more than 30% off the top for lead acid batteries…so that gives me just about 10 KwH to play with.

This, combined with almost 4,000 watts of PV panels has been enough to supply basic power and energy needs within my modest home. My electrical needs are low to begin with, but I do have to watch it and reduce loads especially during the Fall and Winter months, but that’s part of the fun ;) Unless your system is enormous (and enormously expensive) you will have to keep an eye on things anyway…

I have integrated the ability to balance (select) my individual loads to be sourced from either ‘grid’ or ‘solar/batteries/inverter’. I’ve installed a few boxes (Reliance Controls) filled with transfer switches for each individual circuit breaker from the mains panel to accommodate this. It makes the system entirely 100% flexible for balancing loads and demands throughout the year.

DC Cable Size Between Connections

How big should the cables be?

First you will need to calculate the maximum current that may potentially flow through the various interconnecting cables before you choose the proper cable size.

Cables must be sized to carry the maximum expected load. Undersized cables may result in overheating, melted connections, and even become a potential fire hazard.

There are three maximum DC current specifications to figure out:

1. DC Charger charging current.
2. DC Load demand from the inverter.
3. DC current flow within the battery bank interconnects.

Battery Charger – Max Current

My solar battery charger is the ‘Midnite Solar Classic 200’.

According to its specifications, the maximum charge that it can put to the battery bank at 48 volts is 74 amps (~ 3500 watts).

Inverter – Max Battery Current

My inverter is an Outback VFX 3648.

According to the specifications, the maximum overcurrent ampacity is 175 amps and they call for a OBCD-175 breaker on the DC side. Evidently there’s a built-in surge handling capability on the DC side to 175 amps. Therefore the cables between the battery bank and inverter must be sized for at least 175 amps.

Max Current Within Battery Bank

You will need to determine how many amps will flow through interconnecting cables of the battery bank. This involves some basic understanding of series-parallel wiring and how it affects volts and amps. I will assume you have that basic understanding…

First, we know that in my example there may be up to 175 amps pulled from the battery bank (worst case scenario). However I have 6 strings of batteries in parallel, so only one sixth of the maximum will flow through any one string as the strings share the load. So that works out to be just about 30 amps flowing through each of the six strings.

So technically I only need at least 30 amp rated cable throughout the series strings of batteries (6 groups of 4 batteries in series).

That said, I try not to live on the edge, and always oversize with a wide margin where I can.

Actually, for my current installation I’ve sized all of my DC cables to meet the 175 amp worst case scenario. So all interconnecting cables are 2/0 based on the chart below.

Cable Size Ampacity Chart

The following maximum amps versus cable size (AWG) come from the NEC version 2011. As far as I know these values are valid as of today. For more detail though, check with the National Electrical Code as well as your own zoning laws.

– Copper Conductor Ampacity based on 75°C (167°F) reference
– Cable Types: RHW, THHW, THW, THWN, XHHW, USE, ZW

Size (AWG) — Ampacity

14 – 20
12 – 25
10 – 35
8 – 50
6 – 65
4 – 85
3 – 100
2 – 115
1 – 130
1/0 – 150
2/0 – 175
3/0 – 200
4/0 – 230

 
Related:
Battery State of Charge Chart
The Four Essentials of Off Grid Solar
IronEdison.com | Nickel Iron Battery For Off Grid Energy Storage

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27 Comments

  1. @ Ken
    Let this electrical engineer say “job well done explaining to others about wire sizing for amperage capacity”.

    However, with 6 strings of batteries in parrallel let me ask if your charge controller (system) ever have problems keeping the specific gravity of the electrolyte of some of your batteries at the correct level? I have 4 strings in parrallel and to keep the electrolyte specific gravity correct I have to equalize once a month.

    I have 3 Outback 8048 inverters in parrallel output and under max power(24 kwwatts) the max cable amperage is over 500 amps. Now, that requires some serious cabling.

    1. Texas boy,

      My current battery bank is made up of AGM’s, and they cannot be EQ’d. They supposedly resist sulfation fairly well so long as they don’t sit for long periods of time under-charged.

