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Thread: Backfeed Wire Size Question

  1. #1
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    Backfeed Wire Size Question

    So......

    We are putting a 40 amp breaker backeed into a 100 amp sub panel. My plan is to downsize the main breaker in the sub to an 80 to accomplish this.

    The wire feeding the sub panel is only a #8. Assuming it is protected by a 40 or 50 amp breaker where it originates (haven't actually located that panel yet, it's a big compound), is this OK?

    My guess is yes, this is legal, but I haven't run into this situation yet.

    thanks

  2. #2
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    Quote Originally Posted by bbartmasse r View Post
    So......

    We are putting a 40 amp breaker backeed into a 100 amp sub panel. My plan is to downsize the main breaker in the sub to an 80 to accomplish this.
    Perhaps I'm missing something, what you're saying does not make sense.

    *If there is main, you would not backfeed, you would use the main unless there are two sources.
    *downsizing the main to an 80 is not the same as a 40 amp backfed breaker.
    *not sure where you would find an 80 amp main breaker.

  3. #3
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    Quote Originally Posted by Coppersmith View Post
    Perhaps I'm missing something, what you're saying does not make sense.

    *If there is main, you would not backfeed, you would use the main unless there are two sources.
    *downsizing the main to an 80 is not the same as a 40 amp backfed breaker.
    *not sure where you would find an 80 amp main breaker.

    Sorry for the confusion, I will attempt to clarify.

    We are installing a PV system. The breaker I am installing for said system will be placed in a sub-panel as a "back-feed" breaker. It will be 40 Amp.

    The sub-panel is rated at 100 amps, with a 100 amp "main breaker" installed. In order to satisfy the 120% rule I will downsize that breaker to an 80, therefore allowing me to use the 40 amp breaker previously mentioned.

    The catch, and what my question is about, is that the sub-panel is fed only with #8 wire (and what I assume to be a 40-50 amp breaker but I have not confirmed this yet) from another panel somewhere.

    Electrically speaking I do not really see a problem if the #8 feeder is properly protected with the right breaker, but I just wanted to be sure.

    That would mean that the #8 is supplying the panel's loads and serving as a path for the solar to head on out to the grid.

    Thoughts?

    thanks

  4. #4
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    Quote Originally Posted by bbartmasse r View Post
    Sorry for the confusion, I will attempt to clarify.

    We are installing a PV system. The breaker I am installing for said system will be placed in a sub-panel as a "back-feed" breaker. It will be 40 Amp.

    The sub-panel is rated at 100 amps, with a 100 amp "main breaker" installed. In order to satisfy the 120% rule I will downsize that breaker to an 80, therefore allowing me to use the 40 amp breaker previously mentioned.

    The catch, and what my question is about, is that the sub-panel is fed only with #8 wire (and what I assume to be a 40-50 amp breaker but I have not confirmed this yet) from another panel somewhere.

    Electrically speaking I do not really see a problem if the #8 feeder is properly protected with the right breaker, but I just wanted to be sure.

    That would mean that the #8 is supplying the panel's loads and serving as a path for the solar to head on out to the grid.

    Thoughts?

    thanks
    One thought I have is that you need to comply with 705.12(D)(2)(3) in the MDP as well as in the subpanel.

  5. #5
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    If there is a 100A main breaker in the sub but the feeder for the sub is protected elsewhere by a 40 or 50A breaker, then the 100A breaker is basically just an additional disconnecting means and has no bearing on the 120% calculation. At least in my opinion. In other words, you shouldn't need to downsize anything. Unless, that is, the unknown breaker protecting the feeder upstream is oversize in violation of code.

    As ggunn said, you need to look at the series connection(s) upstream as well.

  6. #6
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    I agree. The 100 amp main breaker in the sub-panel does not need to be downsized if the breaker feeding the sub-panel is 80 amps or smaller. You can use the feeder breaker in the calculation for the 120% rule. If the feeder breaker is installed in a 200 amp or larger main panel opposite the main breaker, then you are in good shape. Generally, the 120% rule is applied to all panels in series back to the main service equipment making sure that all backfeed breakers are opposite the main breaker in their respective panels.

  7. #7
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    Quote Originally Posted by PWDickerson View Post
    I agree. The 100 amp main breaker in the sub-panel does not need to be downsized if the breaker feeding the sub-panel is 80 amps or smaller. You can use the feeder breaker in the calculation for the 120% rule. If the feeder breaker is installed in a 200 amp or larger main panel opposite the main breaker, then you are in good shape. Generally, the 120% rule is applied to all panels in series back to the main service equipment making sure that all backfeed breakers are opposite the main breaker in their respective panels.

    Good points all, that makes perfect sense. The bussing of the 100A panel could never be overloaded so no downsizing required. That's great!

    Glad I brought it up and thanks for the responses.

  8. #8
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    So the actual situation is this:

    Backfeeding a 100 amp panel with 100 amp breaker with 40 amps of solar

    Above 100 amp breaker is fed from 60 amp breaker (#6) that is in a 125 amp panel with 100 amp main breaker

    Above is fed from a 100 amp breaker in a 200 amp panel (with properly sized feeds)

    Seems OK to me

  9. #9
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    Quote Originally Posted by bbartmasse r View Post
    So the actual situation is this:

    Backfeeding a 100 amp panel with 100 amp breaker with 40 amps of solar

    Above 100 amp breaker is fed from 60 amp breaker (#6) that is in a 125 amp panel with 100 amp main breaker

    Above is fed from a 100 amp breaker in a 200 amp panel (with properly sized feeds)

    Seems OK to me
    Depending on your Code cycle you might have a problem with the first line.
    Back in 2008 the backfeed calculation was made from the breaker size, not 125% of the nominal inverter output. 100A breaker kills you.
    Is the 40A of solar the nominal AC output? If so, you have to size wire, breaker and backfeed based on 125% of that, 50A, in later Code.
    50A backfeed plus 60A infeed from panel above is only 110A, so that will fit under the 120% rule. But inspector may want to see a 60A main in the last panel too. (He would, IMHO, be wrong though.)
    The 125A panel has a 120% value of 150A, which will not allow a 100A main plus 60A backfeed.

  10. #10
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    We're on 2014 code cycle.

    I'm not sure I understand what you're saying. The inverter has a output nameplate rating current of 32 amps, requiring a 40 amp breaker after the 1.25 multiplier.

    Seems to me putting 40 amp backed breaker onto a 125 amp panel with a downsized main to 100 works fine.

    thanks

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