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Thread: 120 percent rule

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    120 percent rule

    I have 800 amp main service 240/120 delta high leg. I plan on doing a load side tape to a 200 amp fused disconnect. To a 68kva step up transformer 240v primary delta, secondary 480 volt secondary to a 150amp 480v solar sub panel with a 100 amp back feed breaker, feeding Solar edge 66.6kus 480 inverter. Continues out put is 80 amps x1.25 = 100. My Question is where does the 120 percent start and end. Does the first 200 amp fused disconnect have to be within the 120 percent of 800 amps is 960 amps= 160 backfeed space thanks

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    The '120 percent rule' these days (since 2014 NEC) applies only to panelboards. There are other rules for feeder taps. Read all of 705.12 carefully. What code cycle are you on? What kind of equipment is the service equipment and what is the size of the feeder you are tapping?

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    If the project is in California (2014 NEC) the code reference is 705.12(D)(2)(2) and (3), specifically.

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    Quote Originally Posted by 624 View Post
    I have 800 amp main service 240/120 delta high leg. I plan on doing a load side tape to a 200 amp fused disconnect. To a 68kva step up transformer 240v primary delta, secondary 480 volt secondary to a 150amp 480v solar sub panel with a 100 amp back feed breaker, feeding Solar edge 66.6kus 480 inverter. Continues out put is 80 amps x1.25 = 100. My Question is where does the 120 percent start and end. Does the first 200 amp fused disconnect have to be within the 120 percent of 800 amps is 960 amps= 160 backfeed space thanks
    In general the 120% rule starts where an inverter output circuit lands in a panel and it does not end, i.e., it figures into the busbar calculations all the way back to the service. Of course, there are a couple of other ways to qualify busbars under 705.12.

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    I’m under 2014 nec my conductors I’m going to tap into are each phase has two 500 mcm protected by two 400 amp fuses

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    I'm reading that as one fuse per conductor.

    Not surprisingly, the feeder conductors aren't oversized so you have to pursue one of a few options:
    -do a load calc to see if you can downsize the existing fuses
    -insert another overcurrent protection device downstream of your tap
    -upsize the feeder conductors downstream of your tap

    Assuming that your service disconnect is a fused disco with the 400A fuses, you have no issue there.

    With the information you've given, it's not clear that the 120 percent rule comes into play at all. Are there any panelboards between your tap and the service point?

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    There are no panels between my service disconnect and where I want to tap. Can you guys recommend some reading on the change of the 120 percent in the 2014 thanks

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    Quote Originally Posted by 624 View Post
    There are no panels between my service disconnect and where I want to tap. Can you guys recommend some reading on the change of the 120 percent in the 2014 thanks
    The 120% rule itself did not change, but AC combiners that do not comply with it were legitimized. 705.12(D)(2)(3)(c).

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