      What inspired me to write this today was a half-day grid outage a few days ago during a horrendous windstorm. It got me to looking at my off-grid system and battery bank a little more closely. I’ve decided that it’s time to check each individual battery no-load voltage (after a 24 hour rest) to discover whether or not they’re within acceptable range of each other. Hopefully all batteries are still ‘good’ (haven’t done it yet).

      Also of note regarding the paralleled strings: I kept all of the paralleled interconnecting cables the same length to ensure the same voltage potential (due to cable resistance) for each string. Hopefully that has helped with keeping all the batteries in line with each other…

      A previous system at a previous home utilized T-105 batteries, and I had to EQ them once a month for maintenance (and check the water levels). The AGM’s are maintenance free. My next battery bank however may be something different…

      By the way, I know I’ve mentioned it before, but your system sure does pack a punch ;)

        1. Chuck,

          The proper way to keep batteries conditioned is to keep them charged properly according to specs. For those who have lead acid batteries (not the AGM type or Gel type) it is advised to ‘equalize’ (EQ) the batteries on occasion to minimize sulfation. This may be once a month or longer depending on your battery mfgr. recommendations (and your usage).

          Any good charge controller will have the ability to EQ.

          Personally I wouldn’t trust a relatively cheap “conditioner” on a battery bank worth so much money. I’m not saying though that the product referenced doesn’t work, rather simply pointing out that proper charging techniques is the right way to keep batteries in their best condition.

  2. Very nice looking installation! Neatness counts…
    One other thing to consider is to keep the low voltage cables as short as reasonably possible in order to minimize loss in the cables themselves. Appropriate sizing of the cables as described in the article will also minimize these losses.

    1. Very good point. Cable resistance matters!. The fatter the conductor, the less resistance. And shorter is better (did I really just say that?).

      1. As a short guy I appreciated the comment. When they were passing out height as was back in line getting seconds on……………….. brains.

    2. I’m looking at installing a system, and my batteries would be located farther away than ideal. Is there a way to calculate the need for larger cable size due to distance from battery to controller?

  3. @Texas boy,

    Let me ask you a question regarding your Outback inverters.
    How are they with EMI/RFI interference?

    Unfortunately my own Outback inverter delivers lots of interference such that AM radio listening (for example) is full of buzz. I’ve fussed around with grounds, I’ve put ferrite rings all over the place, but nothing helps. Just the way it is with this inverter I suppose (and many others by what I read).

    If I upgrade my system in the future, this will be an area that I will explore. My research so far reveals that the SMA inverters are supposed to be very EMI/RFI quiet.

    1. Ken…
      Thanks for this comment. Don’t have a set up like this, but some day would like to. It’s good to know some of the more “unknown” problems which need to be checked in to.

    2. All of my electronics are in a faraday cage (metal building) which protects the electronics from an EMP and fortunately due to Gausses law no electromagnetic energy can escape either. Without the metal building enclosing the electronics anyone looking for my inverters could find them due to the radiated energy. I took care of over heating issues also. In the event of some national emergency the federal government will require all devices that radiate energy to be turned off. Why so is for another article and there is a good reason.

      1. Texas boy,
        Ok, you’ve piqued my interest.

        ” In the event of some national emergency the federal government will require all devices that radiate energy to be turned off. Why so is for another article and there is a good reason.”

        Assuming there is “some national emergency” that will require all devices that radiate energy to be turned off, how in the world could they expect compliance? I know they could shut down the grid, but to get 100% compliance would require folks like Ken to be scared sh&tless of not complying.

        A hint maybe of what the “good reason” might be? I’m not being critical of your post, I’m genuinely interested in the answer. Fear of interference with radiation detection equipment while searching for a nuke device on home soil?

  4. Ken;
    Have an observation and a comment.

    Running 4 batteries in ‘series’ or ‘strings’ in parallel is very common, and well used through the industry, as with most you probably have the ‘string’ connected to a “buss Bar” than on to the inverter/charge-controller.

    Each string has a capacity of 600 AH and you have an Inverter that has a max demand of 75AMPs which will normally pull from the bank of 6 strings of batteries, correct so far?

