ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Electrical Engineer - Photovoltaic Systems
We are contemplating connecting a three phase PV system with maximum output current of 153A to the load side of an 800A disco. This is governed by 2017 NEC 705.12(B)(2), which in turn evokes 240.21(B)(2), as our tap conductors are between 10 and 25 feet in length.
My problem is with the way 705.12 interacts with 240.21. 705.12(B)(2) says "...any taps shall be sized based on the sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B)." Parsing that sentence in different ways yields different results.
Does all this mean that our tap conductors must have ampacity equal to 1/3 of 800A plus (1.25)(153A)? That doesn't make a whole lot of sense to me, since if the tap were a load the conductors would only need to be rated for 800A/3, and any current injected into the feeder through the tap would serve to reduce, not increase, the current in the tap conductors.
My problem is with the way 705.12 interacts with 240.21. 705.12(B)(2) says "...any taps shall be sized based on the sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B)." Parsing that sentence in different ways yields different results.
Does all this mean that our tap conductors must have ampacity equal to 1/3 of 800A plus (1.25)(153A)? That doesn't make a whole lot of sense to me, since if the tap were a load the conductors would only need to be rated for 800A/3, and any current injected into the feeder through the tap would serve to reduce, not increase, the current in the tap conductors.