The NEC^® has provisions for existing installations where a series rated system is installed. Quite often, a remodel might create the need to use an existing blank space or spare breaker in an older main distribution panel (MDP) to supply a new electric panel or branch circuit.

For many of these older installations, there may not be a series rating label or book at the MDP that provides all of the acceptable overcurrent device combinations to be used in a series combination. Over years, these books seem to disappear.

Section 240.86(A) allows series ratings for existing installations to be selected under engineering supervision. For these situations, the engineer is responsible to ensure that the downstream circuit breakers (that are part of the series combination) remain passive during the interruption period of the line side fully rated, current-limiting device. This is a tall order that many engineers do not wish to be responsible for. It can be difficult through math alone to guarantee that a specific circuit breaker does nothing during a fault condition while an overcurrent device ahead of it eliminates the fault condition first.

When an engineer selects the series combination for an existing installation where the series combinations are no longer available from the manufacturer, specific documentation is required as well as special field labeling mentioned in NEC^® 110.22(B).

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.

(A) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.

For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.