In Section 240.86(B), the NEC^® has specific rules for “tested” series combination systems. The actual testing of overcurrent devices used in a series combination is done by nationally Recognized Testing Laboratories (NRTL) such as UL.

Two circuit breakers, or a fuse and a circuit breaker can be submitted to the testing laboratory by the manufacturer and tested under a simulated high-level fault condition to see if the combination of the two devices safely interrupts the fault before either of the two devices explodes. In the testing, the upstream fuse or circuit breaker is fully rated for the maximum fault current available at the line terminals and the downstream circuit breaker is subjected to more available fault current than the marked interrupting rating of the device.

Once an acceptable tested combination has been determined, UL 67 (Standard for Panelboards) requires the combination to be marked on or in the panelboard. Often, the number of possible series combinations are so many that a separate book is provided with the new panelboard. Unfortunately, this book is usually one of the first things tossed into the trash by the electrician after the panelboard is installed.

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).

2014-2017 Code Language:

240.86 Series Ratings.

(B) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.