Wire lenght/inverter question

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cizzi
cizzi Solar Expert Posts: 109 ✭✭✭✭
I want to know if I can use an existing wiring for a new purpose..

I have a 2/12 AWG wire of about 25 feet from my battery bank to its destination..can I put a 200W inverter at the end of the wire and use about 50W of power from it? It would not be used for more than that.. i ask because i read distance between inverters and battery should be short as possible but given the low power requirements and the gauge of wire already present in the walls would this be safe? thanks

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  • Ecnerwal
    Ecnerwal Solar Expert Posts: 101 ✭✭
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    Re: Wire lenght/inverter question

    I'll assume 12V, since you didn't say.

    200W at 12V is 16.7 amps - got to figure that someone might plug something else into the inverter, one of these days - you say not, but once something is there and wired, future use is likely to become uncontrolled (if only because you sell the house, or something).

    While 12 gauge will carry the amperage OK (wire won't catch on fire), you'll get about 13% voltage drop (1.6V) if it's 12V.

    50 W is 4.1 amps - call it 5 since inverters are not 100% efficient (should have done same above, but never mind that.)

    That gives a more acceptable (but not good) drop of 4.2% or about half a volt. 10 gauge would be about minimum to have an acceptably low drop (assuming 12V, remember) at this distance, even for just 50 W load.

    Planning for possible future use to capacity, I'd run 4 gauge, but I like things done right, once, rather than done over. 6 gauge might _just_ work.

    This is one reason that larger systems veer away from 12V - the amperage required to get much power is huge, and small voltage drops are a large percentage of overall system voltage.

    <Edit - add>

    Of course, if the 12Ga wire is rated for 120V, you could place the inverter at the battery bank and run the 120V down the wire...which would have a lot lower loss, and 12Ga would be nicely oversized for even the maximum load the inverter could run.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,523 admin
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    Re: Wire lenght/inverter question

    This is where the Voltage Drop Calculator comes in handy... You can just type in the voltage, current, and 2 way (length of run * 2) wire run.

    So, 12 awg, 25 foot run (1 way, 2x25=50 foot), 200 watt inverter--you did not say what your battery voltage is--but I think you mentioned 12 volts in another post, 50 watt * 1/0.90 eff * 1/10.5 volt (near dead battery) = 5.29 amps :
    AWG    Feet   Ohms    Panel (Volts)  Panel (Amps) Voltage drop Voltage drop %
    12awg    50'    0.079747ohm    10.5v    5.29a      0.42186 vdrop    4.01771%
    
    So--it is pretty close... Assuming that your inverter turns of at 10.5volts, then it will run the battery down to 10.5+0.42=10.92volts and you will have ~4% loss due to the wiring.

    My suggestion would be to mount the inverter next to the battery (if 12 volts)... However, if you don't have a lot of surge on the inverter, it may work OK for you (and assuming the battery is relatively full).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cizzi
    cizzi Solar Expert Posts: 109 ✭✭✭✭
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    Re: Wire lenght/inverter question

    Yes my system is 12V...

    option 1. attaching inverter next to battery and sending 120V 25 feet away but then my devices are all 12v at the end, how would I go from 120v back to 12vdc? seems inneficient using something like a rectifier.. and convert the tension twice

    option 2.. inverter at the end of the 25 feet wire so I can use some dc and some ac (output of inverter).. in the past year the only things i have running in the evenning are a 13W dc bulb and a blue cold cathode neon that barely draws anything on the meter

    which one of these options is best? i would say the second given the low power requirements and awg 12 wire already passed

    also i was reading my 200w inverter manual and it has overload protection and stuff like that, should someone try to plug a high pulling device it would shut down right away
  • Ecnerwal
    Ecnerwal Solar Expert Posts: 101 ✭✭
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    Re: Wire lenght/inverter question
    cizzi wrote: »
    Yes my system is 12V...

    option 1. attaching inverter next to battery and sending 120V 25 feet away but then my devices are all 12v at the end, how would I go from 120v back to 12vdc? seems inneficient using something like a rectifier.. and convert the tension twice

    If your loads are ALL 12V, why have an inverter at all? What you described in your original question was running DC on the wire to power the inverter, powering a 50 watt load. You did not mention any additional DC loads. In that case, running 120V AC on the wire makes the most sense.

    If you have a 50 Watt load on the inverter, and you also have other 12V loads on the same wire, you definitely want a larger wire to run 12V on. If the total of all your loads on this wire (DC & AC from the inverter) is 50W, it's still marginal (really, it's too small, but not likely to set the house on fire) at 12V.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Wire lenght/inverter question

    although many would frown upon doing this, if that is 12/2 romex and or any wire arrangement that has 3 wires, you can send both back providing the inverter does not require the ground wire and the bare ground wire is otherwise unoccupied. have the inverter close to the battery and it can look something like:
    bare wire >12v- lead and 1 leg of inverter ac
    black wire>other ac leg
    white wire >12v+ lead

    the concerns would be that you do not make wrong connections, ergo mixing up what wire is for what, and that the total current on the common ground wire(not being used for ground and is now just a common wire) is not to exceed the sum of the dc and ac currents upon it. as always, use fuses and switches where applicable.
  • cizzi
    cizzi Solar Expert Posts: 109 ✭✭✭✭
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    Re: Wire lenght/inverter question

    yes all my 2 loads are dc currently but i just want the option to run a small ac load as well using an inverter
  • Ecnerwal
    Ecnerwal Solar Expert Posts: 101 ✭✭
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    Re: Wire lenght/inverter question

    The wire is marginal at best for just running the inverter and 50W load, without any additional DC loads.

    With additional DC loads, it's even more marginal, probably not functional at all.

    As for Neil's suggestion with the weird wiring, I'll join the "many" in frowning on it. I might even scowl. What's technically "possible" and what should be done in a house where some other person might ever encounter the wiring job are very different things. And it might not even be possible, depending on whether this inverter can actually have a grounded neutral or not - most small ones cannot.

    Just run another wire, so the installation is adequate and safe. Run your present DC loads (13W + ? watt) on the 12 gauge wire, and install a 14 gauge wire to run 120V from the inverter (placed near the battery and connected to the battery with short pieces of 6 or 8 gauge and appropriate fuses). Or run 4 gauge or 6 gauge to provide adequate DC to the remote location - but 14 gauge is a lot cheaper. If it means opening up walls, open them up - they are not that hard to fix.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Wire lenght/inverter question

    i'm also going to say to scrap that idea of mine for some inverters may not like a dc lead and one of its ac leads shorted together. your choices really are to 1>run separate wires for ac and dc and keep the inverter near the battery. 2> run extra heavy wire that accommodates the normal dc loads and the inverter loads with the inverter farther away from the battery.