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Thread: Adding string. WHat fuses?

  1. #1
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    Adding string. WHat fuses?

    Double check my calcs here.

    Adding new string of modules to existing two-string array of Mitsubishi 180 panels.
    4 @ #10 THHN plus ground (2 strings) running 100-200 feet or so to DC disconnect and Inverter.
    Because it is a ground rack way down the hill and conduit is already buried 10 years ago and wire pulled..... I aim to parallel combine the two existing strings into one at the array. This will then connect to just one of the existing pairs of #10 wires and free up the second pair for my new string.


    (E) 2 @ 8.03A Isc, 7.45A Imp, 15A max fuse rating. (Mits. 180W) (combined: 16.06A Isc, 14.9A Imp)
    (N) 1 @ 9.98A Isc, 9.50A Imp, 20A max fuse rating. (Hanwha Q CEll 310 DUO)

    What, if any, fusing needed?

  2. #2
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    Seems to me you need 15A fuses on the old strings and 20A fuses on the new one. Am I missing anything?

  3. #3
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    Quote Originally Posted by jaggedben View Post
    Seems to me you need 15A fuses on the old strings and 20A fuses on the new one. Am I missing anything?
    I agree 15A fuses on the individual old strings.

    Except, I don't think any fuse is needed on the new string.

    There are 3 strings. The purpose of any fuse is to protect that string from the combined amps of the other two back-feeding into it.
    I don't see the two old strings at a combined (2@8A) 16A feeding into the new string (max fuse rating of 20A) as being a problem.

  4. #4
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    Quote Originally Posted by Zee View Post
    I agree 15A fuses on the individual old strings.

    Except, I don't think any fuse is needed on the new string.

    There are 3 strings. The purpose of any fuse is to protect that string from the combined amps of the other two back-feeding into it.
    I don't see the two old strings at a combined (2@8A) 16A feeding into the new string (max fuse rating of 20A) as being a problem.
    The counterpoint would be that with two sets of 15A fuses protecting it, you effectively have the new string protected at 30A, which exceeds its max fuse rating. Also remember code required Isc*1.56 so that would be 25A, also exceeding max fuse rating. Mind you, that is worst case scenario by code, not necessarily a real possibility.

  5. #5
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    How about putting 15-amp on all strings so no one ever mixes it up on a service call.

    Also, what inverter is the system using? Jus curious on that side of the system.

  6. #6
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    Quote Originally Posted by Zee View Post
    Double check my calcs here.

    Adding new string of modules to existing two-string array of Mitsubishi 180 panels.
    4 @ #10 THHN plus ground (2 strings) running 100-200 feet or so to DC disconnect and Inverter.
    Because it is a ground rack way down the hill and conduit is already buried 10 years ago and wire pulled..... I aim to parallel combine the two existing strings into one at the array. This will then connect to just one of the existing pairs of #10 wires and free up the second pair for my new string.


    (E) 2 @ 8.03A Isc, 7.45A Imp, 15A max fuse rating. (Mits. 180W) (combined: 16.06A Isc, 14.9A Imp)
    (N) 1 @ 9.98A Isc, 9.50A Imp, 20A max fuse rating. (Hanwha Q CEll 310 DUO)

    What, if any, fusing needed?
    Every string must be individually fused, including the two that you are combining. You cannot just fuse the combined pair.

  7. #7
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    Quote Originally Posted by ggunn View Post
    Every string must be individually fused, including the two that you are combining. You cannot just fuse the combined pair.
    you guys are correct. I do need a fuse as the combined amps @ 156% does exceed the third circuit's max fuse rating of 20A. Thx.

    I just wish i had remembered to order fuseHOLDERS with the darn Combiner Box!

  8. #8
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    Quote Originally Posted by shortcircuit2 View Post
    How about putting 15-amp on all strings so no one ever mixes it up on a service call.

    Also, what inverter is the system using? Jus curious on that side of the system.
    SMA SB x000US - 10 year old, transfromer based inverter

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