Code Change Summary: A new code section clears up the longstanding debate of coordination in overcurrent devices.

First off, this new code section is all about “coordination”, not “selective coordination”. This is evident because of the wording in the new informational note to this section as well as the main heading of the code section (see Article 100 revised definition of “coordination, selective”).

The time frame at which to coordinate overcurrent devices serving the essential electrical system in a hospital has been a tough issue for electrical engineers, designers, and AHJ’s for quite some time. With the new code section, it becomes clear. Overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.

The 0.1 second delay permitted in the new code section means that the downstream overcurrent device will open first during a thermal overload but does not guarantee that it will open first because of a short circuit or ground-fault condition. Because of this, coordination is achieved rather than “selective coordination” . Selective coordination covers the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.

Two new exceptions provide special allowances for transformer primary and secondary overcurrent devices as well as overcurrent devices of the same size connected in series.

Below is a preview of the NEC^®.  See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).