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Wire sizing

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  • Wire sizing

    I have a power house for my solar system (not yet installed) which will contain the batteries, inverters and charge controllers. The building is 10'x12' and divided down the middle by a wall. The batteries will be on one side and the electrical equipment will be on the other.

    My solar wiring diagram has two 2/0 cables connecting each battery's terminal to that of the next (four cables per battery.) At the end of the string (twenty-four 2-volt batteries) I will have 48 volts and 2050 aH. The diagram shows one 1000 MCM cable for the positive and one 1000 MCM cable for the negative, connecting the battery bank to the battery combiner box.

    I've already bought the 2/0 cables (rather than 500 MCM cables to use in their place) but I'm wondering: do parallel cables violate the NEC?

    I'm wondering if two 300 MCM cables could replace the 1000 MCM cable, which I haven't yet purchased.

    The length of the 2/0 cables is 12 inches. The 1000 MCM cables would probably be about three feet long.

    Should I return the 2/0 cables and locally source some 500 MCM? The place I got the 2/0 cables doesn't have anything bigger than 2/0.


  • #2
    Jim tap the breaks and come to a complete stop. The size of the conductors is determined by the Insulation material, and the size of the OCPD (over current protection device like a fuse or breaker). In low voltage systems one additional requirement which has nothing to do with safety, rather performance is voltage drop which is effective by the size of the wires and the distance.

    There are three areas where one does need to size the wiring.

    1. Panels to Controller.
    2. Controller to Battery
    3. Battery to Load Device like an Inverter.

    So you have a 48 volt system right? So what panel wattage are you using and what is the configuration of the panels. About the largest panel wattage you can run is 4000 watts on a single Controller. If you were running 100 volts on the panels like they should be with a 150 volt controller current is 50 amps and minimum size is #6 AWG, and if the 1-way distance is say 25 feet would require #4 AWG.

    Controller to battery is up to 100 amps and that requires #4 AWG. with above assumptions.

    From the battery to say an Inverter of 4000 watts @ 48 volts only requires 4 AWG and good up to 6 feet 1-way distance. Same for battery interconnections.

    So the question becomes why are you using such large wires? To terminate the cable you are mentioning takes a very expensive 16-ton hydraulic compression tool and connectors that will cost you some $30 each. 1000 MCM is for 900 to 1000 amps and at 48 volts would be a 50,000 watt Inverter. The batteries to drive that would completely fill a house. Panels to charge such a best would require an acre of land.

    So what size is your panel wattage, Controller (amps), Inverter (watts), and battery capacity in AH?

    Me thinks you are lost and need to stop and ask for directions because you are on the wrong road going the wrong way.
    Last edited by Sunking; 01-14-2018, 03:06 PM.
    MSEE, PE


    • #3
      Yes, a 48 volt system, ground-mount. Two arrays of 24 panels each. The panels are SolarWorld SW340. Each array is connected a combiner box and then to two MidNite Solar Classic 200 MPPT charge controllers (total of four charge controllers for the system) The wires specified to connect the combiners to the charge controllers are #6 THWN-2 buried in conduit. The distance from the combiners to the controllers is about 90 feet maximum. Wires connecting each double charge controller installation to the quad power center with four Magnum MS4448PAE inverters and the battery combiner box are 2/0 (one 2/0 from each double charge controller for the positive and one 2/0 from each charge controller for the negative)

      For the positives and negatives coming from the battery combiner box into the quad power center with the four inverters, there are a total of eight 4/0, five-foot cables (four positives and four negatives)

      From the quad power center to the load panel there are three #2 THWN-2 plus a ground.

      Each battery will be connected to the next by four 2/0 12-inch lugged wires (two parallel for the positive and two parallel for the negative) There are twenty-four 2 volt, 2050 aH batteries.

      It will only be a couple of feet from the end of the battery string to the battery combiner box. Two 1000 MCM (or equivalent) are specified to connect the battery string to the battery combiner box. Would parallel 350 MCM be equivalent?

      The chart I saw states that 1000 MCM is good for 615 amps, not 900 to 1000.

      Here are some more specs:

      SW340 XL MONO
      PMAX 340 WATTS
      VMPP 38.00 VDC
      VOC 47.60 VDC
      IMPP 9.01 ADC
      ISC 9.69 ADC

      4 MODULES
      PMAX 1,360 WATTS
      VMPP 152.00 VDC
      VOC 190.40 VDC
      IMPP 9.01 ADC
      ISC 9.69 ADC

      4 STRINGS
      PMAX 5,440 WATTS
      VMPP 152.00 VDC
      VOC 190.00 VDC
      IMPP 36.04 ADC
      ISC 38.76 ADC

      5 STRINGS
      PMAX 6,800 WATTS
      VMPP 152.00 VDC
      VOC 190.00 VDC
      IMPP 45.05 VDC
      ISC 48.45 VDC

      AC OUTPUT@ 240VAC
      MAX 4,400 WATTS
      MAX 18.33 AMPS