Using an AC extension cord for DC power... what to do with the ground wire?

OptiOpti Registered Users Posts: 10 ✭✭
I have a small portable battery pack that I would like to connect a solar panel to.
I found a sale on a 50' length of 10 gauge construction grade AC extenstion cable and thought it would do the trick as the go between from the solar panel to the battery.

I was simply going to cut off the AC plugs and wire positive and negitive using two of the three conductors but seeing that there is an extra ground wire, I'm wondering if it will be good or bad to simply combine the ground with the positive or negitive conductor to make one even a thicker gauge which should be good for voltage drop on long runs at low voltage.

First, I don't think it would be a problem to do such a thing but I'll put it out there just in case.

Second, if it's good to go, which conductor should I double up on the positive or negative? My thought was the positive but as I was thinking about it I noticed that shunts for amp meters will be installed on the negative conductor which maybe good for metering but I'm not sure if it makes a difference on what I'm doing.

Any expertise available?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    You can parallel the ground with either + or - wire--it won't make a difference.

    What is the rating of the panels (Vmp/Imp)? And the AH/Voltage of your battery bank?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    bill is right that electrically it won't matter which polarity gets the doubled wiring. i'm thinking it may be easier to double wire the - polarity just because at the most it would only get interrupted by possibly a shunt for measuring current, whereas the + polarity can see more interruptions with things like switches, fuses, etc.

    btw, the doubling of the #10 will give you about a #7 equivalency.
  • boBboB Solar Expert Posts: 965 ✭✭✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    Sure. Parallel the third extra wire with one of the other wires. It can only help to reduce wire losses some.

    boB
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    If you're system is negative ground, then to reduce confusion, I would add the cable "ground" wire to the negative wire.
    Electrically, as mentioned, it makes no difference, but some time down the road, you or someone else may see the "ground" wire connected to another wire and assume it's negative (ground), whether it is or not.
  • OptiOpti Registered Users Posts: 10 ✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?
    BB. wrote: »
    You can parallel the ground with either + or - wire--it won't make a difference.

    What is the rating of the panels (Vmp/Imp)? And the AH/Voltage of your battery bank?

    -Bill
    Thanks Bill, I have a few different sized systems so the panel Vmp/Imp is different depending on such, but the largest is a single 215 watt with an 18.4 Vmp and 11.69 Imp.
    Others are 190 watt, 120 watt and 100 watt.

    The battery packs very from 35AH, 50AH and 100AH

    niel, if doubling the #10 will give me the equivalent of #7, that would be for that pos or neg conductor being doubled not for the whole circuit correct?

    The consensus seems to be that it doesn't make a difference which wire + or - is doubled but in the end have I gained any advantage resistance wise to help at all with voltage drop across the 50' length? If the whole circuit was improved to the equivalent of a #7 gauge wire then I've cut the voltage drop close to half compared to a #10 wire alone. If however, I only get the equivalent to #8.5 or #9 overall, well... any little bit helps though I don't think it counts as a bullet point.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    "niel, if doubling the #10 will give me the equivalent of #7, that would be for that pos or neg conductor being doubled not for the whole circuit correct?"

    correct as it's only the - or the + being equivalent to #7. the other side will still be a #10 wire. this will reduce some of the overall resistance and make an improvement. the actual improvement figures will depend on the amount of current you will be sending through the wires.
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    It is too late tonight to make the actual calculations--But if these panels are all ~17.5 volts Vmp then Imp~24.5 amps (adding all panels in parallel at the end of your 50' cable). And the voltage drop on your 50' of ~8 awg wire is ~2 volts.

    That is about 4x higher than we would recommend--And that could give you some problems if you tried to equalize the batteries at ~15.0 volts or even charging at ~14.5 volts... You may lose some output current from the array because of the voltage drop in the wiring and the limit of the ability to output Imp above Vmp.

    But if you are charging the smaller batteries and not using all panels in parallel at the end of the extension cord--Then you probably would be OK.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?
    Opti wrote: »
    Thanks Bill, I have a few different sized systems so the panel Vmp/Imp is different depending on such, but the largest is a single 215 watt with an 18.4 Vmp and 11.69 Imp.
    Others are 190 watt, 120 watt and 100 watt.

    The battery packs very from 35AH, 50AH and 100AH

    niel, if doubling the #10 will give me the equivalent of #7, that would be for that pos or neg conductor being doubled not for the whole circuit correct?

    The consensus seems to be that it doesn't make a difference which wire + or - is doubled but in the end have I gained any advantage resistance wise to help at all with voltage drop across the 50' length? If the whole circuit was improved to the equivalent of a #7 gauge wire then I've cut the voltage drop close to half compared to a #10 wire alone. If however, I only get the equivalent to #8.5 or #9 overall, well... any little bit helps though I don't think it counts as a bullet point.
    Your calculation for voltage drop gets a little more complicated. Instead of just doubling L you have to calculate the voltage drop for the + and - wires separately with the resistance halved on one of them (it makes no difference which one) and add them together. The ratios, however, are simple; if you doubled both wires your Vd would be cut in half, while doubling only one of them will cut it by 25%.

    I'm not recommending you do this, though. I don't believe there's any way that it could be code compliant.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?
    ggunn wrote: »
    Your calculation for voltage drop gets a little more complicated. Instead of just doubling L you have to calculate the voltage drop for the + and - wires separately with the resistance halved on one of them (it makes no difference which one) and add them together. The ratios, however, are simple; if you doubled both wires your Vd would be cut in half, while doubling only one of them will cut it by 25%.

    I'm not recommending you do this, though. I don't believe there's any way that it could be code compliant.

    RE: post #1 "a small portable battery pack".

    There's no code compliance necessary in this instance.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Using an AC extension cord for DC power... what to do with the ground wire?

    opti,
    if you could be more specific on the pv specs then we can run some calculations on the v drop you'll have. if it turns out to be a high v drop then could you consider shortening the length of the cable being used?
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