120 percent rule

624

Member
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
 

jaggedben

Senior Member
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?
 

ggunn

PE (Electrical), NABCEP certified
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.
 

624

Member
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
 

jaggedben

Senior Member
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?
 

624

Member
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
 

ggunn

PE (Electrical), NABCEP certified
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).
 

vanwalker

Senior Member
I know the answer is here so please direct me.I guess we need a question: Does the 120% rule change when you have Tesla battery 10 or 11 KW I think? I am being told that you can put 100 amp PV breaker in main service panel with 200 amp main breaker. Is there any legal way? No tricks THANKS
 

jaggedben

Senior Member
I know the answer is here so please direct me.I guess we need a question: Does the 120% rule change when you have Tesla battery 10 or 11 KW I think? I am being told that you can put 100 amp PV breaker in main service panel with 200 amp main breaker. Is there any legal way? No tricks THANKS
You should maybe start a new thread, and add some details. We always need to know both the breaker and the busbar, not just '200 amp breaker'. The rule does not change, but Powerwall counts the same as solar. You generally cannot put a 100A of any combination of PV+Powerwall into a 200A panel protected by a 200A main breaker. It would need to be a higher bus rating, or downsize the main breaker, or some of both, or use a different rule.
 
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