Micro Inverter wiring and breaker question.

I am about to run 17 Enphase m215 micro's in one segments of 9 inverters and another of eight inverters, which will be combined into one string at a roof mounted junction box. I am running 240 volt system, 17 inverters are most permissible per string, Enphase specs.

My questions, from roof mounted junction box to circuit breaker panel located approximately 50 feet away is it necessary to run 10 gauge wire or would 12 gauge be acceptable. (Enphase cabling is 12) What size backfed circuit breakers would I need? Does the circuit breaker size change if I use 10 or 12 gauge wire? Thank you very much for the help!

Comments

  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Micro Inverter wiring and breaker question.
    cygone wrote: »
    I am about to run 17 Enphase m215 micro's in one segments of 9 inverters and another of eight inverters, which will be combined into one string at a roof mounted junction box. I am running 240 volt system, 17 inverters are most permissible per string, Enphase specs.

    My questions, from roof mounted junction box to circuit breaker panel located approximately 50 feet away is it necessary to run 10 gauge wire or would 12 gauge be acceptable. (Enphase cabling is 12) What size backfed circuit breakers would I need? Does the circuit breaker size change if I use 10 or 12 gauge wire? Thank you very much for the help!

    The breaker size does not depend on the wire gauge. The breaker should be 17 times the rated maximum output of a single microinverter, rounded up to the next standard size.
  • mike95490mike95490 Solar Expert Posts: 8,429 ✭✭✭✭✭
    Re: Micro Inverter wiring and breaker question.

    That's 15.2A @ 240VAC, I'd use 10ga, just to not waste the power.

    I think you'd need a 20A breaker.
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  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Micro Inverter wiring and breaker question.

    Enphase recommends that you keep voltage rise to less than 2% total with an inclusive voltage rise of less than 1% in the Engage cable itself.

    http://enphase.com/wp-uploads/enphase.com/2011/10/Enphase_Tech_Brief_Vdrop_M215_Microinverter.pdf

    If you are running a string of 17 inverters fed on one end, depending on whether they're mounted in portrait or landscape, the last inverter in the string is already going to see either 1.94% or 3.21% voltage rise respectively.

    Right off the bat, you either need to center feed the array which cuts the voltage rise down to 0.57% for portrait and 0.94% for landscape which leaves you with either 1% or 1.4% voltage rise for your feeder.

    Now let's calculate your voltage rise at the end of your 50ft feed using both 10/12 AWG wire:

    10 AWG VRise = 0.9A * 17 * 0.00129 ohm / ft * 50 ft * 2 = 1.97 V / 240 V = 0.8% rise
    12 AWG VRise = 0.9A * 17 * 0.00207 ohm / ft * 50 ft * 2 = 3.17 V / 240 V = 1.32% rise

    So the conclusion is that if you center feed the array and you're using the portrait cable, you can get away with 12 AWG wire for your feed. Otherwise you need to use 10 AWG wire.

    Personally, I'd opt for the 10 AWG regardless - you're going to get 0.5% more production from your system with that cable and the incremental cost should be minimal.

    We're talking maybe $50 max more for 10 AWG wire for a system that will probably produce about 5000 kWh / year, so you'll get up to 250 more kWh out of the system each year with 10 AWG wire vs 12 AWG. OK not that much since the system doesn't produce max power all day, but still, even if you only get 100 more kWh out of the system a year it's a no brainer.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Micro Inverter wiring and breaker question.
    drees wrote: »
    Enphase recommends that you keep voltage rise to less than 2% total with an inclusive voltage rise of less than 1% in the Engage cable itself.

    http://enphase.com/wp-uploads/enphase.com/2011/10/Enphase_Tech_Brief_Vdrop_M215_Microinverter.pdf

    If you are running a string of 17 inverters fed on one end, depending on whether they're mounted in portrait or landscape, the last inverter in the string is already going to see either 1.94% or 3.21% voltage rise respectively.

    Why would the orientation of the modules have any effect on voltages?
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Micro Inverter wiring and breaker question.
    ggunn wrote: »
    Why would the orientation of the modules have any effect on voltages?
    There's 2 different trunk cables for the M215 - the distance between connectors is different to better fit portrait / landscape installs.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Micro Inverter wiring and breaker question.
    drees wrote: »
    There's 2 different trunk cables for the M215 - the distance between connectors is different to better fit portrait / landscape installs.
    And the difference in voltage drop in the cables makes that much difference?
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Micro Inverter wiring and breaker question.
    ggunn wrote: »
    And the difference in voltage drop in the cables makes that much difference?
    Yes - the landscape connector spacing is 1.7M compared to 1.0M on a portrait cable, so the last inverter on a 17 inverter string is 95 ft away on a landscape install compared to 56 ft away on a portrait install.
  • cygonecygone Registered Users Posts: 6
    Re: Micro Inverter wiring and breaker question.

    Hey, thanks for all the input. I am going to center feed and am using a portrait configuration. I went with the 10awg as you all suggested.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Micro Inverter wiring and breaker question.
    ggunn wrote: »
    The breaker size does not depend on the wire gauge. The breaker should be 17 times the rated maximum output of a single microinverter, rounded up to the next standard size.

    Actually it should be 17 times the maximum output of one inverter, multiplied by 1.25, then rounded up. And in any case, a 20 amp breaker is sufficient to protect the wiring on a M215 circuit with any number of inverters; one does not need to go smaller for fewer inverters. 17 M215s is the max allowed on a circuit according to Enphase.
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