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Wednesday, March 15, 2023
Question:
Hello Charlie, I am interested in the reasoning behind NEC 240.101(A) allowing the long-time trip element setting of a breaker (over 1000V) to be set as high as
six times the ampacity of the conductor. I’m sure 6X was specifically chosen for some reason. Is it related to inrush or some other factor?
Thanks, Jason D. Light
A
Answer:
Hi Jason, thank you for your question. When selecting overcurrent protection for any system and especially for over 1000V systems there are three (3) primary objectives:
Safety – Does the OCPD offer sufficient protection rated to carry and interrupt available fault currents and load over-currents? This helps in avoiding catastrophe in worst case scenarios.
Equipment Protection – This goes hand in hand with Safety OCPD must be sized to protect the equipment from damage.
Selectivity – Reliability is always important and, in some cases, mandatory for certain systems. OCPD plays a major role in reliability.
Remember the requirements in the NEC® are the minimum, meaning one can always go above and beyond. For over 1000V systems, Section 240.101(A) of the 2020 NEC® permits the continuous ampere rating of a fuse to be sized not more than three (3) times the conductor ampacity.
The long-time trip element setting of a breaker can be set not more than six (6) times the ampacity of the conductor. These maximums provide enough latitude for OCPD to be set to handle high inrush currents and operational loads but set below equipment damage thresholds. This helps to achieve the needed level of safety, protection of equipment and maintain reliability.
It is also important to mention that in the 2023 Edition of the NEC® the OCPD requirements for over a 1000V are now located in a separate Article, Article 245. More comprehensive requirements for Circuit-Interrupting Devices for these systems are now found in Section 245.21(A)-(E).