    Now technically one does not have to ‘wire’ each ‘string/series of batteries with the 2/0 wire as you did, a simple 8ga wire would/should do.

    BUT, and there is always a but, what-if, yes those fancy words, what-if one or more of the ‘strings’ of batteries goes offline or fails due to a flaky connection, a battery fails or just cause. That happening would put that 175AMP draw on the remaining ‘strings’ of batteries and subsequently a larger load on the wiring. So rather than pulling 30 amps per string you could increases that by 1/6 fold for each failed ‘string’ up to and including the full 175 amps or 100% load on just one string, however so unlikely the possibility is there. Pulling the 175amps on a single 8ga wire and you have a very serious problem.

    The reasoning for this 260 word dissertation is never use a smaller gage wire that the largest load possible in the configuration, in this instance the full load of 175amps on the battery bank As you did with running 2/0 wire for the system.

    Just my 2¢ worth.

    1. NRP,

      Just a correction from your original opening statement:

      “Each string has a capacity of 600 AH and you have an Inverter that has a max demand of 75AMPs which will normally pull from the bank of 6 strings of batteries, correct so far?”

      Should read:

      “Each string has a capacity of 100 AH while the paralleled group of six strings has a capacity of 600 AH. And you have an Inverter that has a max demand of 175 AMPs which will normally pull from the bank of 6 paralleled strings of batteries, correct so far?”

      The rest of your point is very well advised!

      Additionally, I have inline fuses on EVERY string.

      1. Ken;
        Thanks, I saw that 3 seconds after I posted it……

        Fusing each string is a GREAT idea.

  5. Never forget that with a large battery bank (or even a small one) you are dealing with a lot of current. You could weld with these and people do. One mistake, as simple as using a wrench to tighten or loosen the positive connection but inadvertently touching it to ground will do considerable damage to equipment and people.

    1. (One mistake, as simple as using a wrench to tighten or loosen the positive connection but inadvertently touching it to ground will do considerable damage to equipment and people.)

      Always remove the neg wire first. Less chance of sparks if you ground out on the metal frame.

  6. Great article Ken! I have been piecing together a small solar system the last couple years. It started as a way to power a backyard shed but is evolving into something to keep a fridge, freezer, and a few other possible intermittent loads such as radio gear, TV, fans, lights, etc. It seems that everytime I get to a capacity I’m happy with, I want more. Looking back now, I wish I’d have just gone all in at the beginning. In future articles I’d really appreciate some links to something that could steer a beginner in solar systems in the right direction. Looking back, I really wish I had gone with a 48 volt system instead of 12 volt. I guess really all I’d have to do is rewire the panels and batteries and get a new inverter and charge controller and eat my losses and learn from my mistakes.

    1. Texasprepper the higher voltage panels will give you more power when using an MPPT charge controller.

      I plan on changing (re-wiring) my motor-homes 12-volt panels to 24-volt as soon as it gets warm and if it ever quits raining. All the rain we have been getting this Spring sucks. It’s keeping me from doing much outside work. .

      MPPT controllers make much better use of higher voltage panels and can in effect give you more amps to the batteries then the panels put out.

      PS: Note to everyone be very careful when buying MPPT solar charge controllers on Amazon or E-Bay. Many of them are China-made and MPPT controllers in name only. There are a lot of the lower priced PMW controllers labeled as MPPT controllers. I good MPPT controller cost more (My Morningstar controller cost $550.00) A $50.00 priced controller is not likely an MPPT controller.

      MPPT controllers give you a boost of amps above a regular controller. Basically it makes better use of the suns power. It’s like having more panels then you have. In the long run it’s well worth having the better controller.

      1. Should have added this to the above post.

        The difference in how a PMW and MPPT charge controllers work.

        Lets say you have a 12-volt 100 watt solar panel.

        And while the panel is 12-volts it actually puts out 18 to 22 volts at about 7-amps.

        PMW controllers read the power from the solar panels and send about 13-volts and all the 7-amps to the batteries. It only uses 13 of the 18 to 22 volts the panel puts out and ignores any more then the 13 volts.

