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 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 more than the AIC rating of the breaker, the two OCDP’s have been selected by an engineer based on their trip characteristics 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 under engineering supervision, NEC® Section 110.22(B) requires specific marking on each piece of equipment containing an overcurrent device used in the series combination. This marking is intended to ensure that any future service electrician that might be changing a breaker or fuse is aware that only specific replacement OCPD’s can be used to maintain the engineered series rated combination.
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).
2017-2023 Code Language:
110.22(B) Engineered Series Combination Systems. Equipment enclosures for circuit breakers or fuses applied in compliance with series combination ratings selected under engineering supervision in accordance with 240.86(A) shall be legibly marked in the field as directed by the engineer 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 — ENGINEERED SERIES COMBINATION SYSTEM RATED _______ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.
Which of the following is true for an engineered series combination system?
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