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Thread: Wire gauge from battery bank to inverter?

  1. #1
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    Default Wire gauge from battery bank to inverter?

    Hello, I have a question about what sizing/gauge of wire to use going from the battery bank to the inverter. Most would probably say as big as you can go, but I have a specific requirement to meet. I would like to be able to run my toaster oven off my inverter. Here's some data of my planned system:

    PV size: 240W
    MPPT Charger: 20A
    Battery Bank: 12V 225AH (2x 6V)
    Inverter: 5000W Pure Sine Inverter (10kW peak)

    Toaster Oven: 1200W 120VAC going to be on maybe 40 min a day max.

    With the conversion of DC battery power to AC inverter power, won't my amps jump from 10A to 110.4A?

    If so, would I not need 1/0 gauge wire from the the battery to the inverter?

    The battery bank, inverter and MPPT charger will also be very close together (less than a foot of wire between each).

    Thanks for the help!

    Cheers

  2. #2
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    Quote Originally Posted by Trefmawr View Post
    Hello, I have a question about what sizing/gauge of wire to use going from the battery bank to the inverter. Most would probably say as big as you can go, but I have a specific requirement to meet. I would like to be able to run my toaster oven off my inverter. Here's some data of my planned system:

    PV size: 240W
    MPPT Charger: 20A
    Battery Bank: 12V 225AH (2x 6V)
    Inverter: 5000W Pure Sine Inverter (10kW peak)
    OK this is insane.

    PV to charge controller = #12 AWG (size smaller than a pencil, about the size of a golf tee with a 20 amp fuse)
    Charge Controller to battery #12 AWG
    Battery to Inverter is 2 pair of 750MC copper cable as big as your wrist with a 500 amp fuse.

    Does anything seem wrong with this picture?

    Your extremely small battery can only deliver 40 amps continuous to a inverter that needs 400 amps.

    If you think that sounds good all you need is a 1000 watt solar panel to supply UK with power.
    MSEE, PE

  3. #3
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    Quote Originally Posted by Trefmawr View Post
    Hello, I have a question about what sizing/gauge of wire to use going from the battery bank to the inverter. Most would probably say as big as you can go, but I have a specific requirement to meet. I would like to be able to run my toaster oven off my inverter. Here's some data of my planned system:

    PV size: 240W
    MPPT Charger: 20A
    Battery Bank: 12V 225AH (2x 6V)
    Inverter: 5000W Pure Sine Inverter (10kW peak)

    Toaster Oven: 1200W 120VAC going to be on maybe 40 min a day max.

    With the conversion of DC battery power to AC inverter power, won't my amps jump from 10A to 110.4A?

    If so, would I not need 1/0 gauge wire from the the battery to the inverter?

    The battery bank, inverter and MPPT charger will also be very close together (less than a foot of wire between each).

    Thanks for the help!

    Cheers
    Yes, that is what will happen assuming that the output voltage of the battery stays at 12 volts. But since you have only a 225 AH battery bank (AGM or FLA?) you will be drawing a current of C/2. That will pull down the battery voltage because of internal resistance and that will cause the inverter to try to pull even more amps to make up for it. If the voltage hits the Low Voltage Cutoff (LOC) setting, the inverter will shut down.

    The 240 watt panels will take several days to recharge the battery, assuming that you manage to get the full 20 amps from them. (110 amps times .67 hours = 73 amp-hours.) But because you are drawing at such a high rate Peukert's law tells us that you will have removed more than that capacity from the battery. So let's say 100 AH. That is close to a 50% Depth of Discharge (DOD), which in going to reduce the number of cycles you get from your battery and which needs to be replaced as soon as possible to prevent sulfation. At a 20 amp charge rate (which you will NOT get from 240 watts of panel even in full sun) that will take 5 solar hours. And you have to put more AH back into your batteries than you take out. And since the latter stages of the battery charging will take place at a lower current, your battery will not be back to full charge for at least two days assuming that you do not use the toaster oven again the next day.

    Now for the real question: Is this an off-grid system? If not, then trying to use solar PV for a heating appliance is always a bad idea economically. The power you get from the inverter will cost you $2.00 or more per Kwh compared to what you would pay the power company.
    If you are off grid, then you should try some other heat source than an electric toaster oven. Even a microwave would be better than that.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

  4. #4
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    Quote Originally Posted by Trefmawr View Post
    Hello, I have a question about what sizing/gauge of wire to use going from the battery bank to the inverter. Most would probably say as big as you can go, but I have a specific requirement to meet. I would like to be able to run my toaster oven off my inverter. Here's some data of my planned system:

    PV size: 240W
    MPPT Charger: 20A
    Battery Bank: 12V 225AH (2x 6V)
    Inverter: 5000W Pure Sine Inverter (10kW peak)

    Toaster Oven: 1200W 120VAC going to be on maybe 40 min a day max.

