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Tuesday, February 21, 2017
Question:
To Code Question. What is the background of the 25% “buffer” allowance required for continuous load conductors over the maximum load? Why the exact number of 25%? Was that a calculation that was done, a test or was that arbitrary?
Thanks for your answer.
Jim
A
Answer:
Hey Jim thanks for your thought provoking question. In the NEC-Proceedings-1931 the following statement is included:
"One change adopted for Section 805 drops a distinction formerly made between the instantaneous and the time-limit types of circuit-breakers putting both types on a 125 per cent basis instead of the 160 per cent and 110 per cent settings previously authorized." Thanks to NFPA this document is available at http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards?mode=code&code=70&tab=editions - under the 1931 edition.
It is often stated that continuous loaded conductors will cause additional heat to be transferred to overcurrent devices therefore the overcurrent device needs to be sized larger. The conductor also needs to be sized larger to act as a sufficient heat sink for the overcurrent device. Sections 210.19, 210.20, 215.2, 215.3, 230.42 and the exceptions have rules related to this concept.
It is possible the 125 % value was used as a compromise between the 110 and 160 values previously used but I don't know that for certain. If anyone has additional information specifically why the 125% value was selected, please send it in and we will share it.