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Article 210 provides the requirements for branch circuits, but which of these rules apply to the calculations you must do to correctly size conductors and overcurrent protection devices?
Article 210 begins by telling you it applies to all branch circuits except motor circuits [210.1]. So you’re at a decision point, right away. If your branch circuit is for a motor, go to Article 430. Otherwise, continue on in Article 210.
An important concept to understand is that the overcurrent protection device (OCPD) defines the circuit. If your conductors are rated for greater ampacity than the circuit breaker, that will give you higher efficiency but it won’t change the rating of that circuit [210.3].
Multiwire branch circuits are limited to ratings of 15, 20, 30, 40, and 50A. But the NEC does permit you to install a branch circuit larger than 50A if it supplies non-lighting outlet loads on an industrial premises where conditions of maintenance and supervision ensure only qualified persons maintain them.
Branch circuits must conform to the voltage limits detailed in 210.6(A) through (E). For purposes of calculation, this is relevant because you don’t want to be performing calculations based on a voltage that isn’t permitted for the application. The chances that a person qualified to perform electrical calculations will ever violate this are remote, but Article 210 addresses the issue all the same.
In our next issue, we’ll look at why Section 210.11 (Branch Circuits Required) is relevant to your circuit calculations.