The NEC has two sets of rules for the minimum size of feeder conductors. There’s one set for feeders not over 600V [215.2(A)] and another for those over 600V [215.2(B)]. Each subsection is broken into three sub subsections.
While (A) has no text associated with it, (B) provides eight lines of text about the ampacity of the conductors.
• The first subsection of (A) is titled General. It provides four lines about the ampacity of the conductors, ending in a choice between the larger of 215.2(A)(1)(a) and 215.2(A)(1)(b). This is followed by three Informational Notes and two Exceptions.
• The first subsection of (B) addresses feeders supplying transformers.
215.2(A)(2) provides requirements for the size of the grounded conductor, while 215.2(B)(2) provides requirements for the ampacity of feeders supplying a combination of transformers and utilization equipment.
215.2(A)(3) provides a rule about the ampacity of feeders relative to service conductors while 215.2(B)(3) lets qualified persons under engineering supervision determine feeder conductor sizing.
At first glance, it seems (A) and (B) don’t correspond with each other. But look closer, and you’ll see some harmonization.
