Enter Zipcode

Results 1 to 8 of 8

Thread: Undersized wiring for micro inverters PV system, how big of an issue?

  1. #1
    Junior Member
    Join Date
    Feb 2015
    Posts
    22

    Default Undersized wiring for micro inverters PV system, how big of an issue?

    String of 17 micro inverters 240V ac with Imax 1.1 A, using #12 THHNs, and 2 strings like that home run in 3/4 (so 8 of #12 wires) PVC conduit for about 60' run, to 20 amps breakers for each string. How big of an issue?

  2. #2
    Moderator
    Join Date
    May 2009
    Location
    off grid retirement in Northern Calif
    Posts
    8,509

    Default

    I'm trying to decipher your terminology here ,

    String of 17 micro inverters 240V ac with Imax 1.1 A, using #12 THHNs, and
    2 strings like that home run in
    3/4 (so 8 of #12 wires) PVC conduit for about 60' run,
    to 20 amps breakers for each string.
    How big of an issue?
    String inverters have 10 - 20 panels wired in series for each inverter
    Microinverters have 1 panel per rooftop inverter, and they run HV AC back to the electrical panel.

    I can't figure out how you have 8 wires in the conduit, a cluster of micros would only have
    3 wires, L1, L2, & Gnd or L1, Neutral, Gnd. 2 clusters 6 wires. Not sure if you are pulling the ground inside or out.

    17 panels @ 1.1A ac, would be 18.7A on #12 wire, into a 20a breaker ? That would be a poor setup. These are
    continuous AMPS, for 5 or 6 hours at a time, and that will eventually heat up the #12 wire, and then heat up the
    PVC conduit. #10 wire for these steady amps would be a better choice, and at least 1" PVC to give you enough room
    for them. The 20A breaker will be OK, till the scattered clouds roll in on a warm afternoon, then the lensing will give
    enough surge that the breaker will likely trip, unless it's been cooked and locked ON from steady heat (they are not
    supposed to latch on, but I've seen them stuck and won't turn off). Your choice, save some cash now, and big bill later,
    or a bit more effort now.

  3. #3
    Junior Member
    Join Date
    Feb 2015
    Posts
    22

    Default

    Ground run inside the conduit, the installation was already done before, and "they" have run #12 which still I believe is poor, even tho it's a little less current. We just had to add 4 modules to two clusters of 15 and deal with what we have, didn't know about #12 until opened one of JBs, had to make a decision fast, decided to not repulling the whole home run and changing the conduit. Thanks for your answer. Just trying figure how big is that issue.

  4. #4
    Solar Fanatic
    Join Date
    Sep 2014
    Location
    San Diego
    Posts
    1,734

    Default

    Quote Originally Posted by abelov1984 View Post
    Ground run inside the conduit, the installation was already done before, and "they" have run #12 which still I believe is poor, even tho it's a little less current. We just had to add 4 modules to two clusters of 15 and deal with what we have, didn't know about #12 until opened one of JBs, had to make a decision fast, decided to not repulling the whole home run and changing the conduit. Thanks for your answer. Just trying figure how big is that issue.
    What micro-inverter is rated for 1.1 A output? That does not match Enphase or ABB models. The original wiring might have been fine... 20 A protecting 12 AWG is generally OK even after the ampacity corrections, and 20 A breaker for a 16 A rated output would have been right too. Sounds like the only thing that really went wrong here is that someone decide to expand the system without doing any of the necessary design work.

  5. #5
    Solar Fanatic
    Join Date
    Oct 2012
    Location
    Clearwater Florida
    Posts
    5,568

    Default

    Quote Originally Posted by sensij View Post
    What micro-inverter is rated for 1.1 A output? That does not match Enphase or ABB models. The original wiring might have been fine... 20 A protecting 12 AWG is generally OK even after the ampacity corrections, and 20 A breaker for a 16 A rated output would have been right too. Sounds like the only thing that really went wrong here is that someone decide to expand the system without doing any of the necessary design work.
    Except he has 2 strings of 4 wires (L1,L2,N & Grd) running in the same 3/4" conduit (not sure if the Neutral is required but those other 4 L1 & L2 #12 wires need to be de-rated based on more than 3 energized wires in the same conduit.

  6. #6
    Solar Fanatic
    Join Date
    Sep 2014
    Location
    San Diego
    Posts
    1,734

    Default

    Quote Originally Posted by SunEagle View Post
    Except he has 2 strings of 4 wires (L1,L2,N & Grd) running in the same 3/4" conduit (not sure if the Neutral is required but those other 4 L1 & L2 #12 wires need to be de-rated based on more than 3 energized wires in the same conduit.
    2011 NEC
    Table 310.15(B)(16) - Base ampacity of 12 AWG THHN = 30 A
    Table 310.15(B)(2)(a) - Ambient temp correction for 45 deg C (113 deg F) = 0.87
    Table 310.15(B)(3)(a) - 4 conductor correction = 0.80

    30 * 0.87 * 0.80 = 20.9 A

    The temp correction might be a little bit light for some parts of the country, especially if the conduit is on the roof. However, it is at least possible that 12 AWG was ok for the original installation. Adding more panels to it definitely was not ok.

  7. #7
    Solar Fanatic
    Join Date
    Oct 2012
    Location
    Clearwater Florida
    Posts
    5,568

    Default

    Quote Originally Posted by sensij View Post
    2011 NEC
    Table 310.15(B)(16) - Base ampacity of 12 AWG THHN = 30 A
    Table 310.15(B)(2)(a) - Ambient temp correction for 45 deg C (113 deg F) = 0.87
    Table 310.15(B)(3)(a) - 4 conductor correction = 0.80

    30 * 0.87 * 0.80 = 20.9 A

    The temp correction might be a little bit light for some parts of the country, especially if the conduit is on the roof. However, it is at least possible that 12 AWG was ok for the original installation. Adding more panels to it definitely was not ok.
    You are correct a #12 THHN is rated 30A. I guess I like to be more conservative and always design around the 25 amp rating for a #12 wire even if I use the higher amp rated wire.

  8. #8
    Member
    Join Date
    Feb 2015
    Location
    Darlington, MD
    Posts
    86

    Default Electrician 2 calculator

    I like this calculator, but unsure of the differences after 2005, which it is based on: http://www.electrician2.com/calculat...cpd_ver_1.html . It says with 90 degree C wire, 75 degrees termination, 86 degrees ambient F, 4-9 current carrying conductors in the raceway, OCPD not rated for continuous load that #12 THHN(-2) can handle 16A continuous. If 17A it bumps to #10 with a 30A breaker.
    Jeff, BSEE, 20.1KW, 45-240W w/M190, 36-260W w/M250

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •