Series combination, or series rated systems, are often used in order to save money on an electrical project. A series rated system is less expensive than purchasing OCPD’s with a higher AIC rating that are fully rated for the amount of available fault current (AFC) at the line terminals of the devices.

In a tested series rated system, there will be an overcurrent protective device (OCPD) with an ampere interrupting capacity (AIC) rating equal to or greater than the AFC in series with a downstream circuit breaker that has an AIC rating less than the AFC at that location. Even though the AFC at the downstream circuit breaker is higher than the AIC rating of the breaker, the two OCDP’s have been tested by a Nationally Recognized Testing Laboratory (NRTL) to be able to safely interrupt a fault current condition that subjects the downstream circuit breaker to more AFC than it is rated for.

Series rated systems are permitted to be selected under engineering supervision for existing installations in accordance with NEC^® 240.86(A) or selected as part of a tested combination in accordance with NEC^® 240.86(B).

If a series rated system is selected from a tested combination, NEC^® Section 110.22(C) requires specific marking on each piece of equipment containing an overcurrent device used in the series combination. This marking is intended to alert the service electrician who is about to change a fuse or breaker that only specific replacement OCPD’s can be used to maintain the tested series rated combination.

For a new electric panel, the labeling required by NEC^® 110.22(C) is often provided by the manufacturer along with the series rating book. Unfortunately, the series rating label 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:

110.22(C) Tested Series Combination Systems. Equipment enclosures for circuit breakers or fuses applied in compliance with the series combination ratings marked on the equipment by the manufacturer in accordance with 240.86(B) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating. The marking shall meet the requirements in 110.21(B) and shall be readily visible and state the following:

CAUTION — SERIES COMBINATION SYSTEM RATED ____ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.