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can someone help on calculate the right size for cable?

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  • can someone help on calculate the right size for cable?

    does anyone know how to calculate the right size for the cable :
    1. solar panel to charge controller
    2. charge controller to dc load
    3. charge controller to batteries
    4. baterries to batteries(if more than one battery)
    5.battery to power inverter
    6. power inverter to ac load

    please guide me on this one. i hope there are some calculation that i can refer on. im trying to do for 100w solar power system.im trying to do the solar panel for my room.

    thank you

  • #2
    The wire size you need in each of these situations is based upon your expected maximum current load. I am not familiar with battery systems, however this should hold true for wire sizing in any situation. I will try to answer as best as I can.

    Your DC wire sizing is dependant on the module short circuit current (Isc), derated for extreme irradiance conditions and for continuous photovoltaic source. The Isc is the maximum current possible to be gererated by each string of modules, and is found on your equipment data sheet. Each of the derates is for 125% so total DC design current per string is Iscx1.56. You will then need to use a wire that has an ampacity of at least this amount. NEC Table 310.16 shows the allowable ampacities (i.e. current carrying capacity) for variuos wire sizes. You need to match your wire temperature ratings to your terminal ratings (always 75C). You also need to adjust your ampacities based on your maximum ambient temperature, and if you use 90C rated wire NEC 110.14(C) allows you to use the 90C column when adjusting or correcting your ampacities. So in summary you need to find your max current and factor it up, then find your wire ampacity and correct for temperature, then find a wire that has a current carrying capacity greater than that. As a good rule of thumb, you can pretty much always use 10AWG for single strings.

    When you need to combine stings (be it a combiner box, disconnect, or simply a J-Box) you just add the Iscx1.56 current together. The wire size you transition to here needs to be large enough to carry this resultant current.

    To size conductors from you inverter AC output is a little different. You first need to find your current load. This is found by dividing your power (watts) by your voltage. For typical residential this will be 240, but a commercial three phase system may be configured for 208 or 277V. So if you have a 5000W inverter dividing by 240 gives you 20.8. Your overcurrent protection and conductions are required to be sized at 125% of this load (can't off hand remember the code section) so 26A, with 30A being our next standard breaker size. So these conductors should therefore be sized for 30A. Then its just back to NEC table 310.16 to find you minimum wire size.

    I would suspect all of your other wire sizing questions would fall under this simple answer: Find your maximum expected current with appropriate derates, and then refer to NEC Table 310.16 to find a wire size, corrected for temperature that will handle this amount of current.

    WHewww...

    Hope this helps!

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    • #3
      all the information that u given to me is very useful. but there are something i need to know, why do we need to multiply the short-circuit current with 1.56?

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      • #4
        The 1.56 is a safety factor. The multiplication of the short-circuit current and 1.56 is used to specify the size of a slow-blow fuse in the array output circuit. Should a ground fault occur in the array while the controller is engaged, this fuse will protect the array modules and the conductors from high battery current.

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        • #5
          thank you for sharing the info with me. this is really helping me.

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          • #6
            Be sure to post some pictures of your project if you get the chance! Good luck!

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            • #7
              Originally posted by farhannna View Post
              does anyone know how to calculate the right size for the cable :
              1. solar panel to charge controller
              2. charge controller to dc load
              3. charge controller to batteries
              4. baterries to batteries(if more than one battery)
              5.battery to power inverter
              6. power inverter to ac load

              please guide me on this one. i hope there are some calculation that i can refer on. im trying to do for 100w solar power system.im trying to do the solar panel for my room.

              thank you
              Also as well as getting the proper gage wire on DC be sure to use STRANDED wire and the more strands the better as with DC the current flows on the skin of each strand.
              So a 8 Gage stranded wire will handle more current than a SOLID conductor of the same 8 Gage Size.
              Hope this isn't too confusing but we had a house in this area burn down as they used solid conductor off of the solar panels to the battery and ran it through a SMALL hole in the wall and when it over heated it caught fire. They were so far out in the back country that by the time they called the fire department the house was toasted.

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              • #8
                Originally posted by Hammerhead46 View Post
                ... gage wire on DC be sure to use STRANDED wire and ... SOLID conductor ... when it over heated it caught fire. T

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                • #9
                  hey marineliner...just added those words. thanks for the suggestions

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