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In this series, we’ve been looking at which NEC requirements affect circuit calculations. In our previous issue, we looked at the effects of Article 220 (calculations for services, feeders, and branch-circuits) on calculations.
Article 225 affects circuit calculations only indirectly. Article 230 (Services) presents us with a bit of an anomaly. What is that, you might ask? Article 240 contains the requirements for overcurrent protection except where those requirements are amended or supplemented by the Articles listed in Table 240.3. The table lists Article 230, and you’ll find those requirements in Part VII. For branch-circuits and feeders, they are in Article 240.
You can sum up the Article 230 requirements (relative to calculations) by saying the overcurrent protection device has a rating no higher than the allowable ampacity of the conductor [230.90(A)]. But there are five exceptions to this and one of them might apply to your installation.
Article 240 has few requirements that directly affect your calculations. One of them (for example) is that you must protect conductors (other than flexible cords, flexible cables, and fixture wires) against overcurrent in accordance with their ampacities specified in 310.15. There are seven exceptions (A through G).
Article 240 also requires you to use overcurrent devices with standard ampere ratings [240.6]. But this is about device selection, so it won’t change your calculations per se.
The three remaining Articles (250, 280, and 285) affect your design, but not your load calculations or calculations for sizing overcurrent protection.