People without much training or experience in sizing conductors typically go right to the Article 310 ampacity tables. This is a mistake for several reasons. For one, it skips over the process of accurately determining what your load is.
Recently, we’ve been looking at load characterization for branch circuits. This is crucial to determining what your load is. It requires fact-gathering and engineering judgment to be done correctly.
But once this is done (or if it’s been done for you), what’s the next step? To determine the minimum size conductor you need, you turn to the NEC. Note that you can use a larger conductor, and often it’s a good engineering decision to do so. The NEC is concerned with using a sufficiently-sized conductor to carry the load safely. You might use a larger conductor to carry the load more efficiently.
The NEC is a big book. Drop it on your foot, and it hurts! Where do you begin, if not with those ampacity tables? Fortunately, the NEC is arranged to follow the load calculation sequence.
Not coincidentally, the load calculation sequence is really where the “meat” of the NEC begins. After Chapter One lays out the definitions and the general rules to follow, Chapter Two addresses wiring and protection.
Article 200 is short, and provides general rules for grounded conductors. In Part 6, we’ll look at what Article 200 has to say and which of its rules apply to our calculations.