        An MPPT controller will also send 13-volts to the battery, but it also converts the extra volts to amps so you get more amps into the battery so the battery charges faster.

        MPPT controllers have more parts and more expensive parts in them so they cost more. But you are getting free charging amps that the PMW controllers don’t give you.

        If you can wire 2 12-volt solar panels in series to get 24-volts the MPPT controller will do an even better job of converting all that extra voltage into real amps to charge your batteries.

        My Morningstar controller can run up to 60-volts of panels to charge the 12-volt batteries I have. I wish I knew this last year when I first put them on the motor-home. But a bit of re-wiring will fix it.

  7. My motor-home has 4 L-16E batteries, 550 watts of solar panels, a Morningstar MPPT charge controller with 4 ga wiring ran from the batteries to a fuse box inside the home. From there I use smaller cable 8 to 10 ga to run various things in the motor-home. I’m running 10 ga wire from the panels to the charge controller and batteries, not a super big wire but then the panels don’t put out a lot of amps at any one time.

    So far it has been able to supply all the power needs. But I don’t run too many things that need a lot of amps. My Ham radio station draws the most and that’s no more then 20-watts at any one time. The 100-watt HF radio (a Kenwood TS-50) draws the most and it’s only using 15-watts or so. All the other radios draw 10-amps or less when transmitting. I can only transmit with one radio at a time so no overload problem. I have to figure out how to get all the antennas put on it without making the roof look like a porcupine.

    I plan on getting a 4,000 watt pure sign wave inverter this summer as microwaves don’t like modified wave inverters. That’s going to be the biggest draw of amps and I will run a large cable to it. I also want it to run the refrigerator. The motor-hoe had a bad refrigerator and Camping World wanted $3,100.00 for a new aqua-ammonia one. I went to Home Depot and bought one the same size (59-inches tall and 2-ft wide) for $380.00 but it’s a normal refrigerator that runs in 110 AC, not propane like the old one. I want to make sure it will run from the batteries / inverter.

    Want to get more solar panels at some point but so far the ones I have keep up.

    The motor-home makes a good off-grid setup for use as a possible SHTF situation. While I have it plugged into 110-volts right now I want it to be able to run off-grid as much as possible. This Summer I want to complete it’s conversion to an all off-grid thing. I’m having fun rebuilding the motor-home, I would like to do a bit of traveling with it when it’s all done.

    It’s a 1984 motor-home and was using auto light bulbs for all inside lighting. I checked a few bulbs and they draw 2-amps each. I replaced all the bulbs with LED bulbs (that cost me close to $400.00) and they draw a lot less. T tested the LED bulbs and they draw 1/20th to 1/10 of an amp. That means 20 LED bulbs draw as much as 1 old 12-volt light bulb. And the LED ones are much brighter then the old bulbs. LED auto bulbs are expensive ($10.00 to $15.00 each) but for off-grid solar they are the way to go.

  8. I am enjoying this thread but my 2 cents worth is from the scavenger POV. While I am aware of the benefits of 24 or even higher voltage set ups the VAST majority of the post SHTF scavenging materials will be from 12 volt vehicles. Yes I have to use heaver gauge wiring and shorter runs to keep resistance low enough but I can easily use almost anything I salvage from a vehicle in some way. LED Headlights for your house lights for example.

    IF EMP kills off all the LED’s and such I HOPE you have Faraday Caged back ups for all your MPPT, PWM and inverters as all exposed units are likely to be damaged also. Thus my scavenger idea that even a primitive 12 volt DC system is better than nothing.

    1. Hi. Bek here. Novice. What would be good faraday caging for such an event? Do you need sheet metal or will heavy mesh work? And I presume not grounded? Also was wondering how the RF/ EMF copper fabric works at reducing Such signals. As I recall it is recommended to ground that. But then I assume it would not work as a faraday cage? What about lightening strikes? Thanks

      1. Welcome Bek
        . . . .
        Top of the menu bar there’s a search feature. Type in Faraday and quite a number of articles will load. Lots of good information here for preparedness lifestylers.

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