    With the conversion of DC battery power to AC inverter power, won't my amps jump from 10A to 110.4A?

    If so, would I not need 1/0 gauge wire from the the battery to the inverter?

    The battery bank, inverter and MPPT charger will also be very close together (less than a foot of wire between each).

    Thanks for the help!

    Cheers
    I'd guess <5% voltage drop across each wire at rated current would be a good starting point.

  5. #5
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    Yeah my math is pretty terrible, and I've never taken any physics before so this is all just in theory to me. At first blush it looked like it might be possible to run a toaster oven, but after it's been spelled out it doesn't seem at all possible with the restrictions I have haha!

    Here's my situation:

    Very small RV (slide in truck camper), small wood burning stove (all safety issues addressed, don't worry), 4" insulation, etc.

    I'm going to be running 12v LED lighting, and really the only other power draw will be my laptop and phone charger.

    Ideally I wanted to be able to bake some dinner every other day in the toaster oven, but that is clearly impossible with the setup I have in mind haha!

    I'll be sticking to the gas stove it seems

    Thanks a ton for clearing things up for me though!

    Cheers

  6. #6
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    Quote Originally Posted by 'NuffSaid View Post
    I'd guess <5% voltage drop across each wire at rated current would be a good starting point.
    And would you want that to be the rated demand of a 1200w toaster oven? Or the rated demand of the inverter at full 10Kw surge power, which Sunking calculated?

    And at that point it would be pointless since the battery internal resistance and the contact resistance of the connectors at the ends of each wire would be worse than the voltage drop in the wire itself.

    There is something wrong with this picture!

    Now for the loads you describe in your latest post, you could make something work with a much smaller inverter and a properly sized battery bank. But the chances are very good that you would do better using your tow vehicle alternator (or a small generator) to charge the batteries instead of solar.
    If you will not be able to run the tow vehicle or a generator, then it will make sense to look at exactly what PV will require and cost.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

  7. #7
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    Assume initial contact resistances to be much less than other system impedances and let's go with the nominal current each wire will be carrying as a first cut.

    Since the OP asked, can anyone advise on rules of thumb for PV system-level wiring spec's?
    THX.

  8. #8
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    Quote Originally Posted by Trefmawr View Post
    Ideally I wanted to be able to bake some dinner every other day in the toaster oven, but that is clearly impossible with the setup I have in mind haha!
    Do not give up so easily without a fight or else us Yanks will call you a BITC* instead of BRIT*H
    You can have some of what you want easily with what you propose, just give up the idea of operating 5000 wat inverter off a 12 volt battery period and make a few adjustments. For example.


    • Instead of golf cart flooded lead acid batteries use AGM batteries which can deliver much higher currents and do not spill acid in the RV
    • Down size the inverter to something a 12 volt system can handle to power things like TV, DVD, charge a cell phone, laptop, toys for mum, run lights, etc..
    • Use something like a 12 volt RV microwave for 10 to 15 minutes per day to heat stuff up.
    • Incorporate your vehicle alternator to help the charging of the batteries


    Come on my Brit friend, get a stiff upper lip and grow some brass and ask questions.
    MSEE, PE

  9. #9
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    Haha alright, alright. Although I do have to apologize for any misconception that I am from Britain. Just one of your friendly northern neighbours

    As it is, I was most interested in having my toaster oven in my camper because A) I already own my toaster oven, and B) I love me some oven baked dinner!

    The really big limiter in my situation though is weight. I've only got a half ton truck and my camper weighs about 1100 lb dry (no liquids or gear) which leaves me around 700 lb for the rest of my crap (not including my 190lb fat behind ;D). Of that 700 lb, I've slotted 150lb for battery weight. I'll still have around 500lb of "emergency" weight until I hit my GVWR, but I don't want to come close to that with batteries.

    I'd also be interested in running a compact fridge (115V 1.0A, cycles around 8 min of run time an hour).

    I don't necessarily have to go with 2 6v batteries, it's just they seemed to be the best AHr/dollar.

    So what type of inverter should I be looking at? You said something smaller and made for 12v? Something around a 1000w PSW?

    Thanks again!

  10. #10
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    Quote Originally Posted by Trefmawr View Post
    Haha alright, alright. Although I do have to apologize for any misconception that I am from Britain. Just one of your friendly northern neighbours
    I guess it was the CHEERS greetings that threw me off. Can't say a Yankee is much better than a Brit though

    Let me throw some ideas at you and see what you think about it.

    Would you be willing to run your truck motor while the toaster oven is operating?

    If so you have a possible work around.

    Can you tell me what the wattage of the toaster oven is?

    Would you be willing to buy a smaller toaster oven?

    Have you considered a propane refrigerator?

    Give some feedback to those questions and I think we can come up with some affordable solutions for you.
    MSEE, PE

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