Simplified Motor Control Conductor Protection Based upon the 1984 National Electrical Code

Abstract

Section 430-72 of the 1984 National Electrical Code has increased the level of protection required for conductors and transformers in motor control circuits, and has made these requirements somewhat easier to understand through the addition of Table 430-72(b). However, many variables, such as 1) whether or not the control conductors leave the enclosure, 2) the size branch circuit protective device (BCPD), 3) whether or not a control transformer is employed, and 4) the size of the control transformer, make it very difficult to determine when, where, and what size of a protective device is required. To simplify compliance with these requirements, two matrices were developed. One matrix determines the requirements for control circuits in series with a control transformer, and the other determines the requirements for control circuits not in series with a control transformer. The background of these matrices, along with an explanation of the various footnotes, is discussed.