Rounding Up Rule

Jeffrey Sargent

Overcurrent protection of conductors is a critical element of electrical system safety. The general approach is to use a protective device having a continuous current rating or setting that does not exceed the allowable ampacity of the conductor. Using the standard ratings or settings for fuses and circuit breakers found in 240.6 and aligning those ratings and settings with the allowable conductor ampacities typically selected from Table 310.16 provides the protection against overcurrent required by the NEC®. However, the allowable conductor ampacities and the ratings or settings of standard overcurrent protective devices do not always perfectly align. In those cases, the provisions of 240.4(B)—often referred to as the “rounding up” rule— can be used. However, it needs to be said that this rule does not change the conductor ampacity to a higher value; it only allows for some latitude in using standard conductor sizes with standard ratings and settings of overcurrent protective devices. The first paragraph of 240.4 provides one of the most fundamental electrical safety requirements in the entire NEC. The use of insulated conductors and provision of overcurrent protection for a conductor based on its ampacity are long-standing requirements, having been in the Code since its first edition in 1897.